Acids, Bases and Salts
This domain tests the reactions of acids, the meaning of pH and neutralisation, and how pure salts are prepared and identified. You need accurate salt names and a clear method for each preparation route.
Definitions and reactions
- An acid is a proton donor; a base is a proton acceptor; an alkali is a soluble base.
- pH below 7 is acidic, 7 is neutral, above 7 is alkaline. Universal indicator gives an approximate pH; litmus, methyl orange and phenolphthalein show colour changes.
- Acid + metal → salt + hydrogen.
- Acid + base → salt + water (neutralisation).
- Acid + carbonate → salt + water + carbon dioxide. The salt name comes from the acid: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, nitric acid gives nitrates.
Preparing salts
- Soluble salts of reactive metals or ammonium: titrate acid with alkali using an indicator, then repeat without indicator using the measured volumes.
- Soluble salts of less reactive metals such as copper: add excess insoluble base or carbonate to warm acid, filter off the excess, then crystallise.
- Insoluble salts: mix solutions of two soluble salts so precipitation occurs, then filter, wash and dry.
Worked example
Describe how to prepare pure, dry copper(II) sulfate crystals from copper(II) oxide and dilute sulfuric acid. Warm the acid, add copper(II) oxide a little at a time until no more dissolves, and stir. The excess oxide ensures all the acid is used up. Filter to remove the excess solid. Heat the filtrate gently to the point of crystallisation, then cool to form crystals. Filter the crystals, wash with a little cold distilled water and dry between filter papers. Do not heat the crystals strongly, because hydrated copper(II) sulfate loses water and turns white.
Traps that cost marks
- Saying acid + metal gives salt + water; that is acid + base.
- Using an indicator in the excess-base method, which contaminates the product.
- Forgetting to add excess solid, leaving unreacted acid.
- Naming a sulfate as a sulfide or a nitrate as a nitrite.