What makes a solution acidic or alkaline, and how does the pH scale measure it?
Describe the properties of acids and bases, the ions they produce in solution, and use the pH scale and indicators to classify solutions.
A focused answer to WJEC GCSE Chemistry topic 2.2, covering the properties of acids and bases, the hydrogen and hydroxide ions they produce in solution, the pH scale, and using universal indicator and a pH meter to classify solutions.
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What this topic is asking
WJEC topic 2.2 wants you to know the properties of acids and bases, the ions each produces in water, and how the pH scale and indicators (universal indicator or a pH meter) classify a solution as acidic, neutral or alkaline.
Acids and bases
Common acids include hydrochloric acid (), sulfuric acid () and nitric acid (). Common alkalis include sodium hydroxide () and aqueous ammonia. Many bases are metal oxides or hydroxides.
The ions in solution
The chemistry of acids and alkalis comes down to two ions:
The pH scale
The pH scale measures how acidic or alkaline a solution is:
Measuring pH
You can find the pH of a solution in two main ways:
- Universal indicator is a mixture of dyes that turns a range of colours depending on pH. You add it (or use indicator paper) and compare the colour with a pH colour chart to read off an approximate pH. Acids give red, orange or yellow; neutral gives green; alkalis give blue or purple.
- A pH meter has a probe that gives a precise numerical pH value on a display, which is more accurate than the colour comparison.
Simple indicators such as litmus only tell you acidic (red) or alkaline (blue), not a pH number.
Strong and weak, concentrated and dilute
Two pairs of words are easy to confuse. Strong and weak describe how fully an acid splits up (ionises) into ions in water: a strong acid (such as hydrochloric or sulfuric acid) ionises almost completely, releasing many ions, while a weak acid (such as ethanoic acid, found in vinegar) only partly ionises. Concentrated and dilute describe how much acid is dissolved in a given volume of water, not how fully it ionises. So a solution can be a dilute strong acid or a concentrated weak acid. A strong acid generally has a lower pH than a weak acid of the same concentration, because it produces more hydrogen ions. Keeping these ideas separate is important when you explain pH values in the exam.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC sample3 marksState the ion that all acids produce in aqueous solution and the ion that all alkalis produce. Describe how universal indicator could be used to show that a solution is acidic.Show worked answer →
A Unit 2.2 structured question. Reward: all acids produce hydrogen ions, , in aqueous solution; all alkalis produce hydroxide ions, . To use universal indicator, add a few drops to the solution (or dip indicator paper) and compare the colour with a pH colour chart: an acidic solution turns the indicator red, orange or yellow (pH below 7). Markers credit hydrogen ions for acids, hydroxide ions for alkalis, and the use of the colour change matched to a pH chart. A common slip is to name the wrong ion or to forget to compare with a chart.
WJEC sample2 marksA solution has a pH of 13. State whether it is acidic, neutral or alkaline, and whether it is weak or strong.Show worked answer →
A Unit 2.2 interpretation question. Reward: a pH of 13 is alkaline (above 7), and because it is close to 14 (far from 7) it is a strongly alkaline solution. Markers credit alkaline and strongly alkaline. A common error is to call any high number "acidic" by reading the scale the wrong way round.
Related dot points
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- Describe tests for hydrogen, oxygen, carbon dioxide and chlorine, flame tests for metal ions, and tests for metal cations using sodium hydroxide solution.
A focused answer to WJEC GCSE Chemistry topic 2.2, covering the standard tests for hydrogen, oxygen, carbon dioxide and chlorine, flame tests for metal ions, and identifying metal cations by the colour of their hydroxide precipitates with sodium hydroxide.
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A focused answer to WJEC GCSE Chemistry topic 2.2, covering the tests for halide ions with acidified silver nitrate, sulfate ions with acidified barium chloride, and carbonate ions with dilute acid and limewater.
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Sources & how we know this
- WJEC GCSE Chemistry specification (from 2016) — WJEC (2016)