To describe how strong an acidic or basic substance is, we use the pH scale. At 25°C, the pH scale goes from 0 to 14, with acidic substances having pH less than 7, and basic substances having pH greater than 7. Strongly acidic substances have pH closer to 0.
Very basic substances have pH closer to 14. A neutral substance is neither acidic nor basic and has a pH of 7.
The pH is a measure of how much free hydrogen is present in a solution. If there is a lot, the pH is very low. If there is hardly any, the pH is higher.
Every time you take a step along the pH scale, say from pH 3 to pH 4, the hydrogen present decreases by a factor of 10. Say you have 10 mL of a solution with a pH of 1. If you add 90 mL of water, the new volume will be 100 mL and you will have diluted
the solution by a factor of 10. The pH of the new solution will be 2.
Indicators
Indicators are chemicals that are used to show the pH of a solution. Some indicators are not very precise and only tell us whether a solution is acidic or basic. Litmus, which is made from plants called lichens, is an example of this kind of indicator.
It is red in acidic solution and blue in alkaline
The pH scale (pH is short for
‘power of hydrogen’)
strong acids neutral strong bases
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH
dishwashing powder
detergents
baking sodasea waterbloodpure watertap water
coffeewineorange juicevinegar
stomach acid
Fig 2.4.8
Fig 2.4.9 Blue litmus paper turns red when in
contact with an acid. The juice of a lemon contains citric acid.
The colour changes of some indicators
bromothymol
colourless change pink red-orange change yellow
red change blue
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 deep red red
indicator pH
yellow change blue Fig 2.4.10
Prac 2 p. 64 Worksheet 2.2 A neutralisation reaction
solution and thus only gives us a broad range of possible pH values. Other indicators are far more precise. Universal indicator is an example of this because it can undergo many colour changes and gives us a good estimation of the pH of a solution.
You may have seen universal indicator used to check the pH of a swimming pool or spa. Figure 2.4.10 shows the colour changes of some other common indicators.
Many plants, like beetroot, red cabbage, hydrangeas and hibiscus, produce dyes that can be used as indicators. Hydrangeas, for example, have blue flowers in acidic soil and pink flowers in alkaline soil.
2.4
UNIT
[ Questions ]
Checkpoint
Acids
1 Define the term ‘acid’.
2 Describe two properties of an acid.
3 Describe how sulfuric acid reacts with living tissue.
4 Identify three different acids and give their chemical formulas.
5 Identify the acid present in vinegar. Explain why it is a weak acid.
Bases
6 Define the term ‘base’.
7 Define the term ‘alkali’.
8 Describe two properties of a base.
Acids and metals
9 To non-chemists, ‘salt’ is sodium chloride, NaCl.
Define what chemists mean by the term ‘ salt’.
10 Construct word equations for the reactions of the following metals with nitric acid:
a aluminium c iron
b zinc d lithium
Neutralisation
11 Describe some everyday examples of neutralisation.
12 a Identify some examples of stinging creatures.
b Explain what can be done to neutralise the sting.
c Explain why lifesaving clubs in New South Wales make available buckets of dilute vinegar when bluebottle jellyfish are being blown onshore.
d Explain why vinegar would not relieve beestings.
13 Copy and complete the following table to distinguish the reactants and products of the following reaction types.
Reaction type Example Reactant(s) Product(s) (word equation
and chemical formulas) Acid + metal
Acid + hydroxide Acid + oxide Acid + carbonate
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Acids and bases Acids and bases
Worksheet 2.3 pH levels of common drinks
deep red red red orange yellow
green green
green blue blue violet
red orange blue violet
1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH
Fig 2.4.11 Colour changes of universal indicator with pH
Prac 3
The pH scale
14 Identify the approximate pH of:
a a strong acid d a weak base b a weak acid e a strong base c pure water
15 Define the term ‘indicator’.
16 Identify an important use for an indicator.
17 At pH 8, identify the colour of:
a universal indicator b red litmus c blue litmus
Think
18 Identify three fruits containing citric acid.
19 Distinguish between a dilute solution of nitric acid and a concentrated solution of nitric acid.
20 When sodium hydroxide reacts with fats, soap is produced. Explain why bases feel soapy to touch.
21 Describe how you could test for:
a hydrogen gas b carbon dioxide gas
22 The normal pH in the mouth is about 6.5. The pH in the stomach is around 2 to 3. Explain why you get a burning sensation in the oesophagus, throat and mouth when you vomit.
[ Practical activities ]
2.4
UNIT
Prac 1 Unit 2.4Acids and metals
AimTo observe the reaction of an acid with a metal
Equipment
3 test tubes with stoppers, test-tube rack, matches, 100 mL beaker, small pieces of aluminium, magnesium, zinc, iron and tin, 0.1 M solutions of hydrochloric, sulfuric and acetic acids, lab coat, safety glasses
Method
1 Copy the results table opposite into your book.
[ Extension ]
Investigate
1 Many advertisements for shampoos and skin lotions mention their pH.
a Research which pH is best for your skin and which is best for your hair.
b Does the age of a person and their type of skin or hair change the answer to this question?
2 Sulfuric acid is one of the most important chemicals in the world. The sulfuric acid production of a country is said to be a good indicator of the state of its economy. Investigate what sulfuric acid is used for and analyse the reasons why sulfuric acid is a measure of the economy.
3 Investigate some other acids that are used either in cooking or in medicine. This could include salicylic acid or tartaric acid,
4 The ideal pH of a swimming pool is around 7.2.
At this pH, most bacteria and green algae can’t grow.
Describe how pH levels of pools are tested, why they change and how the pH is kept at 7.2.
UNIT UNIT
2.4 2.4
23 Azaleas grow only in soil with pH greater than 7.
a Select which substance should be added to basic soil to lower its pH. Choose either water, an acid or a base.
b Explain your answer.
24 A certain food is found to be slightly acidic. It contains either hydrochloric acid or acetic acid. Evaluate which acid it is more likely to contain.
Analyse
25 Identify the salt produced by each of the following neutralisation reactions.
a nitric acid + strontium hydroxide b sulfuric acid + copper carbonate c hydrochloric acid + silver oxide
d nitric acid + magnesium hydrogen carbonate 26 a Construct word equations for the following reactions.
i hydrochloric acid + iron(II) hydrogen carbonate ii nitric acid + silver hydroxide
iii sulfuric acid + barium oxide
b Use the formulas given throughout this chapter to write the reactions as chemical equations.
27 Identify which acid and base you could combine to make these salts:
a barium chloride b calcium nitrate c iron(III) sulfate
28 You are given 10 mL each of two solutions. Solution A has a pH of 2. Solution B has a pH of 4. Calculate how much water you would have to add to solution A to make its pH the same as that of solution B. This is a hard one!
29 Evaluate the importance of acids and bases in our daily lives.
30 Explain how classifying acids and bases can make science easier.
Hydrochloric Sulfuric Acetic
acid acid acid
Aluminium Magnesium Zinc Iron Tin
>>
2 Pour 2 cm of hydrochloric acid into each test tube.
3 To the first test tube add one of the metals. If there is an obvious reaction, hold the stopper on the tube for about 15 seconds. Light a match and, removing the stopper quickly, hold the lit match to the mouth of the tube.
Record your observations.
4 Repeat step 3 for the other metals. You do not have to repeat the gas test for every reaction.
5 Repeat steps 2–4 for the other acids.
Questions
1 Construct word equations for the reactions of the metals with one of the acids tested.
2 For the other two acids, identify the salts produced in each reaction.
3 From the speed of the reaction with each metal, arrange the metals tested in order from most active to least active.
Hold stopper on tube for 15 seconds.
A ‘pop’
indicates hydrogen is present.
A second person lights a match and holds it to the mouth of the tube as the stopper is removed.
‘pop’
i ii iii
Fig 2.4.12
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Acids and bases Acids and bases
Prac 2 Unit 2.4