Acids + Metals
The reaction and salt-naming here are for everyone; the redox explanation at the end is Higher Tier only.
Acids react with metals that are more reactive than hydrogen to produce a salt and hydrogen gas:
acid + metal → salt + hydrogen
For example:
Mg + 2HCl → MgCl2 + H2
The bubbles are hydrogen gas — you can test for it with a lit splint, which gives a squeaky pop. (AQA limits these reactions to magnesium, zinc and iron with hydrochloric and sulfuric acids.)
The first part of the salt’s name comes from the metal; the second part comes from the acid:
- Hydrochloric acid (HCl) → chloride salts
- Sulfuric acid (H2SO4) → sulfate salts
- Nitric acid (HNO3) → nitrate salts
So zinc + sulfuric acid gives zinc sulfate; iron + hydrochloric acid gives iron chloride.
Why these are redox reactions Higher
Hydrogen sits in the reactivity series too. A metal above hydrogen can displace it from the acid: the metal loses electrons (oxidised) and the hydrogen ions gain them (reduced). So acid–metal reactions are redox reactions. Taking zinc and hydrochloric acid:
Zn + 2HCl → ZnCl2 + H2
Zn → Zn2+ + 2e− (oxidation)
2H+ + 2e− → H2 (reduction)
Remember that hydrogen is diatomic (H2), so you need two H+ ions to make one molecule — that is why the half equation starts with 2H+ and gains 2e−.
Copper is below hydrogen, so it cannot displace it — which is why copper does not react with dilute acids.
🧪 Exam-style questions
Which salt is produced when zinc reacts with sulfuric acid?
Copper does not react with dilute hydrochloric acid. Use the reactivity series to explain why.
Show answer
- Copper is less reactive than hydrogen (it is below hydrogen in the series). 1 mark
- So it cannot displace hydrogen from the acid, and no reaction occurs. 1 mark
Neutralisation & Making Salts
Acids are also neutralised by bases. A base is any compound that neutralises an acid; the products always include a salt and water.
A base is a substance that neutralises an acid. The bases you meet here are metal oxides, metal hydroxides and metal carbonates.
An alkali is a base that is soluble in water (for example sodium hydroxide). Most metal oxides and carbonates are insoluble bases.
All bases neutralise acids. Alkalis are the soluble ones that dissolve in water; insoluble bases stay as solid powders — the kind you add in excess to make a salt.
There are three neutralisation patterns to know. The salt is named exactly as before — from the metal and the acid.
acid + metal oxide → salt + water
acid + metal hydroxide → salt + water
acid + metal carbonate → salt + water + carbon dioxide
Carbonates are the odd one out — they also give off carbon dioxide, which you would see as effervescence. For example:
CuO + H2SO4 → CuSO4 + H2O
CaCO3 + 2HCl → CaCl2 + H2O + CO2
Working out the formula of a salt
To write the formula of a salt, balance the charges of the positive ion (from the metal) and the negative ion (from the acid) so the overall charge is zero. Some common ions to know:
| Positive ions | Negative ions |
|---|---|
| Na+, K+, Li+, H+, NH4+ | Cl−, NO3−, OH− |
| Mg2+, Ca2+, Cu2+, Zn2+, Fe2+ | O2−, SO42−, CO32− |
| Al3+, Fe3+ |
So magnesium (Mg2+) and chloride (Cl−) need two chlorides to balance one magnesium: MgCl2. Sodium (Na+) and sulfate (SO42−) need two sodiums: Na2SO4.
🧪 Build a salt formula
Pick a positive ion and a negative ion from the table, then add ions of each until the total positive charge exactly cancels the total negative charge. The formula appears when the charges balance in the simplest ratio.
Prepare a soluble salt (e.g. copper sulfate) by neutralising an acid with an insoluble base (e.g. copper oxide):
- Gently warm the dilute acid (e.g. sulfuric acid) with a Bunsen burner — this speeds up the reaction.
- Add the insoluble base (copper oxide) a little at a time, stirring, until no more dissolves and some solid remains. This excess guarantees all the acid has been neutralised, giving the maximum yield of salt.
- Filter to remove the unreacted excess base. The filtrate is a solution of the pure salt.
- Pour the filtrate into an evaporating basin and crystallise: heat to evaporate off some water, then leave the solution to cool so crystals form slowly.
- Pat the crystals dry between filter paper (or leave to dry) to get a pure, dry sample.
Hazards, risks and precautions
Required practical questions often end with a safety mark: “Give one safety precaution the student should take. Explain why.” Answering it well needs three linked ideas:
A hazard is anything that could cause harm — concentrated acid, a hot Bunsen flame. The risk is the chance of that hazard actually causing harm, which depends on how you work with it. A precaution reduces the risk — and it must be chosen to match the specific hazard.
Bottles carry hazard symbols so the hazard is known before anything is opened. The ones worth recognising:
| Symbol | Hazard | Matching precaution |
|---|---|---|
| Corrosive — attacks and destroys living tissue, including skin and eyes (concentrated acids and alkalis). | Wear safety goggles and gloves; wash any spill off the skin immediately. | |
| Toxic — can cause death if swallowed, breathed in or absorbed through the skin (e.g. chlorine). | Use small amounts in a fume cupboard; wash hands afterwards. | |
| Flammable — catches fire easily (e.g. ethanol, hydrogen). | Keep well away from naked flames; put lids back on bottles. | |
| Oxidising — provides oxygen, so other substances burn more fiercely (e.g. hydrogen peroxide). | Store and use well away from flammable substances. | |
| Moderate hazard (irritant) — not corrosive, but reddens or blisters the skin. The dilute acids and alkalis used at GCSE are usually in this class. | Wear safety goggles; rinse any splash with plenty of water. |
The mark is for a matched pair, not a safety slogan — “be careful” and “wear a lab coat” score nothing. Name the hazard, give the precaution that targets it, and say what harm it prevents: “wear safety goggles because the dilute acid is an irritant and could damage the eyes”. Two details worth getting right in this practical: the dilute acids used at GCSE are usually classed as irritant rather than corrosive, so say “irritant” to be safe — and the heating stages carry their own hazards, so stop heating before the solution spits and let the evaporating basin cool before moving it.
🧪 Exam-style questions
Name the salt and the gas made when calcium carbonate reacts with hydrochloric acid.
Show answer
- Salt: calcium chloride; gas: carbon dioxide. 1 mark (both needed)
acid + carbonate → salt + water + carbon dioxide. Equation: CaCO3 + 2HCl → CaCl2 + H2O + CO2.
A salt is made from aluminium ions, Al3+, and sulfate ions, SO42−. Deduce the formula of this salt.
Show answer
- Balance the charges: two Al3+ give 6+, three SO42− give 6−, so the ratio is 2 : 3. 1 mark
- Formula: Al2(SO4)3 — the bracket and subscript 3 show three sulfate groups. 1 mark
Describe how you would prepare a pure, dry sample of copper sulfate crystals from copper oxide (an insoluble base) and dilute sulfuric acid. This is a levels-of-response question — give the steps in a logical order, with enough practical detail to follow. Plan, then compare with the model answer.
Show a model answer
How it is marked (levels of response):
- Level 3 (5–6): a logically ordered method with the key practical detail (excess base, filter, crystallise, dry) that would reliably give a pure, dry sample.
- Level 2 (3–4): several correct steps, but with a gap or the order muddled.
- Level 1 (1–2): one or two relevant practical points.
Indicative content:
- Warm the dilute sulfuric acid gently (to speed up the reaction).
- Add the copper oxide a little at a time, stirring, until no more dissolves and some solid remains — i.e. the base is in excess, so all the acid is used up.
- Filter to remove the unreacted excess copper oxide; the blue filtrate is the copper sulfate solution.
- Crystallise: heat the filtrate in an evaporating basin to evaporate off some of the water, then leave it to cool slowly so crystals form.
- Dry the crystals (e.g. pat between filter paper or leave in a warm place).
Conclusion (needed for Level 3): following the steps in this order — excess base, filter, crystallise, dry — gives a pure, dry sample of copper sulfate, because the excess guarantees the acid is all neutralised and filtering removes the unreacted solid. 6 marks
Equation: CuO + H2SO4 → CuSO4 + H2O. Do not accept evaporating to dryness, which would drive off the water of crystallisation and spoil the crystals.