Acids, Bases and Salts
Acids, Bases and Salts tests properties, indicators, pH, neutralisation, common salts and water of crystallisation. Questions often combine identification with a neutralisation calculation or a practical test.
Core rules
- Acids produce H+ ions in water; bases produce OH- ions. Strong acids and bases ionise almost completely; weak ones partly.
- Indicators: blue litmus turns red in acid; red litmus turns blue in base. Phenolphthalein is colourless in acid and pink in base. Methyl orange is red in acid and yellow in base.
- pH scale is 0 to 14. pH below 7 is acidic, 7 neutral, above 7 basic. pH measures hydrogen ion concentration, not amount of acid.
- Neutralisation: acid + base -> salt + water. Examples include HCl + NaOH -> NaCl + H2O.
- Common salts: NaCl, Na2CO3, NaHCO3, bleaching powder. CuSO4.5H2O contains water of crystallisation; heating it drives off water and changes colour.
Worked example
50 mL of 0.1 M HCl is mixed with 50 mL of 0.1 M NaOH. Moles of HCl = 0.1 x 0.050 = 0.005 mol; moles of NaOH are the same. They react in a 1:1 ratio, so neither is in excess. The resulting solution contains NaCl and water, so its pH is about 7. The reasoning is to compare moles, not volumes alone.
Traps
A dilute strong acid can have pH closer to 7 than a concentrated weak acid. Phenolphthalein is not pink in acid. Not every salt is neutral; some salts are acidic or basic. Do not taste or smell laboratory chemicals. Water of crystallisation is part of the crystal, not free water.