Acids, alkalis and reactions
Chemistry · Key Stage 3 · The School
The pH scale
pH runs 0 to 14: below 7 acidic, 7 neutral, above 7 alkaline. Universal indicator gives a colour for each; litmus only tells you which side of 7 you are on. Strong acids (pH 1, stomach acid) and strong alkalis (pH 14, drain cleaner) are BOTH corrosive — dangerous is not the same as acidic.
Neutralisation
Acid + alkali → salt + water. Hydrochloric acid + sodium hydroxide → sodium chloride + water. The name of the salt comes from the acid used: hydrochloric gives chlorides, sulfuric gives sulfates, nitric gives nitrates. Learn that pattern and you can name the product of any neutralisation.
Where it is used
Indigestion tablets neutralise excess stomach acid. Farmers add lime to acidic soil. Toothpaste neutralises the acid bacteria make on your teeth. Bee stings are acidic; wasp stings alkaline. Every one of these is the same reaction doing a different job — which is why the equation is worth knowing rather than just memorising.
Naming the salt, and proving you made it
Work one through. Sulfuric acid + potassium hydroxide. The acid gives the salt its surname — sulfuric gives sulfates — and the alkali gives its first name, so the salt is potassium sulfate, and the other product is always water. Written out: sulfuric acid + potassium hydroxide → potassium sulfate + water. Now a different pair: an acid plus a metal CARBONATE gives three products, not two, because the carbonate releases carbon dioxide. Hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide. That third product is the fizz you see, and bubbling it through limewater turns the limewater cloudy, which is the test that proves it was carbon dioxide and not just air.