Acids, Bases, Salts and Qualitative Analysis
Acids and bases
An acid releases hydrogen ions (H+) in water; a base neutralises an acid; an alkali is a soluble base that releases hydroxide ions (OH−). The pH scale runs from 0 to 14: below 7 is acidic, 7 neutral and above 7 alkaline. Litmus, methyl orange and phenolphthalein are common indicators. Neutralisation: acid + base → salt + water.
Preparing salts
Soluble salts are made by reacting an acid with a metal, metal oxide, metal hydroxide or metal carbonate, then filtering and crystallising. Insoluble salts are made by precipitation, mixing two solutions that form an insoluble product. Titration uses a burette to find the exact volume of acid needed to neutralise a known volume of alkali.
Qualitative analysis
Flame tests identify metal ions: sodium yellow, potassium lilac, calcium brick-red, copper blue-green. Sodium hydroxide solution gives coloured precipitates, such as blue copper hydroxide. Carbonate reacts with acid to give carbon dioxide, which turns limewater milky. Hydrogen gives a squeaky pop; oxygen relights a glowing splint; ammonia turns damp red litmus blue; chlorine bleaches damp litmus.
Worked example
25.0 cm3 of 0.10 mol/dm3 sodium hydroxide is neutralised by 0.10 mol/dm3 hydrochloric acid. Moles of NaOH = 0.025 × 0.10 = 0.0025 mol. The equation is 1:1, so 0.0025 mol HCl is needed. Volume = moles ÷ concentration = 0.0025 ÷ 0.10 = 0.025 dm3, which is 25.0 cm3.
Traps that cost marks
Strong is not the same as concentrated. A base is not always an alkali. Never add water to concentrated acid; add acid to water. In titration calculations, convert cm3 to dm3 by dividing by 1000.