The characteristic properties of acids and bases

  • A base is an oxide or hydroxide of a metal. An alkali is a soluble base. Aqueous acids contain H⁺ ions and aqueous alkalis contain OH⁻ ions.
  • Litmus: red in acid, blue in alkali. Methyl orange: red in acid, yellow in alkali. Thymolphthalein: colourless in acid, blue in alkali.
  • Universal indicator paper compares acidity by colour and pH: red (strongly acidic, pH 0 to 2), orange or yellow (weakly acidic), green (neutral, pH 7), blue (weakly alkaline) and purple (strongly alkaline, pH 12 to 14). A lower pH means a higher H⁺ concentration.
  • Acid + metal → salt + hydrogen. Example: Mg + 2HCl → MgCl₂ + H₂.
  • Acid + base → salt + water. Example: CuO + H₂SO₄ → CuSO₄ + H₂O.
  • Acid + metal carbonate → salt + water + carbon dioxide. Example: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
  • Neutralisation: acid + alkali → salt + water. Ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l).
  • Bases also react with ammonium salts. Warming an ammonium salt with sodium hydroxide gives ammonia, which turns damp red litmus paper blue.

Acid strength and the proton theoryExtended

  • ExtendedAn acid is a proton (H⁺) donor and a base is a proton acceptor.
  • ExtendedA strong acid is completely dissociated in aqueous solution. Hydrochloric acid is strong: HCl(aq) → H⁺(aq) + Cl⁻(aq).
  • ExtendedA weak acid is only partially dissociated. Ethanoic acid is weak: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq). The ⇌ sign shows the reaction is reversible.
  • ExtendedAt the same concentration, a weak acid has a higher pH than a strong acid because fewer H⁺ ions are present.

Oxides

  • Metal oxides are basic, for example CuO and CaO. They react with acids to form a salt and water.
  • Non-metal oxides are acidic, for example CO₂ and SO₂. They react with alkalis.
  • ExtendedAmphoteric oxides react with both acids and bases to form a salt and water. Aluminium oxide (Al₂O₃) and zinc oxide (ZnO) are amphoteric.

Preparation of soluble salts

Choose the method by checking the solubility rules. A salt is named from its metal (or ammonium) and the acid used: hydrochloric acid gives chlorides, sulfuric acid gives sulfates and nitric acid gives nitrates.

  • Sodium, potassium and ammonium salts are soluble. All nitrates are soluble.
  • Chlorides are soluble except those of lead and silver. Sulfates are soluble except those of barium, calcium and lead.
  • Carbonates are insoluble except those of sodium, potassium and ammonium. Hydroxides are insoluble except those of sodium, potassium and ammonium, while calcium hydroxide is partially soluble.
  • From excess metal, excess insoluble base or excess insoluble carbonate: add the solid to warm acid until no more reacts, so all the acid is used up.
  • Filter off the excess solid. Heat the filtrate to evaporate some water until crystals start to form, then leave it to cool and crystallise. Filter, wash with a little cold distilled water and dry between filter paper.
  • From an alkali: use titration. Add acid to a measured volume of alkali with indicator, note the volume needed to neutralise it, repeat without indicator, then crystallise.
  • A hydrated substance is chemically combined with water. An anhydrous substance contains no water.

Insoluble salts and water of crystallisationExtended

  • ExtendedPrepare an insoluble salt by precipitation: mix two soluble salt solutions whose ions combine to form the insoluble product. Example: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).
  • ExtendedFilter off the precipitate, wash it with distilled water and dry it.
  • ExtendedWater of crystallisation is the water in hydrated crystals, for example CuSO₄•5H₂O (blue) and CoCl₂•6H₂O (pink). Heating removes it to leave the anhydrous salt.

Exam tips

  • Use the phrase 'an excess of the solid' when describing salt preparation and explain that it makes sure all the acid reacts.
  • Learn the observations for each acid reaction: fizzing for metals and carbonates, and a solid dissolving for bases.
  • Write ionic equations for neutralisation and precipitation with state symbols, and cancel the spectator ions.
  • Check the solubility of the product first. If it is insoluble, you need precipitation, not crystallisation.
  • Give colours of indicators exactly, for example methyl orange is red in acid and yellow in alkali.

Common mistakes

  • Using a strong acid with a metal that is below hydrogen in the reactivity series, such as copper, which will not react.
  • Confusing a base and an alkali. All alkalis are bases but only soluble bases are alkalis.
  • Heating the solution until it is completely dry, which gives a powder rather than good crystals.
  • Forgetting to filter off the excess solid before crystallising the salt.
  • Writing CO₂ as a product of acids reacting with metal oxides. Carbon dioxide forms only with carbonates.