CHEMISTRY PHYSICAL AND CHEMICAL CHANGES
Physical and Chemical Changes
Physical Change
A change which alters some physical properties (e.g. colour, state, texture, magnetic or electrical conditions, density) and does not involve a change in the molecular structure, i.e. no new substance is formed.
Chemical Change
A change which alters the specific properties of a material by bringing about a change in which the original substance gives rise to one or more new substances with different composition and properties.
Physical change Chemical change
Characteristics 1. No new substance is formed.
2. Composition of the original substance remains unchanged.
3. The change is temporary and reversible.
4. There is no change in the weight of the substance.
5. There is no gain or loss of energy.
6. Energy required for completion of a physical change is released when the change is reversed.
Example: Ice changes to water
1. New products are formed.
2. Composition of the reactants changes, resulting in the formation of one or more new substances.
3. The change is permanent and irreversible.
4. The weight of a substance undergoing a chemical change usually changes.
5. It is always accompanied by the absorption or liberation of energy.
6. Energy in the form of heat, light and pressure is required for a chemical change.
Examples Change of state of matter:
1. Melting: Solid to liquid 2. Freezing: Liquid to solid 3. Vaporisation: Liquid to gas 4. Liquefaction: Gas to liquid
Chemical reactions:
1. Addition of acids to substances 2. Fermentation of substances 3. Respiration
4. Dehydration of carbohydrates
Exothermic Change
A chemical change which occurs with the release of heat energy is called an exothermic change.
Carbon burns in oxygen to form carbon dioxide, and heat energy is produced.
C + O2 → CO2 + Heat
Endothermic Change
A chemical change which occurs with the absorption of heat energy is called an endothermic change.
Formation of carbon disulphide C + 2S + Heat → CS2
CHEMISTRY PHYSICAL AND CHEMICAL CHANGES
Photochemical Reaction
A chemical reaction which proceeds with the absorption of light energy is called a photochemical reaction.
Example: Photosynthesis in plants 6CO2 + 6H2O C6H12O2
Electrochemical Reaction
A chemical reaction which proceeds with the absorption of electric energy is called an electrochemical reaction.
Example: Fused potassium chloride on passing electric current breaks into its charged particles (ions).
2KCl K+ + Cl−
Types of Chemical Change
1. Direct
combination or synthesis
A chemical reaction in which two or more substances combine to form a single product is called a combination reaction or synthesis.
When iron and sulphur are heated together, they combine to form a single product, iron sulphide.
2Fe(s) + S(s) → FeS(s) Iron Sulphur Iron sulphide
2. Decomposition A chemical reaction in which a single compound splits into two or more simple substances is called a decomposition reaction.
Electrolysis of acidulated water
On passing electric current through acidulated water, water produces two volumes of hydrogen gas and one volume of oxygen gas.
2H2O(l) 2H2 (g) + O2(g)
3. Thermal
decomposition
During thermal decomposition, a chemical compound breaks into simpler compounds. The simpler compounds do not reunite to form the original compound on cooling.
2 KClO3 2KCl + 3O2
2NaNO3 2NaNO2 + O2
4. Reversible reaction
A reversible reaction is one in which the direction of a chemical change can be easily reversed by changing the conditions under which the reaction occurs.
The reaction between iron and water to produce ferric oxide and hydrogen gas is a reversible reaction. This is indicated by using a two-way arrow.
3Fe + 4H2O Fe3O4 + 4H2
Iron Water Ferric oxide Hydrogen 5. Thermal
dissociation
A reaction in which a substance dissociates into two or more simpler substances on the application of heat is called a thermal dissociation reaction. It is a reversible reaction.
Ammonium chloride dissociates in the presence of heat to form ammonia and hydrogen chloride gas.
NH4Cl(s) NH3 + HCl(g)
Ammonium chloride Ammonia Hydrogen chloride gas
CHEMISTRY PHYSICAL AND CHEMICAL CHANGES
6. Displacement A reaction in which the more reactive element displaces the less reactive element from its compound is called a displacement reaction.
Zinc displaces copper in copper sulphate to form zinc sulphate.
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) Zinc Copper sulphate Zinc sulphate Copper
More electropositive metal Less electropositive metal 7. Double
displacement
A reaction in which ions of the reactants exchange places to form two new compounds is called a double displacement reaction.
In a double displacement reaction, the two reactants taking part are generally water soluble, and one of the products is soluble and the other being insoluble separates out as a solid.
Sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Sodium hydroxide Hydrochloric acid Sodium chloride Water 8. Double
decomposition
A type of chemical change in which two compounds in a solution react to form two new compounds by the mutual exchange of radicals. Usually, a solid is formed as a result of the reaction.
These reactions are of two types—precipitation and neutralisation.
Precipitation reaction
The insoluble solid formed during a double displacement reaction is called a precipitate. A reaction in which a precipitate is formed as one of the products is called a precipitation reaction.
Sodium sulphate reacts with barium chloride to form barium sulphate and sodium chloride solution.
Na2SO4(aq) + BaCl2 → BaSO4(s) + 2NaCl(aq)
Sodium sulphate Barium chloride Barium sulphate Sodium chloride Neutralisation reaction
The reaction between an acid and a base to form a salt and water is called a neutralisation reaction.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Sodium hydroxide Hydrochloric acid Sodium chloride Water 9. Isomerisation It is a special type of reaction in which a simple rearrangement of the
atoms occurs in the molecule of a substance, and a new substance is formed due to a change in the molecular structure only.
Ammonium cyanate on heating produces urea.
NH4CNO NH2CONH2
Ammonium cyanate Urea
10. Polymerisation When two or more molecules of the same compound associate to form a bigger molecule, the reaction is called polymerisation. These reactions usually occur at high pressure.
3C2H2 C6H6
Acetylene Benzene
nC2H4 → (-cH2-cH2-)n
Ethene Polythene 11. Oxidation and
reduction reaction (Redox)
Oxidation reaction
A reaction which involves the addition of oxygen or the removal of hydrogen is called an oxidation reaction.
Hydrogen sulphide reacts with chlorine gas to form hydrogen chloride and sulphur. In this reaction, chlorine acts as an oxidising agent and removes hydrogen from hydrogen sulphide to form sulphur.
CHEMISTRY PHYSICAL AND CHEMICAL CHANGES
H2S + Cl2 → 2HCl + S Hydrogen sulphide Chlorine Hydrogen chloride Sulphur Ferrous chloride reacts with chlorine gas to form ferric chloride. In this reaction, ferrous chloride gains an electronegative radical chlorine and is therefore oxidised to ferric chloride.
2FeCl2 + Cl2 → 2FeCl3
Ferrous chloride Chlorine Ferric chloride Reduction reaction
The addition of hydrogen to a substance is called reduction. The removal of oxygen from a substance is also called reduction. A reaction which involves the addition of hydrogen or the removal of oxygen is called a reduction reaction.
Ferric oxide reacts with carbon monoxide to form iron and carbon dioxide. In this reaction, carbon monoxide acts as a reducing agent and removes oxygen from ferric oxide to form iron.
Fe2O3 + 3CO → 2Fe + 3CO2↑ Ferric oxide Carbon monoxide Iron Carbon dioxide Chlorine gas reacts with hydrogen sulphide to form hydrogen chloride and sulphur. In this reaction, hydrogen sulphide acts as a reducing agent and supplies hydrogen to chlorine to form hydrogen chloride.
H2S + Cl2 → 2HCl + S Hydrogen sulphide Chlorine Hydrogen chloride Sulphur Redox reaction
The oxidation and reduction reactions occur together. When oxidation occurs in one substance, reduction occurs in the other substance. Such reactions are called redox reactions. In the term
‘redox’, ‘red’ stands for reduction and ‘ox’ stands for oxidation.
Burning
Burning is a chemical change in which combustible substances combine with oxygen and produce new products called oxides with the liberation of heat and light.
Conditions of Burning
1. Substance must be combustible.
2. The combustible substance must be placed in an environment which supports combustion.
3. The temperature of the substance must be raised to its ignition point.
Ignition Temperature
It is the lowest temperature to which a substance must be heated before it can be burned.
Reactions of metals with air
Metals combine with air (oxygen) to form their respective oxides. These oxides are basic in nature.
Metal + Oxygen → Metallic oxide 4Na + O2 → 2Na2O
2Ca + O2 2CaO 4Al + 3O2 → 2Al2O3
4Fe + 3 O2 + 2xH2O → 2Fe2O3.xH2O
CHEMISTRY PHYSICAL AND CHEMICAL CHANGES
Reactions of non-metals with air Carbon
Carbon + Oxygen → Carbon dioxide C + O2 → CO2
Phosphorus
Phosphorus + Oxygen → Phosphorus pentoxide 4P + 5O2 → 2P2O5
Carbon dioxide
Carbon dioxide + water → Carbonic acid CO2 + H2O → H2CO3
Respiration
Respiration is a process which occurs in living beings while breathing air in and out. CO2 and H2O are formed with the release of energy in the form of heat alone.
C6H12O6 + 6O2 → CO2 + 6H2O + Heat Glucose in our blood Water vapour
Comparison of Burning and Respiration
Burning Respiration
1. Oxygen is needed to combine with carbon and hydrogen present in carbon compounds.
1. Oxygen is needed to combine with carbon and hydrogen present in food materials.
2. CO2 and H2O are formed with the release of energy in the form of heat and light.
2. CO2 and H2O are formed with the release of energy in the form of heat alone.
3. It is a fast process. 3. It is a slow process.
4. It occurs at higher temperatures. 4. It occurs at body temperature through enzymes.
CHEMISTRY PHYSICAL AND CHEMICAL CHANGES Oxygen Cycle
Oxygen is removed from the air by the process of
Decomposition of waste materials
Combustion of fuels
Breathing by living animals
Corrosion of metallic objects
Photosynthesis
Photosynthesis is a process in which carbon dioxide and water are converted into simple carbohydrates (glucose, C6H12O6) and oxygen. The process is powered by solar energy, which is absorbed by
chlorophyll molecules present in the cells of green plants and algae.
light energy
2 2 chlorophyll 6 12 6 2 2
6CO +12H O C H O +6H O+6O
Carbon Cycle
The various processes by which the percentage proportion of carbon dioxide in the atmosphere is kept constant constitute the carbon cycle. About 0.03% to 0.04% of carbon dioxide is present in the
atmosphere.
Carbon dioxide is produced in industries by heating of carbonates and bicarbonates. It is also produced by reacting carbonates and bicarbonates with acids.
CHEMISTRY PHYSICAL AND CHEMICAL CHANGES
Carbonates
CaCO3(s) CaO(s) + CO2(g)
Bicarbonates
2KHCO3(s) K2CO3(s) + H2O(g) + CO2(g)
Removal of carbon dioxide
Carbon dioxide is used in the process of photosynthesis. It is also absorbed by water, alkalis and limewater.
2KOH + CO2 → K2CO3 + H2O Ca(OH)2 + CO2 → CaCO3 + H2O (Milky)
CaCO3 + CO2 + H2O → Ca(HCO3)2