Acids, bases and salts
Acids, bases and salts tests pH, neutralisation, salt preparation and qualitative tests for ions and gases.
Acids, bases and pH
An acid is a proton (H+) donor; a base is a proton acceptor. A strong acid is fully ionised in water, while a weak acid is only partially ionised. The pH scale runs from 0 to 14: below 7 is acidic, 7 is neutral and above 7 is alkaline. Universal indicator gives an approximate colour and pH; litmus only shows acid or alkali; methyl orange and phenolphthalein give sharp end-points in titrations.
Preparing salts
Soluble salts can be made by reacting an acid with an excess of an insoluble base, carbonate or reactive metal, then filtering off the excess and crystallising. Sodium, potassium and ammonium salts are made by titration because all reactants are soluble. Insoluble salts are made by precipitation, mixing solutions of two soluble salts and filtering the precipitate.
Tests
Carbonates react with acid to give CO2, which turns limewater milky. Ammonium salts give ammonia gas with warm sodium hydroxide; ammonia turns damp red litmus blue. Chlorides give a white precipitate with acidified silver nitrate; sulfates give a white precipitate with acidified barium chloride. Flame tests identify metal ions.
Worked example
To prepare copper(II) sulfate crystals, warm dilute sulfuric acid and add copper(II) oxide until no more dissolves. Filter off the excess black oxide. Evaporate the blue filtrate to the point of crystallisation, then cool and filter the crystals. The equation is CuO + H2SO4 → CuSO4 + H2O. Adding excess oxide ensures all the acid is used up.
Traps
Not all metals react with dilute acid; copper and silver do not. A dilute strong acid can still have a low pH, but pH 0 is not the same as no acid. In ion tests, the reagents must be acidified to avoid false precipitates. Do not evaporate to dryness if hydrated crystals are wanted.