Acids, Bases and Salts
What is tested
This domain tests the behaviour of acids and bases, indicators, the pH scale, neutralisation and important salts. Expect questions on why dilute acid conducts electricity, how plaster of Paris sets, and what happens when an acid reacts with a metal or carbonate.
Core rules
- Acids are sour, turn blue litmus red and release H+ ions in water. Bases are soapy, turn red litmus blue and release OH- ions. Not every base is soluble; soluble bases are alkalis.
- Acid + metal → salt + hydrogen. Acid + carbonate → salt + water + carbon dioxide. Acid + base → salt + water is neutralisation.
- pH below 7 is acidic, 7 is neutral and above 7 is basic. Dilution of an acid raises its pH toward 7 but does not make it basic.
- Water of crystallisation gives hydrated salts their shape and colour: CuSO4·5H2O is blue; Na2CO3·10H2O is washing soda.
- Chlor-alkali electrolysis of brine gives NaOH, Cl2 and H2. Bleaching powder is CaOCl2; baking soda is NaHCO3; plaster of Paris is CaSO4·½H2O.
Worked example
A solution has [H+] = 1 × 10^-3 mol/L. pH = -log(10^-3) = 3, so the solution is acidic. If it is diluted 100 times, [H+] becomes about 10^-5 mol/L and pH rises to about 5, still acidic.
Traps
Phenolphthalein is colourless in acid and pink in base; methyl orange is red in acid and yellow in base. Do not say all bases are alkalis. Do not confuse baking soda with washing soda.