Reactions of Acids with Metals, Bases and Carbonates
Four reactions, four sets of products. Getting the products right is most of the marks — and only one of them makes a gas you can see bubbling as carbon dioxide.
1. The four general equations
acid + METAL → salt + HYDROGEN acid + METAL OXIDE (base) → salt + WATER acid + METAL HYDROXIDE (alkali) → salt + WATER acid + METAL CARBONATE → salt + WATER + CARBON DIOXIDE
Learn these four by heart — tutors flagged them as essential recall. Every salt-preparation question starts from one of them.
Only the CARBONATE reaction produces carbon dioxide. That is how you identify it: effervescence plus a gas that turns limewater milky.
Water is a product of three of the four — but not the metal reaction, which gives hydrogen.
2. Which salt forms
The metal comes from the base/metal/carbonate; the ending comes from the acid.
| Acid | Salt ending |
|---|---|
| Hydrochloric, HCl | chloride |
| Sulfuric, H₂SO₄ | sulfate |
| Nitric, HNO₃ | nitrate |
Examples:
- zinc + hydrochloric acid → zinc chloride + hydrogen
- copper oxide + sulfuric acid → copper sulfate + water
- calcium carbonate + nitric acid → calcium nitrate + water + carbon dioxide
3. Acid + metal
acid + metal → salt + hydrogen Zn + 2HCl → ZnCl₂ + H₂
Only metals ABOVE hydrogen in the reactivity series react. Copper, silver and gold do not.
Observations: effervescence (bubbling), the metal dissolving/getting smaller, and the mixture warming up (exothermic).
Test for the gas: a lighted splint gives a squeaky pop — hydrogen.
The gas is HYDROGEN, not carbon dioxide. Only carbonates give CO₂.
4. Acid + metal oxide or hydroxide (neutralisation)
acid + base → salt + water CuO + H₂SO₄ → CuSO₄ + H₂O NaOH + HCl → NaCl + H₂O
No gas is produced — so no effervescence.
Observations with an insoluble oxide: the solid dissolves, often with a colour change (black copper oxide → blue copper sulfate solution).
The ionic equation for any strong acid–alkali neutralisation:
H⁺(aq) + OH⁻(aq) → H₂O(l)
A BASE is a metal oxide or hydroxide; an ALKALI is a soluble base. Both react the same way with acid.
5. Acid + carbonate
acid + metal carbonate → salt + water + carbon dioxide CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Observations: vigorous effervescence, the solid dissolving, and a gas that turns limewater milky.
This reaction is the test for a carbonate — add dilute acid and test the gas with limewater.
6. Recognising the reaction from the observations
| Observation | Reaction |
|---|---|
| Effervescence, gas gives squeaky pop | acid + metal |
| Effervescence, gas turns limewater milky | acid + carbonate |
| No gas, solid dissolves | acid + base |
Both metals and carbonates give bubbles — the gas test is what tells them apart.
7. Neutralisation in context
Everyday examples:
- Indigestion tablets (a base) neutralise excess stomach acid
- Lime (calcium oxide/hydroxide) neutralises acidic soil
- Toothpaste neutralises acids produced by bacteria
pH during neutralisation: adding alkali to acid raises the pH from below 7 towards 7 at the endpoint, and beyond 7 if excess alkali is added.
8. Mistakes that cost marks
Wrong products — especially forgetting water, or adding carbon dioxide where there is none.
Saying acid + metal gives carbon dioxide.
Saying copper reacts with dilute acid.
Wrong salt name for the acid used.
Unbalanced equations.
Giving observations without the gas test.
Omitting state symbols in ionic equations.
Frequently asked questions
What does acid + metal produce? Salt + hydrogen.
What does acid + base produce? Salt + water.
What does acid + carbonate produce? Salt + water + carbon dioxide.
Which reaction gives carbon dioxide? Only the carbonate one.
How do I test for hydrogen? A lighted splint gives a squeaky pop.
How do I test for carbon dioxide? It turns limewater milky.
Which metals don’t react with dilute acid? Those below hydrogen — copper, silver, gold.
What salt does sulfuric acid make? A sulfate.
What is the ionic equation for neutralisation? H⁺(aq) + OH⁻(aq) → H₂O(l).
What’s the difference between a base and an alkali? An alkali is a soluble base.
Quick revision checklist
- I know all four general equations
- I know which acid gives which salt ending
- I know only carbonates give carbon dioxide
- I know acid + metal gives hydrogen
- I know which metals don’t react
- I can give observations for each reaction
- I can use the gas tests to tell them apart
- I can write balanced equations for all four
- I know the ionic neutralisation equation
- I can give everyday examples of neutralisation
These notes cover the reactions of acids in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
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