Acids, Bases and Salts
Questions here test indicators, the pH scale, preparation and uses of common salts, and equations for acid–base reactions.
Core rules
- Acids give H+ (or H3O+) ions in water; bases give OH−; a base soluble in water is an alkali.
- Strong acids (HCl, H2SO4, HNO3) ionise completely; weak acids (CH3COOH, H2CO3) ionise only partly.
- Indicators: litmus turns red in acid and blue in base; methyl orange is red in acid and yellow in base; phenolphthalein is colourless in acid and pink in base; turmeric turns red in base.
- pH below 7 is acidic, 7 neutral, above 7 basic; a fall of one pH unit means a tenfold rise in H+ concentration.
- Acid + metal → salt + hydrogen; acid + carbonate or bicarbonate → salt + water + carbon dioxide; acid + base → salt + water; metal oxide + acid → salt + water.
- Always add acid to water while stirring, because dilution is strongly exothermic.
Worked example
Sodium carbonate with dilute hydrochloric acid: Na2CO3 + 2HCl → 2NaCl + H2O + CO2. Reason it out: the carbonate ion takes two H+ ions to form H2CO3, which breaks into water and carbon dioxide. The gas turns limewater milky (Ca(OH)2 + CO2 → CaCO3 + H2O), confirming a carbonate.
Salts and traps
- Washing soda is Na2CO3·10H2O, baking soda NaHCO3, bleaching powder CaOCl2, plaster of Paris CaSO4·½H2O.
- The chlor-alkali process on brine gives NaOH, Cl2 and H2.
- Water of crystallisation explains the blue colour of CuSO4·5H2O, which turns white on heating.
- Trap: concentrated and strong are different - concentration is the amount per unit volume, strength is the extent of ionisation.
- Tooth decay begins when the mouth pH falls below about 5.5.