How do acids react with metals, bases and carbonates, and what does the pH scale tell us?
The reactions of acids with metals, bases and carbonates, neutralisation, salts, the pH scale, strong and weak acids, and making soluble salts.
A focused answer to OCR Gateway GCSE Chemistry A topic C3.3 on acids and bases, covering the reactions of acids with metals, bases and carbonates, neutralisation and salts, the pH scale, the difference between strong and weak acids, and making soluble salts.
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What this dot point is asking
OCR wants you to describe the three key reactions of acids (with metals, with bases, and with carbonates), explain neutralisation and salts, use the pH scale, distinguish strong and weak acids (and strength from concentration), and describe how to make a soluble salt. Acids and bases are a core part of C3 and the practical work.
The reactions of acids
The salt produced depends on the acid: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, and nitric acid gives nitrates.
Neutralisation and the pH scale
The pH scale measures how acidic or alkaline a solution is, from to :
- Below 7 is acidic (lots of ions).
- 7 is neutral.
- Above 7 is alkaline (lots of ions).
The lower the pH, the higher the hydrogen ion concentration. pH can be measured with universal indicator (a colour scale) or, more precisely, a pH probe and meter. As an acid is neutralised by adding alkali, the pH rises from low, through 7 at the end point, to high.
Strong and weak acids (Higher)
Strength (how fully it ionises) is different from concentration (how much acid is dissolved in a given volume). A dilute strong acid can have a higher pH than a concentrated weak acid.
Making a soluble salt
A soluble salt can be made by reacting an acid with an insoluble base (such as a metal oxide or carbonate):
- Add the base in excess to warm acid and stir until no more reacts (the acid is used up).
- Filter off the excess solid base.
- Crystallise the filtrate by evaporating some water and leaving it to cool, giving pure salt crystals.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20195 marksDilute sulfuric acid reacts with magnesium, with copper oxide, and with sodium carbonate. For each reaction, name the products and, for the magnesium reaction, write a balanced symbol equation. (Magnesium sulfate is .)Show worked answer →
A C3.3 structured question on the three reactions of acids. Reward: acid + metal gives a salt + hydrogen, so sulfuric acid + magnesium gives magnesium sulfate + hydrogen, with the equation . Acid + base (metal oxide) gives a salt + water, so sulfuric acid + copper oxide gives copper sulfate + water. Acid + carbonate gives a salt + water + carbon dioxide, so sulfuric acid + sodium carbonate gives sodium sulfate + water + carbon dioxide. Markers credit the correct products for each of the three reaction types and the correctly balanced equation for the magnesium reaction. A common slip is forgetting the carbon dioxide from the carbonate.
OCR 20224 marksExplain the difference between a strong acid and a weak acid in terms of ionisation, and explain why a strong acid has a lower pH than a weak acid of the same concentration.Show worked answer →
A Higher tier question on acid strength. Reward: a strong acid is fully ionised in water, meaning all of its molecules split up to release hydrogen ions (). A weak acid is only partially ionised, so only a small fraction of its molecules release hydrogen ions at any time. Because the strong acid produces a higher concentration of ions for the same concentration of acid, and pH depends on the concentration (the higher the concentration, the lower the pH), the strong acid has a lower pH than the weak acid. Markers credit fully versus partially ionised, the strong acid giving a higher concentration, and linking higher concentration to lower pH. Note: concentration and strength are different ideas.
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