High School Science Part II

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Air and Water : 21 :

18 Air and Water

You have already studied that air is a mixture of gases, which we cannot see. It is the main abiotic component of the environment. Air is an extremely important natural resource. Air is also necessary for all the living organisms, because they breathe in air. A human being breathes about 22,000 times in a day and takes about 16 kg of air into the body during this process.

Further, the air envelope, which surrounds the surface of the earth, presses the earth’s surface and creates a pressure on it. This air pressure, also called atmospheric pressure, is found to be useful. It would be interesting to know about air in this context also.

Like air, water is another abiotic component of the environment, which is also essential for all living beings. Water is the most abundant and renewable natural resource. It covers about three quarters of earth crust. Water occurs in nature in the free state as well as in the combined state. The different properties of water make it useful, important and essential in our daily life. We will also learn about water and its properties in this lesson.

OBJECTIVES

After completing this lesson, you will be able to:

• tabulate various component of air according to their amount;

• explain the importance and utility of various components (O₂, N₂, CO₂) of air; • measure atmospheric pressure and its variation with height;

• discuss the various atmospheric phenomena; • list different sources of water;

• list simple methods for making water potable; • describe various properties of water;

• recognise the utility of water for various purposes; • argue in favour of rainwater harvesting.

18.1 COMPOSITION OF AIR

Ancient philosophers considered air as one of the most vital element. Mayow in 1674 proved that air is not an element but the mixture of two substances, one of which is active and the other is non-active. Lavoiser in 1789 named the active element as oxygen and said that it is 1/5 of the total volume of air. The non-active

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: 22 : Air and Water

element is nitrogen and it is about 4/5th_{of the total volume of air. The ratio of} oxygen and nitrogen in the air is about 1:4 by volume.

The major components of air are nitrogen (N₂) and oxygen (O₂), while the minor components are argon (Ar), carbon dioxide (CO₂) and some are trace gases like neon (Ne), helium (He), krypton (Kr) and xenon (Xe). The composition of dry air at sea level is given in table 18.1.

Table 18.1 : Composition of air

Gas Composition Gas Composition

(% by volume) (% by volume)

N₂ 78.03 Ne 0.0015

O₂ 20.09 He 0.000524

Ar 0.94 Kr 0.000014

CO₂ 0.033 Xe 0.000006

Water is excluded from this table because its concentration in air varies drastically from location to location.

Let us perform a simple activity to study the presence of oxygen and nitrogen in the air.

ACTIVITY 18.1

Aim : To show the presence of oxygen and nitrogen in air

What is required? About 5cm long test tube, a beaker, water, graph paper,

cotton wool and a small piece of yellow phosphorus.

What to do?

• Take the small piece of phosphorus on cotton wool. • Insert the cotton wool inside a test tube.

• Now place the tube in inverted position in the beaker.

• Pour the water in beaker in such a way that 5 cm length of tube should be above the water.

• With the help of stand, hold the test tube in this position for one hour (Fig 18.1).

Fig. 18.1 Experimental set-up to show that air contains oxygen and nitrogen Phosphorus 0 0 1 1 2 2 3 3 5 5 4 4 Water Water Phosphorus Cotton wool plug

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What to observe?

After one hour you will see that water level in the test tube rises up by 1 cm.

Why is it so?

The oxygen present in air within the test tube slowly reacts with phosphorus and forms the oxide of phosphorus. The oxide dissolves in water to form phosphoric acid, which can be shown as follows:

P₄ (s) + 5O₂ (g) P₄O₁₀(s)

phosphorus oxygen oxide of phosphorus

P₄O₁₀ (s) + 6H₂O (l) 4H₃PO₄ (l)

oxide of phosphorus water phosphoric acid

The pressure of air within the tube falls, because the oxygen completely reacts with phosphorus. To make up this loss in pressure, the air from outside exerts pressure and hence, forces the water to rise upward within the tube.

From this activity it is clear that air consist 1 part oxygen and 4 parts nitrogen because the level of water rises by 1 cm out of the initial volume of 5 cm. Let us perform another activity that shows the presence of carbon dioxide in air.

ACTIVITY 18.2 Aim : To show the presence of carbon dioxide in air

What is required? A test tube, freshly prepared lime water, a cork with two

holes, two glass tubes bent at right angles.

Lime water turns milky

Suction by mouth

Fig. 18.2 To show that air contains carbon dioxide

What to do?

• Take about 4 mL freshly prepared lime water in a test tube.

• Fix a cork (having two holes) in the mouth of the test tube.

• Fix the two glass tubes in two holes in such a way that only one tube should be dipped in lime water but the other one should be above the lime water as shown in Fig. 18.2. • Suck the air from the tube, which is not dipping in limewater. Due to suction

the pressure of the air within the test tube falls.

To make for this loss in pressure, the air from outside enters into the tube through the tube dipping in limewater and air bubbles are liberated in limewater. You will see that after a minute the limewater turns milky. We know that only carbon dioxide can turn limewater milky. So this activity clearly shows the presence of carbon dioxide in the air.

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CHECK YOUR PROGRESS 18.1

1. Is air an element or a mixture?

2. What are the major constituents of air? 3. What is the ratio of major constituents of air?

4. Does the percentage of CO₂ remain constant in air or it vary from place to place?

5. Does the percentage of water remain constant in air or it vary from place to place?

18.2 IMPORTANCE OF VARIOUS COMPONENTS OF AIR

Oxygen, nitrogen and carbon dioxideare useful for human beings and plants. Without oxygen and nitrogen it is impossible to survive.

18.2.1 Importance and utility of oxygen

Since we live on the surface of the earth, we are surrounded by air, which contains oxygen. Oxygen is the main part of the air i.e. about 21%. We know that the life is not possible without oxygen. Therefore, oxygen is very much essential for life. The importance and utility of oxygen are as follows:

(a) General uses

• Oxygen is absolutely necessary for respiration. • It is the supporter of combustion.

• Liquid O₂ is used as oxidant in rocket fuel called as LOX (Liquid oxidant).

• In nature it dissolves in water. The dissolved oxygen keeps the water fresh and is a source of respiration for aquatic life.

• In some situations it is used for artificial respiration such as: – in the submarines and by deep sea divers.

– climbers, during high altitude climbing and also aviaters during high altitude flying.

– firemen during fire fighting.

• Corrosion is the term usually applied to the deterioration of metals by an electrochemical process. The most common example of corrosion is the formation of rust on iron. Oxygen gas and water must be present for iron to rust. It clearly indicates that oxygen is necessary for corrosion. • Oxygen combines with almost all elements to form oxides.

(b) Medical uses

• Carbogen: It is a mixture of 95% oxygen and 5% carbon dioxide. It

stimulates natural breathing. It is given to the patients suffering from asthma or for reviving patients from drowning or gas poisoning.

• Anaesthesia: It is a mixture of oxygen and nitrous oxide, which is

used in surgical operations. (c) Industrial uses

• In steel industry: Since oxygen produces more heat as compared to

air (because air contains some non reactive substances); it is used in place of air for the purification of iron.

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• For cutting and welding purposes: Oxygen is mixed with hydrogen

(hydrogen torch) or acetylene known as oxyacetylene torch. These are used for cutting and welding purposes.

• It is also used for the manufacture of sulphuric acid from sulphur and nitric acid from ammonia (NH₃).

18.2.2 Importance and utility of nitrogen

Nitrogen is the main constituent of proteins. A number of amino acids containing nitrogen join together to form protein. It is essential for the life of living beings. Its main uses are:

• It dilutes the activity of oxygen: If the amount of oxygen is increased in the air then the process like metabolism, combustion and corrosion will became very fast and becomes harmful. The presence of nitrogen dilutes the concentration of oxygen and thus, the combustion of fuel during burning and combustion of food during respiration takes place at moderate rate.

• The compounds of nitrogen are of vital importance to plants as they help them to manufacture proteins. Living beings obtain protein from plants.

18.2.3 Importance and utility of carbon dioxide

The percentage of carbon dioxide in air varies from place to place. The areas where more fuel containing carbon is burnt have more carbon dioxide. It is necessary for the production of food i.e. photosynthesis in plants. Its main uses are:

• During photosynthesis plants absorb carbon dioxide and water vapour from air. In the presence of chlorophyll and sunlight, they are converted to carbohydrates.

• It also provides Ca2+_{and Mg}2+_{ions in the soil, which are necessary for} the growth of plants. It dissolves in water and can also dissolve rocks containing calcium carbonate (CaCO₃) or magnesium carbonate (MgCO₃). The salts formed are Ca (HCO₃)₂ and Mg(HCO₃)₂. These salts give taste of natural water and also supply these ions to the plants. • It is also used in food preservation. In the presence of CO₂ the grains

are prevented from being destroyed by insects. • CO₂ is a green house gas. It traps infrared radiations .

• Solid CO₂is also known as dry ice which is used as refrigerant. • As it can be dissolve in water, it is used for the preparation of soft drinks. • CO₂ is used in fire extinguishers to put off fire.

1. Oxygen is essential for life, why? Give one example.

2. Carbon dioxide acts as food for plants. Name the process in which it happens. 3. What is dry ice?

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18.3 THE AIR AND ITS PRESSURE

We know that the air is a mixture of gases and particles of these gases have weight due to gravity. It shows that the air has weight. Anything that has weight pushes and presses against things. The air presses down on the earth’s surface and creates a pressure on it. So there is a force exerted by gas particles of air, which act downwards on the surface of the earth.

The force of air column acting per unit area of a surface results in a pressure exerted by atmosphere. This pressure is called atmospheric pressure. The

atmospheric pressure is about 1kg wt cm-2_{or 10 ton wt m}-2 Let us perform an activity to show that air exerts pressure

ACTIVITY 18.3 Aim : To show that air exerts pressure

What is required? A glass tumbler, a piece of cardboard and water. What to do?

• Fill the glass tumbler with water.

• Put the piece of cardboard on the top of the glass tumbler. • Hold the glass tumbler firmly with the palm of your hand.

• Grip the base of glass tumbler with your other hand. Turn the glass tumbler quickly upside downs as shown in figure 18.3.

• Remove the palm of your hand carefully below the cardboard.

What do you observe?

You will find that the cardboard and the water remain in their place. Can you think of the reason behind this? The water in the glass tumbler stays because air is exerting a pressure on the cardboard. The pressure of air against the cardboard is greater than pressure of water against the card board. If you turn the glass side ways and in any other position, the water still remains in the glass showing that air exerts pressure in all directions.

Fig. 18.3 Air exerts pressure

In our everyday life, atmospheric pressure plays an important role in the working of many things, for example, working of a straw, working of a syringe or ink dropper, working of a lift pump etc. Think and try to explain how atmospheric pressure helps in the working of these above mentioned things?

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18.3.1 Normal or standard pressure and its units

By international agreements, the normal or standard pressure is the pressure exerted by 76cm of mercury column. It is shown that,

Normal pressure = hDg

Where h = height of mercury column= 76 cm of Hg

D = density of mercury = 13.6 x 103_{kg m}-3 g = acceleration due to gravity = 9.8 m s-2

Therefore, the pressure exerted by a column of mercury at a height of 76 cm = 0.76 x (13.6 x 103_{) x 9.8 N m}-2

= 1.014 x 105_{N m}-2 Thus,



1 atmosphere = 1.014 x 105_{N m}-2



The unit of pressure used in meteorology is known as 1 bar where by definition



1 bar = 105_{N m}-2



Thus, 1 atmosphere = 1.014 bar



Another unit used for atmospheric pressure is known as torr Where, 1 torr = 1mm of mercury = 133.3 N m-2

Thus,



1 atmosphere = 760 torr



1 torr =1mm Hg

1 atmosphere = 760 mm Hg =760 torr

The SI unit of pressure is the Pascal (Pa), defined as one Newton per square metre. 1Pa = 1N/m2_{. The relationship between atmosphere and} Pascal is,

1 atmosphere = 101.325 kPa or 1.01325 x 105_Pa Since 1000 Pa = 1kPa

1 atmosphere = 1.01325 x102 _kPa

18.3.2 Measurement of atmospheric pressure

The instrument used to measure atmospheric pressure is called barometer. There are different types of barometers such as, Simple barometer, Fortin barometer, Aneroid barometer etc.

Simple barometer consists of long glass tube, closed at one end and filled

with mercury. If the tube is carefully inverted in a dish of mercury in such a way that no air enters the tube, then some mercury will flow out of the tube into the dish, creating a vacuum at the top as shown in fig 18.4. The weight of the mercury remaining in the tube is supported by atmospheric pressure acting on the surface of the mercury in the dish.

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Fortin’s barometer consists of a long vertical glass tube about 80 cm long.

It is completely filled with mercury and inverted over a cistern containing mercury. A small ivory peg is fitted into the lid of the cistern. While reading the atmospheric pressure, the tip of ivory peg should touch the level of mercury on which the atmospheric pressure acts (Fig 18.5).

An Aneroid barometer is more portable and cheaper than mercury type. No liquid is used here. The main features are as shown in the figure 18.6.

Fig. 18.4 Simple barometer Fig. 18.5 Fortin’s barometer Fig. 18.6 Aneroid barometer

B : Sealed metal box of corrugated sheet which is partially evacuated and sealed. Increase in atmospheric pressure causes the top to cave in while decrease allows it to expand.

L : A lever which magnifies the movement of the metal box. C : A chain wrapped round the spindle of the pointer.

P : It is pulled by lever. This moves the pointer over a scale S.

18.3.3 Variation of air pressure with height

The atoms and molecules of the gases in the atmosphere like those of all other matter, are subject to earth’s gravitational pull. As a consequence, the atmosphere is much denser near the surface of earth than at higher altitudes. In fact, the density of air decreases very rapidly with increasing distance from earth.

Thus, atmospheric pressure decreases with altitude. Often at higher altitudes, people find their nose bleeding because blood pressure is much more than the pressure outside (i.e. atmospheric pressure).

1. What is the unit of pressure?

2. At high altitude the people find their nose bleeding. Why? 76cm S V S2 Brass tube Ivory pointer S1 Sto rm y Rain Change Fa_ir V ery d ry S C PIVOT L B p PARTIAL VACUUM

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18.4 ATMOSPHERE

The region of air around earth is called atmosphere. The atmosphere protects us and all living things from harmful radiations like ultraviolet rays etc. We can divide the atmosphere into different layers according to temperature, pressure variation and composition. The main layers of the atmosphere (Fig. 18.7) from the surface of earth upward are troposphere (0-10km), stratosphere (10-50km), mesosphere (50-85 km) and thermosphere (85-500 km).

The most active region is the troposphere, the layer of the atmosphere, which contains about 18% of the total mass of air and practically all the atmosphere’s water vapours. It is the thinnest layer of atmosphere and here all the dramatic events of the weather (such as rain) occur.

Fig. 18.7 Layers of the atmosphere

18.4.1 Evaporation

We know that air contains water vapour. Their amount in the air is not the same everywhere. It is the maximum in the low latitudes and over oceans. The atmosphere over polar regions and land has less amount of water vapour. It is also more in summer than in winter.

Though water vapour comprises a very small part of the atmosphere, it plays an important role in heating and cooling of the atmosphere and in the day to day change in weather. In fact clouds, rain, snow, fog, frost and dew that we experience, result from water vapour in the atmosphere.

150 200 250 300 1× 10–2 Temperature/Kelvin 1× 100 _{1× 10}2 _{1× 10}4 _{1× 10}6 15 km 50 km 85 km 500 km Thermosphere Altitude Mesosphere Stratosphere Troposphere Mesosphere Stratosphere Troposphere Temperature Principal chemical species N , O , N, O2 2 N , O , N2 , O + 2 + + + O N , O , O2 2 3 N , O , N2 + 2 + + O 100 nm and more 220 nm and more 330 nm and more N , O , H O2 2 2 Ar, CO2

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But how does water vapours come in the atmosphere? It comes in the atmosphere through a process called evaporation due to solar heat. In fact evaporation is a process in which water from any source change into vapour state due to heat.

18.4.2 Cloud formation

Condensation of water vapour in the atmosphere leads to the formation of clouds. Clouds are formed when moist air rises upwards and is cooled as it rises. When the dew point i.e. the temperature at which the water begins to change into water drops, is reached, condensation of water vapour and the formation of very tiny droplets of water or ice crystal occurs. They cling to the dust particles in the air. These millions of minute water droplets or tiny ice crystals almost hang in the air rather than fall. They are blown as clouds by the wind. Clouds are of different types according to their shapes and height. If you watch the sky carefully you will be able to see that clouds are of different types.

18.4.3 Rain

When clouds are cooled owing to rising up or when they are blown into cooler region of the atmosphere, the small droplets of water in them become still cooler and they come closer to each other. A number of droplets combine to form big drop of water. These drops are so big that they can no longer float in the air, they fall downwards on the earth. As they fall, they pickup more and more small drops of water on their way down. The falling of these big drops of water from the clouds is known as rain. This process is called as precipitation.

The instrument used to measure rainfall is called rain gauge. Rainfall is measured in centimeters. The maximum rainfall occurs in the countries near equatorial regions and South-East Asia. In these regions, annual rainfall is 200 cm or more. The lowest rainfall occurs in Tundra Pradesh, central Asia and hot deserts, where it is less than 25cm. The medium rainfall (between 25cm to 200cm) occurs in west European countries, Tega regions and China.

18.4.4 Relative humidity

The existence of water vapour in the atmosphere, is known as humidity. Humidity of the air is related to its temperature. For example, during summer, you must have experienced days when both the temperature and humidity are high.

Relative humidity is the ratio of the mass of water vapour actually present in a certain volume of air at room temperature to the mass of water vapour required to saturate the same volume of air at that temperature.

The instrument used to measure relative

humidity is called hygrometer. Fig. 18.8 Hygrometer

120 110 100 90 80 70 60 50 40 30 20 10 o_F 120 110 100 90 80 70 60 50 40 30 20 10 o_F

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1. Name the instrument used to measure: i) atmospheric pressure.

ii) relative humidity. iii) rainfall.

2. Name the process through which water vapours come in air.

18.5 WATER - ITS SOURCES AND PROPERTIES 18.5.1 Sources of water

Other than air, water is the most important substance needed by living beings. Living beings cannot live long without water. The water is available in plenty on earth. It fills the seas, rivers and lakes, which cover more than three-fourth of the earth surface. It is also find inside the crust of the earth. Most of the water that we get from the wells comes from this source.

The natural sources of water are rain, spring, well, river and sea.

(a) Rain water: The rain water is considered to be the purest form of natural water

(distilled water) free from impurities. Why do we say so? We know that water from sea and rivers get evaporated into water vapour by the heat of sun. During this process of evaporation, impurities are left behind. When the water vapours go high up in the air they condense to form clouds. The water drops come down as rain.

(b) Spring water: Springs are formed by percolation of rain water into soil. Spring

supply water to wells and lakes.

(c) Well water: The rain water seeps through the soil and goes down. On digging

the well this underground water is available to us. This is known as well water. This water is not pure and contains impurities such as suspended particles, bacteria and other microorganisms.

(d) River water: Rivers form by melting

of snow on the mountains and also sometimes from the rain water. It is also not pure and not fit for drinking.

(e) Sea water: Out of these sources, sea

water is the largest natural source of water. However, it is the source of common salt and is the most impure form of water. All the impurities dissolved in river water are carried into the sea. As such, sea water can not be used for drinking purpose. There is a constant cyclic movement of water throughout the globe, which is called water cycle. Fig 18.9 shows the diagram of water cycle in biosphere.

1.3×10 kg16

6×10 kg14

Pore and ground water 4.2×10 kg Shallow plus soil moisture

18

5.3×10 kg deep18

Lakes and rivers

1.27×10 Kg17 Ice 2.9×10 Kg19 1.37×10 kg21 Uptake and photosynthesis Evaporation 4.23×10 kg y17 –1 Evaporation 3.86×10 kg y21 –1

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18.5.2 Purification of water for drinking

Water from different sources contains different substances in different amounts. In addition to salts found in sea water, water from other natural sources may contain foreign materials like suspended solids, minerals, certain compounds viruses, bacteria, eggs of insects and other animals, algae, protozoa and other aquatic plants. Such water is not safe for drinking and causes many harmful effects in the body.

There are different ways of purifying water for drinking. These are:

• By boiling during which bacteria and other germs die. When boiled water is

allowed to cool, heavy impurities collect at the bottom and dissolved salts form a thin layer on the surface called scum. Now if we filter the water, the filtered water is safe for drinking.

• By decantation and filtration.

• By chlorine treatment in which small living organisms and bacteria are killed.

18.5.3 Properties of water

Water, which seems to us to be a common ordinary material, is really a highly unusual substance with many unique properties which makes its use important and essential in our daily life.

18. 5.3a Water acts as universal solvent

Water is certainly one of the best and most useful solvents that we have. It has a unique property to dissolve many substances starting from solids such as common salts, sugar, to gases like oxygen, carbon dioxide etc. Indeed, so many substances dissolve in water that is why it is called as a universal solvent. This property is useful for plants to take in their food materials and minerals from the soil. It helps us to absorb food that we eat, in the form of water solution. Many chemical reactions also take place only in solution form in water i.e. aqueous solution.

18.5.3b Lather formation

Water forms lather with soap which is used for cleaning purpose. But sometimes water from some sources like rivers or hand pumps does not produce any lather with soap. Why? This is because of the presence of dissolved salts in water. Water, which we get from taps, contains lesser amounts of dissolved salts in it than water that we get from hand pumps. The dissolved salts are usually bicarbonates, sulphates and chlorides of calcium and magnesium. These salts prevent lathering but how?

The soap is a sodium salt called sodium stearate. This is soluble in water. However the calcium and magnesium stearates are insoluble and so when soap is added to hard water, which contains calcium and magnesium ions, a precipitate of Ca or Mg stearate is formed. This appears as a greasy scum. The formation of scum wastes soap (does not forms lather) and makes it more difficult to clean things.

Sodium stearate + Calcium sulphate Calcium stearate + Sodium sulphate

(Soap) (Scum)

Hence we can say that,

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• Water which does not form lather is called hard water.

• The hardness of water is due to the presence of salts of magnesium and calcium in water.

18.5.3c Conversion of hard water into soft water

Hard water does not form lather with soap –can this hard water be converted into soft water? Yes, hard water can be converted into soft water. Let us see how ? The removal of Ca and Mg ions which are responsible for hardness is called the softening of water.

Hardness of water is of two types namely, • Temporary hardness

• Permanent hardness

a) Temporary hardness

Temporary hardness of water is due to the presence of soluble bicarbonates of

calcium and magnesium. It is also called carbonate hardness. It can be removed by boiling and by soda lime process.

(i) By boiling: On boiling hard water, the calcium or magnesium bicarbonate

present is decomposed and give magnesium or calcium carbonate. These carbonate salts are insoluble in water. They settle down easily and water can be decanted. Decantation is the process of separation of solid from the liquid by allowing the former to settle down and pouring off the latter.

Heat

Ca (HCO₃)₂ CaCO₃ + H₂O + CO₂

calcium bicarbonate calcium carbonate

(Soluble) (Insoluble)

Heat

Mg (HCO₃)₂ MgCO₃ + H₂O + CO₂

magnesium bicarbonate magnesium carbonate

(Soluble) (Insoluble)

(ii) By soda lime (Clark’s method):When a calculated amount of lime is added

to hard water, then the soluble bicarbonates are converted to insoluble carbonates as follows:

Ca(HCO₃)₂ + Ca(OH)₂ 2CaCO₃ + 2H₂O

lime

Mg(HCO₃)₂ + Ca(OH)₂ 2MgCO₃ + 2H₂O

b) Permanent hardness

Permanent hardness of water is due the presence of soluble chlorides and sulphates of calcium and magnesium. It is also known as non-carbonate hardness. It can be removed by addition of washing soda and by the ion exchange method. A brief description of removal of permanent hardness of water is given below:

(i) By addition of washing soda: The hard water is treated with the calculated

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and sulphate of calcium and magnesium to form precipitate of calcium and magnesium carbonate.

The reactions are as follows:

CaCl₂ + Na₂CO₃ CaCO₃ + NaCl

calcium chloride sodium carbonate calcium carbonate sodium chloride

MgSO₄ + Na₂CO₃ MgCO₃ + Na₂SO₄

magnesium sulphate sodium carbonate magnesium carbonate sodium sulphate

The precipitate settles down and the water can be removed by decantation.

(ii) By ion exchange method: Two

types of ion exchangers can be used, namely, inorganic ion exchanger and organic ion exchanger. In inorganic

ion exchange process, complex

compounds known as Zeolite are used to soften the hard water. The salts causing the hardness of water are precipitated as insoluble zeolite of calcium and magnesium.On the large scale, this process is carried out in tanks as shown in figure 18.10

By using organic ion exchanger the

water obtained is free from cations and anions and is known as deionized water or demineralized water.

18.5.3d Polar nature of water

Water is very effective solvent for ionic compounds. Although water is an electrically neutral molecule, it has a small positive charge (on the H atoms) and a negative charge (on the O atoms), Therefore, it is a polar solvent.

O

δ-Hδ+ Hδ+

Let us perform an activity, which proves the polar nature of water

ACTIVITY 18.4 Aim: To study the polar nature of water

What is required? Burette, water, ebonite rod (negatively charged), glass rod

(positively charge) and burette stand.

What to do?

• Take a burette and fill it with water.

• Fix the burette vertically in a burette stand.

Fig. 18.10 Obtaining soft water on a large scale using tanks

Tap to take out soft water

Zeolite or permutt

Coarse sand Inlet for hard water

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• Open the stopcock of the burette and allow the water to flow.

• Take a ebonite rod (negatively charged by rubbing one end with fur) near the water

You will see that the stream of water is attracted towards negatively charged rod (Fig. 18.11a). Why? Because the water molecules have positive charge.

Similarly, now we take a glass rod near water, which is positively charged. You will see the rod again attracts the stream of water. This indicates that water molecule also has negative charge (Fig. 18.11b). This proves the polar nature of water.

18.5.3e Surface tension

Surface tension is the property of all the liquids. Due to this tension water drops try to occupy a minimum surface area. Hence, the water droplets always tend to take the shape of a sphere.

The tension exerted by molecules of water present on the surface layer is called as surface tension.

To understand this let us perform an activity.

ACTIVITY 18.5 Aim: To study surface tension

What is required? Glass and razor blade. What to do?

Take a glass full of water. Put a safety razor blade (having a coating of very thin layer of wax) gently on the surface of water.

You will find that it remains there although it is heavier than water.

Have a close look at the surface of water. You will find a thin film of water on the lower surface of the blade.

Why is it so? The upper layer of water is acting like a tight sheet. Why is the

sheet tight? Due to intermolecular forces i.e. forces between the molecules of the liquid surface and the blade, there is a tension or force acting on the surface of the thin film of the liquid.

18.5.3f Capillarity – Rise of water

When a capillary tube with a fine bore is dipped in water, water rises in the capillary. The extent to which the water rises depends on the diameter of the capillary. The smaller the diameter of the capillary the higher will be the rise of water in the capillary tube.

+ + + + + + + – – – – – – – Water Burette Stream of water Negatively charged Thermocol or Ebonite rod Positively charged glass rod

Fig. 18.11 To show that water is polar in nature

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This property of rise of water inside a capillary is called capillarity. This is the property, by which water from the soil enters the leaves and branches of the plants through its stems.

When a piece of cloth or bloating paper is placed on water, it soaks the water by this process of capillary action. The thread strands in the cloth and cellulose of the bloating paper serves like very fine bore tubes for the water to rise.

18.5.4 Density of water

Water behaves in an unusual way when it is heated from 0 o_{C. As the temperature} rises from 0 o_{C to 4} o_{C it actually contracts. However, at 4} o_{C upwards it expands} like any other liquid. This means that water takes up least space at 4 o_{C. It has the} greatest density at this temperature and will sink through warmer or colder water around it.

Density of a substance is defined as its mass per unit volume. Measurements on different volume of water at 4 o_{C show that,}

1 m3_{of water has a mass of 1000 kg.}

2 m3_{of water has a mass of 2000 kg. And so on.} Thus, the density of water at 4 o_{C is found to be,}

Mass of water 1000 kg of water

Density = ————— = ——————

Volume of water 1m3

= 1000 kg m-3 _{or 1g cm}-3

Because of this property of water, we can explain why it takes months for a lake to freeze while a small bucket of water can freeze over night on a bitterly cold day.

18.5.5 Specific gravity or relative density

The relative density of a substance is the ratio between density of a substance and density of water at 4 o_{C. It tells us that how many times more dense is the substance} than water i.e.

Density of a substance

Relative density (RD) = —————————————

Density of water at 4 o_C

As Relative Density is a ratio, it has no unit. For a particular substance its numerical value is always constant irrespective of whatever system of units are used. It is known as specific gravity. Another formula for calculating relative density or specific gravity is,

Mass of substance

RD = —————————————

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1. How much earth surface is covered by water? 2. Name any two sources of water.

3. Is rainwater pure or impure?

4. What is the role of chlorination during purification of water? 5. If water does not form lather, which type of water is it?

6. Name the type of hardness due to presence of bicarbonate of Ca2+_{or Mg}2+_. 7. Name the type of hardness due to presence of chloride or sulphate of Ca2+

or Mg2+_.

8. Which type of hardness is removed by the following (i) boiling

(ii) ion exchange method

9. Is water a polar or nonpolar solvent? 10. What is the unit of density?

18.6 UTILITY OF WATER

Water is used for many purposes, including growing crops, metallurgical operations to obtain metals such as copper, generating electricity, watering lawns, cleaning drinking and recreation. We can say that water is essential for life of living organisms. Without water, plants and animal cells cannot function and they ultimately die. Let us discuss the role of water for domestic use, agricultural use, industrial use and for the generation of electricity.

18.6.1 Domestic uses of water

Water plays an important role in domestic purposes, for example, it is used for cooking food, to wash utensils, clothes and clean the floor of houses. It is also used for whitewashing. It is used to take bath. Water dissolves the waste material of body such as stool, urine etc. and hence provides a good medium for extracting the body waste. The salts and the nutrients of the food dissolve in water. Therefore, these nutrients are easily absorbed by our body.

18.6.2 Agricultural uses of water

Water plays a similar important role in the plants life as in the human body. In agriculture sector water is used for the irrigation of crops, it helps in the germination of seeds and growth of plants. The nutrients provided by fertilizers to the soil are soluble in water. These dissolved nutrients are easily absorbed by the plants. Water is required for the preparation of food by plants (photosynthesis). It also acts as medium for the transport of nutrients and minerals from one part of the plant to other parts. It helps in maintaining firmness and structure of plant parts by providing appropriate pressure to the plant tissue. It is required for respiration by aquatic plants.

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18.6.3 Industrial uses of water

Water is used as a coolant in industries, for example the production of NH₃ in Haber’s process. It is also used in production of ice and as coolant in vehicles. It is used for the production of steam in industrial boilers and in steam engines. It is used as solvent in many industrial processes. Water is used to prepare many chemical compounds, for example H₂SO₄ is prepared by dissolving SO₃ in water and HNO₃ by dissolving NO₂ in water. Water is also used to prepare fuels like hydrogen gas and water gas.

18.6.4 Uses of water to generate electricity

There are many different ways to harness the energy from water. The most common way of capturing this energy is hydroelectric power. Electricity is generated by falling water.

Water is used in thermal power stations or nuclear power station to produce steam for the generation of the electricity.

18.6.5 Rain water harvesting

Over the years rising population, growth in industrialization and expending agriculture have pushed up the demand for water. Efforts have been made to collect water by building dams and reservoirs and creating ground water structures such as wells. Some countries have also tried to recycle and desalinate water. Wise conversion of water has become the need of the day. The idea of ground water recharging is gaining its importance in many of the cities. This is being done through rain water harvesting.

Rainwater harvesting essentially means collecting rain water on the roofs of building and storing it underground for latter use. Not only does this recharging arrest underground depletion of water but also raises the declining water level and can help augment water supply.

While many people may not realize it, but those few centimeters of annual rainfall are a valuable resource. Harvesting rainwater not only helps reduce the possibility of flooding, but it also decreases the community’s dependence for ground water for demestic uses. Rain water is perfectly suited for landscape irrigation, use in room coolers, washing and many other home applications. When rain water is used in room coolers and for washing needs, hardness deposits do no accumulate and there is no problem with soap scum. Harvested water may also be used for personal consumption, but it must be filtered and treated prior to use. By reducing runoff and rain water that falls on your house or field, you can put a valuable water resource to work around your house.

Thus, the benefits of harvesting rain water can be summarized as follows. • Conserves valuable ground water.

• Reduces local flooding and drainage problems.

• Decreases landscaping and property maintenance needs. • Provides excellent quality water for many household uses.

• It can be used for domestic purposes such as for vegetables, flowers, trees and shrubs and seedling in a green house etc.

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1. What are the uses of harvesting rain water?

LET US REVISE

• The major components of air are nitrogen and oxygen. The air also contains argon, carbon dioxide and some trace gases like neon, helium, krypton and xenon. It also contains water vapour.

• The weight i.e. the force of air column acting per unit area results in a pressure exerted by atmosphere called the atmospheric pressure.

• Atmospheric pressure plays an important role in our every day life in the working of common things like ink dropper, to straw, to lift pumps.

• The state of atmosphere in relation to the amount of water vapour is known as humidity.

• Mass per unit volume of a substance is known as its density.

• Next to air, water is the most abundant substance available to us. The natural source of water is rain, spring, wells, rivers and sea.

• The following properties of water make it suitable for use in our everyday life:

• ability to dissolve many things i.e. to behave as a universal solvent. • lather formation.

• surface tension. • capillarity.

• density of water at 4o_{C being 1 g cm}-3_.

• Relative density is the ratio between the density of a substance to the density of water at 4o_C.

• Water resources in a country is managed for proper and judicial use by constructing dams, canals, reservoir, wells and tube wells. Water collected in dams is not only used for irrigation but also to generate electricity.

• Rain water can be conserved by recharging it to ground or using it for various other purposes. This is known as rainwater harvesting.

TERMINAL EXERCISES A. Multiple choice type questions.

1. Air is

a) compound b) element c) mixture d) non of these

2. Major components of air are a) CO₂ + H₂O

b) N₂ + O₂ c) CO₂ + He d) H₂O + Xe

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3. Carbogen is the mixture of a) O₂ + CO₂

b) O₂ + N₂ c) O₂ + CO d) CO₂ + CO

4. The instrument used to measure humidity is a) barometer

b) hygrometer c) lactometer d) none of these

5. Water has maximum density at

a) 0 o_C _{b) 10}o_C _{c) 5}o_C _{d) 4}o_C

B. Descriptive type questions.

1. Name the various components of air. 2. Air is considered a mixture, why?

3. Prove by an activity that air is a mixture of different gases. 4. List the utility of oxygen and nitrogen in our lives.

5. What is atmospheric pressure? How is it measured using simple barometer? 6. How does the atmospheric pressure depend on altitude? Write down the units

of atmospheric pressure.

7. Name the different types of barometers. Explain the working of Aneroid barometer with diagram.

8. Give an activity, which proves that air exerts pressure. 9. What is relative humidity? How is it measured?

10. What is evaporation? How does it help in the formation of clouds? 11. What are the different source of water? Explain any two.

12. Why is water called as universal solvent? On what basis the following components dissolve in water: NaCl, Sugar and NH₃?

13. Explain water cycle with the help of a suitable diagram.

14. What are the different ways to purify drinking water? What is the role of chlorination?

15. Water is a polar solvent. Give an activity to prove it.

16. What do you mean by hard and soft water? Explain the types of hardness in water.

17. How are the temporary and permanent hardness removed from water? 18. Explain the following properties of water

(i) Surface tension (ii) Density

19. What is rainwater harvesting? How is it beneficial for everyday life?

20. List the utility of water for, domestic purpose, agriculture, industry and generation of electricity.

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ANSWERS TO CHECK YOUR PROGRESS 18.1

1. Mixture

2. Nitrogen and oxygen

3. 4:1

4. It varies from place to place. 5. It varies from place to place.

18.2

1. Needed for respiration by plants and animals 2. Photosynthesis 3. Solid CO₂ 4. Nitrogen 18.3 1. Pascal 2. Low pressure. 18.4 1. (i) Barometer (ii) Hygrometer (iii) Rain gauge 2. Evaporation.

18.5

1. Three fourth 2. Rain and sea

3. Pure

4. To kill microorganism 5. Hard water

6. Temporary

7. Permanent

8. (i) Temporary, (ii) permanent

9. Polar

10. g cm-3

18.6

1. It conserves valuable ground water.

2. It reduces local flooding and drainage problems

3. It decreases landscaping and property maintenance needs 4. It provides quality water for many household needs 5. It can be used for domestic purposes

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GLOSSARY

Atmospheric pressure: The force of air column acting per unit area results

in a pressure exerted by atmosphere.

Barometer: Instrument used to measure atmospheric pressure. Cloud: Condensation of water vapour in the atmosphere.

Dew point: The temperature at which the water begins to change into water

drops.

Density: Mass per unit volume of any substance. Evaporation: Vaporization of water due to solar heat.

Greenhouse effect: Trapping of infrared radiations, increased concentration

of CO₂.

Humidity: The state of atmosphere in relation to the amount of water vapour

it contains is known as humidity.

Hard water: Water that do not form lather with soap. Hygrometer: Instrument used to measure relative humidity.

Pascal (Pa): The SI unit of pressure. One Pascal in one Newton per square

meter. One atmosphere is equal to 1.01325 x 105_Pa.

Relative humidity: It is ratio of the mass of water vapour actually present in

a certain volume of air at room temperature to the mass of water vapour required to saturate the same volume of air at the same temperature.

Rain: The falling of big drops of water from the clouds is known as rain. Soft water: water that form lather with soap.

Surface tension: Tension exerted by the molecules of water present in the

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Mineral Resources–Metals and Non-metals : 43 :

19 Mineral Resources–

Metals and Non-metals

We use large number of materials but very few of them occur naturally. Most of these are obtained by transforming a relatively small number of naturally occurring raw materials chemically into more useful substances. You have learnt in earlier lessons that environment provides all the necessary support for the existence of mankind through its various components and resources both biotic and abiotic. In this lesson you will learn about some abiotic resources and how they are used to obtain useful substances and materials.

OBJECTIVES

After completing this lesson, you will be able to:

• differentiate between metals and non-metals on the basis of their properties; • distinguish between minerals and ores;

• recognize various metallurgical processes in the extraction of common metals– Fe, Al and Cu;

• explain the corrosion of metals;

• list various uses of metals and their alloys;

• explain the preparation, properties and uses of various non-metals–H, Si and P; • describe the allotropes of P and S and state the use of sulphuric acid.

19.1 MINERAL RESOURCES 19.1.1 Minerals and ores

There are 83 naturally occurring elements. The remaining ones can be produced in the laboratory only by nuclear reactions about which you learned in lesson 14. Out of all the naturally occurring elements oxygen (O), silicon (Si), aluminium (Al) and iron (Fe) are four most abundant elements ( arranged in the decreasing order) and account for more than 87% of earth’s crust (lesson 2, fig. 2.2). Only few elements are found in the free or native state i.e. in the uncombined state. Cu, Ag, Au and Platinum are some of them.

The remaining elements are found only in chemically combined state, that is, in combination with one or more other elements as minerals. A mineral is a

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: 44 : Mineral Resources-Metals and Non-Metals

naturally occurring homogeneous inorganic substance having a definite chemical composition and characterstic crystalline structure, colour and hardness. Although all minerals are sources of metals, it is not always possible to recover metals from them economically. A mineral from which it can be done is called an ore. Thus,

ore is a mineral from which a metal can be extracted profitably. For example,

copper pyrite (CuFeS₂) is a mineral of copper. Copper can be profitably extracted from it only if its copper content is 4% or more. If so, it is called the ore of copper otherwise a mineral.

19.1.2 Metals and non-metals

There are 83 naturally occurring elements. They can be broadly divided into two categories namely, metals and non-metals. The two differ widely in their physical and chemical properties. The properties which distinguish metals from non-metals are given in the tabular form below.

Table 19.1(A): Differences in physical properties of metals and non-metals Metals

They are solids at ordinary temperature and usually non-volatile (exception: mercury which is a liquid).

Their density is generally high.

They possess metallic lustre and take a high polish.

They are malleable and ductile. They can be beaten into thin sheets and drawn into wires (exception: Bi)

They form alloys with other metals and some non-metals.

They are good conductors of heat and electricity (exception: lead which is a poor conductor of electricity). Property State Density Metallic lustre Malleability and ductility Alloy formation Thermal and electrical conductivity Non-metals

They exist in all the three states, i.e. solid, liquid and gas. They are either gases or volatile liquids at low temperatures (exception: diamond and boron which are hard solids).

Their density is generally low.

Generally they do not possess metallic lustre (exception: graphite and iodine).

They are neither malleable nor ductile.

They do not form alloys (except carbon, silicon and phosphorus).

They are poor conductors of heat and electricity (exception: graphite and gas carbon).

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Table 19.1(B): Differences in chemical properties of metals and non-metals Property Nature of oxides Hydrides Electrochemical nature Action of acids Solubility Metals

They generally form basic oxides which form alkalis with water For example: 4Na + O₂→ 2Na₂O sodium sodium oxide Na₂O + H₂O → 2NaOH

sodium hydroxide (alkali) They either form no compound with hydrogen or form unstable hydrides.

They are electropositive elements and form cations. They are liberated at cathode during electrolysis. Some metals like chromium and manganese form some anions also with other elements like oxygen.

They generally dissolve in mineral acids forming a salt with the evolution of a gas. For example:

Zn + H₂SO₄→ZnSO₄ + H₂ zinc sulphuric zinc hydrogen acid sulphate They generally dissolve by a chemical reaction.

Non-metals They form acidic oxides only.

They form stable hydrides with hydrogen.

They are electronegative elements and form anions. They are liberated at the anode during electrolysis (exception: hydrogen).

They either do not dissolve in mineral acids or form the corresponding oxacids*. For example:

P₄+20HNO₃ → 4H₃PO₄+ 20NO₂+4H₂O

Many non-metals dissolve without any chemical change-taking place. For example: chlorine, bromine or iodine dissolves in water.

* Acids which contain oxygen are called oxoacids or oxyacids.

19.1.3 Activity series of metals

In the last section you learned that metals are electropositive in nature. In all their chemical reactions metals give off their electrons i.e. they act as reducing agents and themselves get oxidized. This process may be written as:

M Mn+ _{+ ne}

-Metals can give off their electron to the atoms of non-metals, hydrogen ions and even to ions of other metals. Electropositive character varies in different metals. A metal that can lose electrons more easily is more electropositive and would be more active in nature. Such a metal when dipped in a solution of salt of a less

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active metal would displace it (less active metal). Thus if a zinc rod is dipped in a solution of copper sulphate it would displace copper which is precipitated.

Zn(s) + Cu2+ _(aq) _Cu(s) ₊ _Zn2+_(aq)

zinc cupric ion copper

zinc ion

This happens because zinc is more electropositive in nature than copper. The arrangement of metals in the decreasing order of their activities is known as activity

series or electrochemical series. It is also known as reactivity series. A portion

of this series is given below:

Ca, Na, Mg, Al, Zn, Cr, Fe, Pb, (H), Cu, Hg, Ag

Hydrogen is the only non-metal that stands in this series. It is because hydrogen also shows electropositive character like metals. Any metal can displace all the metals that are on its right-side and which follow it. In other words, a metal can be displaced by only those metals which precede it and are on its left-hand side. Thus, Zn can displace any of Cr, Fe, Pb, H, Cu, Hg and Ag, metals which follow it and are placed on its right-hand side. But zinc can be displaced only by those metals which precede it and are on its left-hand side i.e. Ca, Na, Mg and Al.

19.1.4 Mineral resources in India

India is very fortunate to have been gifted by nature with rich mineral resources. It is the chief producer of mica and has monopoly for monazite, a mineral of thorium and has vast reserves of many important minerals. You will learn more about our mineral wealth in the discussion of various metals later in this lesson.

1. Name two most abundant elements in earth’s crust. 2. Which two of the following metals occur in native state?

Na, Cu, Zn, Ag, Fe

3. An element A forms basic oxide whereas another element B forms an acidic oxide. Which of them is expected to be malleable and ductile?

4. A non-metallic element forms a cation and is liberated at cathode during electrolysis. Which element could it be?

5. Which of the following metals can displace hydrogen from solutions of acids? Ag, Zn, Cu

19.2 METALS

We see a large variety of machines around us. Today man cannot live without machines. Some of them perform simple mechanical operations and substitute for the weak muscles of man like bulldozers, diggers, machine tools, and agricultural equipments. Others do very precise jobs that man can never do himself like high precision machine tools. Yet others substitute in many cases for human brain like computers. The pace of progress in any field- industry, transport or agriculture, is controlled by the number of machines that are produced and are usefully employed. But all machines are made of metals. Indirectly, the rate of development of a nation can be measured by the rate at which it produces and

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consumes various metals. In this section we will learn about various metallurgical operations which are used for extraction of metals from their ores and also about extraction, purification and properties of three important metals namely iron, aluminium and copper.

19.2.1 Basic metallurgical processes

Metallurgy is the branch of science dealing with extraction of metals from their

ores.The three main steps in extraction of a metal from its ores are (i) concentration of ore, (ii) production of the metal and (iii) purification of metals. Now we will learn about these steps.

19.2.1a Concentration of ore

The preliminary treatment of an ore to separate the waste materials from it is called its concentration. The waste materials usually associated with the ore are clay and silicates. They are collectively called the gangue. The method used for concentration of ore depends on the nature of

gangue associated with it as well as its own nature. Various methods used are:

(a) Magnetic separation

This method is used when a mineral is magnetic in nature and is attracted towards a magnetic field whereas the gangue is not. A schematic diagram of the process is shown in Figure 19.1 For example, separation of magnetite (Fe₃O₄) and pyrolusite (MnO₂) from gangue can be done using strong electromagnets.

(b) Hydraulic washing

This method is used due to a large difference in densities of ore which is heavy and of the gangue which is light. The lighter gangue particles are washed away in a stream of water while heavier minerals stay back.

(c) Froth floatation

This method is used for the concentration of sulphide ores. The powdered ore is mixed with water and oil and air is bubbled through it as shown in Figure 19.2.

This produces froth which floats on the surface. The oil preferentially wets the sulphide mineral particles which are carried by air bubbles to the surface of the mixture and are skimmed off. The froth is allowed to collapse and dried to recover the mineral.

Fig. 19.1 Schematic diagram of magnetic separation

Fig. 19.2 Froth floatation process ELECTRO-MAGNETS Agitator Non-magnetic particles Magnetic partciles Air Gangue

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(d) Calcination

It is used to remove volatile matter like moisture or water of crystallization from the ore by simple heating.

(e) Roasting

It is used to remove volatile impurities and to convert a sulphide or carbonate ore into oxide by heating it in a stream of air. For example

Cu₂S(s) + O₂(g) 2Cu₂O(l) + 2SO₂(g)

2ZnS(s) + 3O₂(g) 2ZnO(s) + 2SO₂(g)

19.2.16 Production of metal

After enriching the ore, it is converted into free metal. Metals in their combined state are always present as cation in their minerals. Therefore, production of a

free metal is always a reduction process. The method of reduction depends

upon the activity of the metal. Different methods employed are shown in the following table in which metals have been arranged in the decreasing order of their activity. Highly reactive metals are reduced by carrying out electrolysis of their molten salts like chlorides or oxide. The less reactive metals are produced by reduction of the metal oxide with a more reactive metal, or reduction with coke (carbon) and carbon monoxide. Table 19.2 lists the reduction processes which are employed for extraction of some metals.

Table19.2: Reduction processes employed to obtain some metals from their ores

Removal of impurities associated with ores

Some impurities are also associated with most of the ores which are called

gangue. They are usually infusible in nature and are removed with the help of

a flux. Flux is a material that is added during its reduction in a furnace to convert infusible impurities (gangue) into a fusible substance that is called slag which is separated from the molten metal and removed. The nature of flux depends upon the nature of impurity to be removed. An acidic flux (like sand) is added to remove basic impurities like FeO:

FeO + SiO₂ FeSiO₃

basic gangue acidic flux fusible slag

A basic flux (e.g. CaO) is added to remove acidic impurities (e.g. sand):

SiO₂ + CaO CaSiO₃

acidic gangue acidic flux fusible slag

Metals

Lithium, sodium, magnesium, calcium

Aluminium

Chromium, iron, manganese, zinc

Copper, gold, silver

Reduction process

Electrolytic reduction of their chlorides Electrolytic reduction of its oxide Al₂O₃ Reduction of the metal oxide with a more reactive metal, or reduction with coke (carbon) and carbon monoxide

Roasting of their sulphide ores

→ →

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19.2.1c Refining of metals

The metals obtained by the metallurgical processes described in the preceding sections usually need further treatment to remove the impurities. The refining process removes undesirable impurities from the metal. The following are the most commonly used methods of refining.

(a) Distillation or sublimation

This removes nonvolatile impurities from volatile metals like zinc and mercury which are obtained in the pure state.

(b) Liquation

Readily fusible metals like lead, tin and bismuth are refined by this method. Impure metal is melted on the sloping surface (called hearth). Pure metal melts and flows down and is collected separately while impurities are left behind.

(c) Electrolytic refining

It is the most commonly used method of refining of metals and produces highly pure metals. Impure metal is made the anode (positive electrode) and a piece of pure metal the cathode (negative electrode). These electrodes are dipped in a solution of a soluble salt of the metal. When electric current is passed, impure metal from the anode gradually passes into the solution and pure metal from the solution gets deposited on the cathode. Thus, effectively pure metal from anode passes to cathode gradually. Impurities either collect below the anode as anode

mud or dissolve in the solution. Copper, aluminium etc. are purified in this way. (d) Oxidation

Impurities like carbon, phosphorus, silicon etc. are removed as oxides by passing air through molten impure metal. Iron is purified by this method.

(e) Zone refining

This method is used for obtaining extremely pure metals and silicon. The metal (or silicon) rod which has already been purified extensively is placed in a quartz tube filled with a noble gas. It is moved slowly through a heating coil that melts only a small portion of the

metal rod. Pure metal crystallizes from the melt. The impurities are more soluble in the molten metal and are carried to the end of the rod. The metal rod is moved repeatedly through the heating coil. Impurities collect at the end of the metal rod (Figure 19.3). This end of the metal rod is cut off and discarded. The remaining metal is

extremely pure. Fig. 19.3 Zone refining

Metal rod Heating coil

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After learning about steps involved in extraction of metals from their ores we will now learn how three important metals, iron, aluminium and copper are extracted from their ores.

19.2.2 Iron

Iron is the chief metal used for making machinery and therefore plays very important role in growth of industry. It has been in use since long time. In India the art of making iron had reached an advanced state. This is testified by Ashok’s Iron Pillar in Delhi and iron joints used in the temple of Puri which remained rust-free even centuries after they were made. Aristotle in 340 B.C. provided a description

of the manufacture of a type of steel called wootz steel which was first produced in India but later on became famous as Damascus steel. It was renowned for its suppleness (bending easily), its ability to maintain a cutting edge and its use in making swords.

19.2.2a Ores of iron

Haematite (Fe₂O₃) is the most abundant ore of iron which is a reddish brown ore. Some other ores of iron are magnetite (Fe₂O₄), lemonite (2Fe₂O₃.3H₂O), siderite (FeCO₃) and pyrite (FeS₃). Out of these, haematite and magnetite are more suitable for the extraction of iron. Huge deposits of haematite are available in India in Mayurbhanj, Singhbhum and Mysore. Iron and steel are manufactured at Asansol, Jamshedpur, Durgapur, Bhilai and other places.

19.9.2b Extraction of iron

The following are the main steps in the extraction of iron from its ores:.

a. Concentration of ore

Ore is first broken into small pieces and if necessary it is washed with water. Usually the ore is rich enough and does not need any concentration.

In case the carbonate (siderite) or sulphide (pyrites) ores are used, they are roasted to convert to oxide and in case hydrated oxide (limonite) is used it is calcinated to remove the water of crystallization. In case of oxide ore (haematite or magnetite) these treatments are not required.

b. Reduction to iron

The oxide ore is reduced chemically by carbon monoxide which is formed in a blast furnace by reaction of coke with air (shown in the box). A mixture of iron ore, limestone, CaCO₃, and coke, called charge is fed into the furnace. The main reactions that occur are given below.

2C (s) + O₂(g) 2CO(g)

Fe₂O₃(s) + CO(g) 2FeO(s) + CO₂(g)

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Blast Furnace

Blast furnace is a chimney-like structure made of steel plates and lined inside with firebricks.

A mixture of iron ore, limestone, CaCO₃, and coke, called charge is introduced into the furnace from the top. A blast of hot air, fed from near the bottom into it. The oxygen in the air reacts with coke to form mainly carbon monoxide and some carbon dioxide. These reactions are highly exothermic in nature and a lot of heat is generated. The hot gases rise in the furnace and react

with iron oxide ore. Before the iron ore falls to the bottom of the furnace most of it is reduced to iron. At the bottom of the furnace the temperature is high enough to keep it in the molten state. It is taken out from the outlet near the bottom of the furnace. Lime stone decomposes into calcium oxide which then reacts with impurities (SiO₂ and Al₂O₃)

CaCO₃(s) CaO(s) + CO₂(g)

CaO(s) + SiO₂ (s) CaSiO₃(l)

CaO(s) + Al₂O₃(s) Ca(AlO₂)₂(l)

The mixture of calcium silicate and calcium aluminate, known as slag, remains in molten state and being lighter than molten iron floats on it and is removed from time to time from a separate outlet.

The iron obtained from blast furnace is called cast iron. It is also called pig

iron. It contains about 95% iron and about 5% carbon along with many other

impurities in small amounts like silicon, phosphorus, manganese and sulphur. It has a relatively low melting point and can be cast into desired shapes.

19.2.2c Properties of iron

(A) Physical properties

(i) Pure iron is a silver white metal. (ii) It is strongly attracted by magnets.

Fig. 19.4 Manufacture of iron in a blast furnace

CO, CO2 Charge (ore, limestone,coke) 200°C 700°C 1200°C 1500°C 2000°C

Hot air blast

Molten iron Slag

Solid char

ge descends

Hot gases rise

3Fe₂O₃+ CO → 2Fe₃O₄ +CO₂ CaCO3 → CaO + CO2

Fe3O4 + CO → 3FeO +CO2

C + CO₂ → 2CO FeO+ CO → Fe +CO₂ Iron melts Molten slag forms

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Commercially it is obtained in three forms: cast iron, wrought iron and steel. (B) Chemical properties

(i) When exposed to atmosphere, iron gets rusted. Rust is hydrated ferric oxide in which the amount of water associated varies. It is therefore represented as Fe₂O₃.xH₂O. For the rusting process oxygen (air) and water are required. It is accelerated by acidic substances like CO₂,SO₂ and retarded by alkalis. Presence of NaCl and other salts also increases the rusting rate. Completely homogeneous iron does not rust.

ACTIVITY 19.1 Aim: To study the rusting process

What is required? A few test tubes, corks, common salt, sodium hydroxide or

washing soda, iron nails, small piece of zinc from used cells and a small piece of copper wire.

How to do? Take 7 test tubes. Paste labels on each of them and mark them as

1,2 and so on. Prepare the test tubes as given below. (Fill about half the test tube with the liquid/solution mentioned.

Practically no rust is formed A small amount of rust is formed. A reddish brown coloured solid is formed on and around the nail. A little more amount of rust is formed than in test tube 2. A reddish brown coloured solid is formed on and around the nail. Amount of rust formed is much more. A reddish brown coloured solid is formed on and around the nail.

Practically no rusting occurs.

White coloured precipitate is formed.

A large amount of rust is formed. A reddish brown coloured solid is formed on and around the nail.

Test tube no. 1 2 3 4 5 6 7 Liquid solution

Boiled water + iron nail and cork the test tube

Tap water + iron nail

Solution of common salt in tap water + iron nail

Solution of common salt and vinegar in tap water + iron nail

Solution of common salt and sodium hydroxide or washing soda in tap water + iron nail Solution of common selt + iron nail with a small piece of zinc attached to it.

Solution of common salt + iron nail with a small piece of copper wire around it.