What are acids and bases, and what does the pH scale tell us?
Acids, bases and alkalis, the pH scale and indicators, and neutralisation as a reaction between an acid and a base.
A focused answer to the WJEC GCSE Science Double Award Unit 5 topic on acids and bases, covering acids, bases and alkalis, the pH scale and indicators, and neutralisation as the reaction between an acid and a base.
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What this dot point is asking
WJEC Double Award Unit 5 wants you to describe acids, bases and alkalis, the pH scale and indicators, and neutralisation.
Acids, bases and alkalis
Common acids include hydrochloric, sulfuric and nitric acid. Common alkalis include sodium hydroxide and potassium hydroxide. Pure water is neutral (pH 7).
The pH scale and indicators
Universal indicator turns red in strong acid, green at neutral and purple in strong alkali, with orange, yellow and blue in between.
Neutralisation
The general word equation is:
For example, hydrochloric acid + sodium hydroxide to sodium chloride + water. Neutralisation is used to treat acidic soil with lime and to treat indigestion (an alkali neutralises excess stomach acid).
Acids and alkalis in terms of ions
A deeper way to understand acids and alkalis is in terms of ions in solution. An acid releases hydrogen ions () when dissolved in water, and the more concentrated these are, the lower the pH. An alkali releases hydroxide ions () in water. During neutralisation, the hydrogen ions from the acid react with the hydroxide ions from the alkali to make water: . This is why the products of neutralisation are always a salt and water, and explains at the particle level why the pH moves to 7.
Strong and weak acids
Acids can be strong or weak. A strong acid (such as hydrochloric, sulfuric or nitric acid) fully releases its hydrogen ions in water, giving a very low pH. A weak acid (such as the citric acid in lemons or the ethanoic acid in vinegar) only partly releases its hydrogen ions, so it has a higher pH than a strong acid of the same concentration. This is why some acids are safe to eat while others are corrosive, and it is a common point when comparing the pH of different acids.
Common indicators
As well as universal indicator, simple indicators give a single colour change. Litmus turns red in acid and blue in alkali. Phenolphthalein is colourless in acid and pink in alkali, and is useful for showing the end point of a neutralisation. Knowing the colours of these common indicators in acid and alkali, alongside the universal indicator colours, helps you answer questions about testing whether a solution is acidic or alkaline.
Try this
Q1. What is the pH of a neutral solution? [1 mark]
- Cue. 7.
Q2. What two products are formed when an acid neutralises a base? [2 marks]
- Cue. A salt and water.
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 style4 marksDescribe the pH scale and how universal indicator is used to find the pH of a solution.Show worked answer →
A Unit 5 describe question worth 4 marks. Reward: the pH scale runs from 0 to 14 (1); below 7 is acidic, 7 is neutral and above 7 is alkaline (1); universal indicator is added and changes colour depending on the pH (1); the colour is matched to a colour chart to read off the pH (for example red for strong acid, green for neutral, purple for strong alkali) (1). Markers credit the 0 to 14 scale, the acid/neutral/alkali regions, and matching the indicator colour to a chart. A common error is to get the acid and alkali ends the wrong way round.
WJEC style3 marksWrite a word equation for the neutralisation of hydrochloric acid by sodium hydroxide, and state the pH at the end point.Show worked answer →
A Unit 5 neutralisation question. The word equation is hydrochloric acid + sodium hydroxide to sodium chloride + water (1 mark for reactants, 1 for products). At the end point the solution is neutral, pH 7 (1). Markers credit the salt and water products and pH 7 at neutralisation. A common error is to forget water is a product, or to give a salt with the wrong name.
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