ZIMSEC O Level · Combined Science · Form 3, Form 4
Acids and bases
- Acids have a pH below 7 and turn blue litmus red. Examples: hydrochloric acid, sulfuric acid, citric acid (in lemons).
- Bases that dissolve in water are alkalis; they have a pH above 7 and turn red litmus blue. Examples: sodium hydroxide, calcium hydroxide (slaked lime).
- pH 7 is neutral (pure water). Universal indicator shows the pH by colour.
Neutralisation
acid + base → salt + water
Uses: indigestion tablets neutralise excess stomach acid; farmers add lime to acidic soils; toothpaste neutralises acids made by bacteria.
Other reactions of acids
- acid + metal → salt + hydrogen (the gas gives a squeaky “pop” with a lit splint)
- acid + carbonate → salt + water + carbon dioxide (turns lime water milky)
Worked example: Name the salt formed when hydrochloric acid reacts with sodium hydroxide.
Hydrochloric acid gives chlorides, and the metal is sodium, so the salt is sodium chloride.
Naming salts
Hydrochloric acid → chlorides; sulfuric acid → sulfates; nitric acid → nitrates.
Exam tip: Learn the gas tests: hydrogen “pops”, oxygen relights a glowing splint, carbon dioxide turns lime water milky.
Check yourself
- What is the pH of a neutral solution?
- Complete: zinc + sulfuric acid → ______ + ______.
- Why do farmers add lime to some soils?
Show answers
- 7
- zinc sulfate + hydrogen
- To neutralise acidic soil so that crops grow well.
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