Acids, Bases and Salts
What the exam tests
Acids-Bases-Salts tests the operational definitions of acids and bases, indicators, the pH scale, neutralisation, and the preparation and uses of common salts such as baking soda, washing soda, bleaching powder and plaster of Paris. Expect equation-writing and explanation of everyday applications.
Core rules
- Acids release H+ (H3O+) in water; bases release OH−. This is the Arrhenius view used at this level.
- Acids turn blue litmus red; bases turn red litmus blue. Phenolphthalein is colourless in acid and pink in base.
- pH below 7 is acidic, 7 is neutral, above 7 is basic. pH measures strength, not amount.
- Acid + metal → salt + hydrogen. Acid + carbonate/hydrogencarbonate → salt + water + CO2. Acid + base → salt + water.
- Neutralisation is the reaction of H+ and OH− to form water.
- Baking soda is NaHCO3; washing soda is Na2CO3·10H2O; plaster of Paris is CaSO4·½H2O; bleaching powder is Ca(OCl)Cl.
- Water of crystallisation is the fixed water in a crystal, as in CuSO4·5H2O.
Worked example
A white powder is used as an antacid and in baking. Identify it and write its reaction with dilute HCl. The powder is sodium hydrogencarbonate, NaHCO3. The equation is NaHCO3 + HCl → NaCl + H2O + CO2. Reasoning: an antacid must neutralise excess stomach acid, and baking uses CO2 released by the same reaction.
Traps that cost marks
- Saying phenolphthalein is pink in neutral solution; it is colourless.
- Confusing pH with concentration; a dilute strong acid can have a higher pH than a concentrated weak acid.
- Writing plaster of Paris as CaSO4·2H2O instead of the hemihydrate.
- Adding water to concentrated acid; always add acid to water.