Acids, Bases and Salts
Acids, bases and salts carries easy theory marks plus one reliable practical-style question: identifying a substance from its behaviour with an indicator, or writing a neutralisation equation.
Properties
Acids taste sour, turn blue litmus red, release H⁺ ions in water, react with metals to give hydrogen, with carbonates and hydrogencarbonates to give carbon dioxide, and with bases to give salt and water. Bases turn red litmus blue and feel soapy; alkalis are soluble bases (NaOH, KOH). Strong acids ionise almost completely (HCl, H₂SO₄, HNO₃); weak acids ionise only partly (ethanoic acid, carbonic acid).
Indicators and pH
Litmus, methyl orange (red in acid, yellow in base) and phenolphthalein (colourless in acid, pink in base) are synthetic indicators; turmeric and china rose are natural ones. pH below 7 is acidic, 7 is neutral, above 7 is basic; pH falls as acidity rises.
Neutralisation
Acid + base → salt + water. Acid + metal carbonate → salt + water + carbon dioxide. The salt is named from the metal and the acid: hydrochloric acid gives chlorides, sulphuric acid gives sulphates, nitric acid gives nitrates.
Common salts and uses
- Sodium chloride - food, and raw material for chlorine, sodium hydroxide and baking soda.
- Baking soda (NaHCO₃) - antacid, baking powder, soda-acid fire extinguisher; gives CO₂ with acid and on heating (2NaHCO₃ → Na₂CO₃ + H₂O + CO₂).
- Washing soda (Na₂CO₃·10H₂O) - laundry and water softening.
- Bleaching powder - disinfectant; plaster of Paris (CaSO₄·½H₂O, from gypsum heated to 373 K) - setting fractured bones.
Worked example
A white kitchen powder fizzes with dilute HCl and the gas turns lime water milky. On strong heating it gives a residue, water and CO₂. The fizz shows a carbonate or hydrogencarbonate; the heating reaction proves NaHCO₃, because Na₂CO₃ is stable to heat.
Traps
Saying all bases are alkalis (the safer statement is that all alkalis are bases); confusing plaster of Paris with gypsum, CaSO₄·2H₂O; forgetting that phenolphthalein is colourless in acid.