The big idea: When an acid reacts with a base, the two cancel each other out — this is neutralisation. The product is always a salt and water.
A salt is the ionic compound formed when the H⁺ of the acid is replaced by a metal ion (or NH4⁺). The other ion of the salt comes from the acid (e.g. Cl⁻ from HCl, SO4²⁻ from H2SO4).
What is the salt called?: Name the salt from the two parts that join:
- metal (from the base/metal/carbonate) → first word - acid anion → second word
- HCl → chloride - HNO3 → nitrate - H2SO4 → sulfate
So NaOH + HCl makes sodium chloride, and Mg + H2SO4 makes magnesium sulfate.
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Acids react with metals, with bases (metal oxides and hydroxides) and with carbonates. Each follows a fixed pattern — learn the three, and you can predict the products of almost any acid reaction.
| Acid reacts with… | General pattern | Example |
|---|---|---|
| Metal | acid + metal → salt + hydrogen | 2HCl + Mg → MgCl2 + H2 |
| Base (metal oxide/hydroxide) | acid + base → salt + water | 2HCl + Mg(OH)2 → MgCl2 + 2H2O |
| Carbonate | acid + carbonate → salt + water + carbon dioxide | 2HCl + CaCO3 → CaCl2 + H2O + CO2 |
How to tell them apart: The extra product is the giveaway:
- a metal → also gives hydrogen gas (bubbles, 'pop' test) - a base → gives only a salt and water - a carbonate → also gives carbon dioxide (effervescence, turns limewater milky)
Practice with real exam questions
Answer exam-style questions and get AI feedback that shows you exactly what examiners want to see in a full-marks response.
Predict the products from the pattern, then balance by adjusting the numbers in front of each formula. Never change a formula — only the coefficients. Add state symbols where asked.
The method
- Identify what the acid reacts with — metal, base or carbonate.
- Write the products from the matching pattern (salt + H2 / water / water + CO2).
- Build the salt's formula by balancing the ionic charges (e.g. Mg²⁺ with Cl⁻ → MgCl2).
- Balance the whole equation with coefficients, then add state symbols.
Worked example — acid + carbonate
Write the balanced equation for hydrochloric acid reacting with calcium carbonate, CaCO3.
Solution
- Pattern: acid + carbonate → salt + water + carbon dioxide.
- Salt: the acid is HCl, so the salt is a chloride; Ca²⁺ with Cl⁻ gives CaCl2.
- Balance: the left has 1 Cl but CaCl2 needs 2, so put a 2 in front of HCl:
Final answer
2HCl + CaCO3 → CaCl2 + H2O + CO2.
How this is tested: 'Write / construct a balanced equation for …' an acid reaction is a reliable Paper 2 mark, and Paper 1A often asks you to pick the correctly written equation among acid + metal, acid + base and acid + carbonate.
For the marks you need: the right products (the correct extra product), a correct salt formula, and a balanced equation.
The two classic slips: - Forgetting the extra product: a metal also gives H2; a carbonate also gives CO2 (and water). - Getting the salt formula wrong: balance the charges first (Mg²⁺ needs 2 Cl⁻ → MgCl2), then balance the equation.
IB-style question — sulfuric acid + a hydroxide (a)
(a) Write a balanced equation, with state symbols, for the reaction of aqueous sodium hydroxide with dilute sulfuric acid. [2]
How to score the marks
- Mark 1 — products. Acid + base → salt + water; the salt is sodium sulfate, Na2SO4, plus H2O.
- Mark 2 — balanced with state symbols. Sulfuric acid is diprotic, so two NaOH are needed:
Final answer
2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l).
IB-style question — acid + metal (b)
(b) Magnesium ribbon is added to dilute hydrochloric acid and fizzing is seen. Write a balanced equation for the reaction and name the gas. [2]
How to score the marks
- Mark 1 — products and gas. Acid + metal → salt + hydrogen; the salt is magnesium chloride, MgCl2. The gas is hydrogen, H2 (the fizzing).
- Mark 2 — balanced. Mg²⁺ needs 2 Cl⁻ → MgCl2, so 2 HCl are needed:
Final answer
Mg + 2HCl → MgCl2 + H2; the gas is hydrogen.