Bases and alkalis
A base neutralises an acid. Metal oxides and hydroxides are examples. An alkali is a base that dissolves in water, such as sodium hydroxide. Copper oxide is an insoluble base. Acid + base → salt + water.
Lesson 4.6 · AQA Combined Science
Neutralisation and salt formulae
Carbonates make one extra product
Acid + metal carbonate → salt + water + carbon dioxide. Carbon dioxide turns limewater milky. Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide.
Choose the salt
Hydrochloric acid gives chlorides, sulfuric acid sulfates, and nitric acid nitrates. Copper oxide + sulfuric acid → copper sulfate + water. Sodium hydroxide + nitric acid → sodium nitrate + water.
Make a neutral formula
Ion charges must add to zero. Mg²⁺ needs two Cl⁻ ions: MgCl₂. Two Na⁺ ions balance one SO₄²⁻ ion: Na₂SO₄. Ca²⁺ needs two NO₃⁻ groups: Ca(NO₃)₂. Brackets keep a group together.
| Positive ions | Negative ions |
|---|---|
| Na⁺, K⁺ | Cl⁻, NO₃⁻ |
| Mg²⁺, Ca²⁺, Cu²⁺ | SO₄²⁻, CO₃²⁻ |
Remember
Acid + base → salt + water. Carbonates also give CO₂. Balance ion charges to write a salt formula.
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