Chemical Substances: Acids, Bases, Salts, Metals and Carbon Compounds
Chemical Substances is tested through acids, bases, salts, metals and carbon compounds, asking you to recognise formulae, explain reactions and apply mole ratios.
Acids, bases and salts
Acids give H⁺ ions in water and turn blue litmus red; bases give OH⁻ ions and turn red litmus blue. The pH scale runs 0–14: below 7 acidic, 7 neutral, above 7 basic. Strength depends on ionisation, not concentration alone. Indicators include litmus, methyl orange and phenolphthalein. Neutralisation is acid + base → salt + water, e.g. HCl + NaOH → NaCl + H₂O. Learn formulae: washing soda Na₂CO₃·10H₂O, baking soda NaHCO₃, plaster of Paris CaSO₄·½H₂O, bleaching powder Ca(OCl)Cl.
Metals and carbon compounds
The reactivity series is K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au. A more reactive metal displaces a less reactive one from its salt solution. Carbon forms covalent bonds, shows tetravalency and catenation, and gives homologous series such as alkanes, alkenes and alkynes. Functional groups decide properties: –OH alcohol, –COOH carboxylic acid, –CHO aldehyde, –CO– ketone. Ethanol reacts with sodium and undergoes oxidation; ethanoic acid reacts with carbonates and alcohols.
Worked example
25.0 cm³ of 0.10 M HCl neutralises 20.0 cm³ of NaOH. Find the NaOH concentration. Moles HCl = 0.0250 × 0.10 = 0.00250 mol. The equation HCl + NaOH → NaCl + H₂O is 1:1, so moles NaOH = 0.00250 mol. Concentration = 0.00250 ÷ 0.0200 = 0.125 M. The reasoning is: convert volume to dm³, use the balanced mole ratio, then divide by the base volume.
Traps
pH measures hydrogen-ion concentration, not the amount of acid. Diluting an acid raises pH towards 7 but never above 7. Balance equations before using mole ratios. In displacement, only metals above the displaced metal in the series react. Do not confuse plaster of Paris with gypsum.