IGCSE Chemistry Acids, Bases and Salts Notes
Acids donate H⁺ ions; bases accept them. Combining them neutralises to give a salt and water. Salt preparation is a classic IGCSE practical.
Key concepts
- Acid
- Substance that releases H⁺ ions in water; pH < 7.
- Base / alkali
- Substance that accepts H⁺; an alkali is a soluble base; pH > 7.
- Neutralisation
- H⁺ + OH⁻ → H₂O; produces a salt and water; pH moves toward 7.
- Salt preparation
- Acid + metal / metal oxide / carbonate / alkali → salt (+ water / H₂ / CO₂).
- Common indicators: litmus (red/blue), universal indicator (full colour scale), methyl orange (red/yellow), phenolphthalein (colourless/pink).
- HCl → chlorides; H₂SO₄ → sulfates; HNO₃ → nitrates.
- Reactive metal + acid → salt + hydrogen (fizzing).
- Metal carbonate + acid → salt + water + carbon dioxide (fizzing; limewater turns milky).
Worked example
Describe how to make dry copper sulfate crystals from copper oxide and sulfuric acid.
- 1Warm dilute H₂SO₄ in a beaker; add excess CuO until no more dissolves (ensures all acid reacts).
- 2Filter off the excess CuO.
- 3Heat the filtrate gently to evaporate about half the water — until crystallisation point.
- 4Leave to cool; crystals form. Pat dry between filter paper.
Answer: Excess copper oxide → filter → evaporate → crystallise → dry.
Common mistakes
✗ Writing 'salt' instead of the specific salt name.
✓ State the actual salt, e.g. sodium chloride or copper sulfate.
✗ Heating solutions to dryness in crystallisation.
✓ Only evaporate to the crystallisation point, then cool — heating to dryness destroys the crystal shape.
✗ Forgetting water as a neutralisation product.
✓ Acid + alkali always gives salt + water.
Quick check
Try these, then reveal the answers.
What salt is made from HCl and NaOH?Reveal answer
Sodium chloride (NaCl).
What gas is produced when a metal carbonate reacts with acid?Reveal answer
Carbon dioxide.
What is the pH of a strong alkali?Reveal answer
13 or 14.
Exam tips
- Remember: acid + metal oxide/hydroxide → salt + water (no gas); acid + carbonate → salt + water + CO₂.
- Choose the correct method: soluble salt from insoluble base uses excess + filter + crystallise.
- For titration, add indicator to alkali, run acid from burette until colour just changes.
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Frequently asked questions
What is neutralisation?+
A reaction in which an acid and a base combine to give a salt and water, moving the pH toward 7.
Why do we use excess base when making a salt?+
So all the acid is used up. The excess base is then filtered off, leaving a pure salt solution.
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