IGCSE Chemistry Acids, bases and salts notes

An acid is a compound that releases hydrogen ions, H⁺(aq), when dissolved in water. A base is the oxide or hydroxide of a metal that neutralises an acid. Bases that dissolve in water are called alkalis, and their solutions contain hydroxide ions, OH⁻(aq). Common alkalis include aqueous sodium hydroxide, potassium hydroxide, calcium hydroxide and ammonia.

The pH scale compares the concentration of hydrogen ions. A solution of pH 7 is neutral; below 7 it is acidic (the lower the pH, the greater the concentration of H⁺(aq)); above 7 it is alkaline (the higher the pH, the greater the concentration of OH⁻(aq)). Universal indicator shows a continuous range of colours, from red through green to purple.

Supplement: a strong acid is completely dissociated in aqueous solution — every molecule has released its proton. A weak acid is only partially dissociated, so an equilibrium mixture of molecules and ions forms. This is the proton-donor view of acids: an acid is a species that donates H⁺, and a base is a species that accepts it.

1. 1Strong: hydrochloric acid

   HCl(aq) → H⁺(aq) + Cl⁻(aq). The single arrow shows complete dissociation.

2. 2Weak: ethanoic acid

   CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq). The reversible arrow shows partial dissociation.

3. 3Consequence

   At the same concentration, the weak acid has a lower H⁺(aq) concentration, so a higher pH and a slower reaction with metals and carbonates.

Oxides of non-metals are usually acidic: they react with water to form acids or neutralise alkalis. Sulfur dioxide and carbon dioxide are typical examples. Oxides of metals are usually basic: copper(II) oxide and calcium oxide react with acids to give a salt and water.

Supplement: amphoteric oxides react with both acids and bases, producing a salt and water in each case. Aluminium oxide and zinc oxide are the two examples you must know. For instance, zinc oxide reacts with hydrochloric acid to form zinc chloride and water, and with sodium hydroxide to form sodium zincate and water.

The solubility rules tell you which salts dissolve in water. Learn them as five statements: salts of sodium, potassium and ammonium are soluble; all nitrates are soluble; chlorides are soluble except silver and lead; sulfates are soluble except barium, calcium and lead; carbonates and hydroxides are insoluble except sodium, potassium and ammonium (calcium hydroxide is partially soluble).

1. 1Excess-solid method

   Warm the dilute acid and add the insoluble solid a little at a time, stirring, until no more reacts and some solid remains.

2. 2Filter

   Filter off the unreacted excess solid. The filtrate is the salt solution.

3. 3Crystallise

   Heat the filtrate to evaporate some water until crystals form on a glass rod, then leave it to cool so crystals grow. Dry them between filter papers.

When both reactants are soluble, as with an acid and an alkali, no excess can be filtered off, so a titration is used. First titrate to find exactly how much alkali neutralises a measured volume of acid, using a single-drop indicator. Then repeat the mixing with the same volumes but no indicator, and crystallise the solution to obtain the pure salt.

Supplement: an insoluble salt is prepared by precipitation. Mix two solutions that each contain one of the ions needed; the insoluble salt forms as a solid suspension. Filter it off, wash the residue with distilled water to remove soluble impurities, and dry it. To prepare lead chloride, for example, mix aqueous lead nitrate with aqueous sodium chloride (or dilute hydrochloric acid).

A hydrated substance contains water chemically combined within its crystals. An anhydrous substance contains no water of crystallisation. The fixed amount of water in a formula is called the water of crystallisation: copper(II) sulfate crystals are CuSO₄•5H₂O and cobalt(II) chloride crystals are CoCl₂•6H₂O.

Heating a hydrated salt drives off its water of crystallisation and leaves the anhydrous salt. Hydrated copper(II) sulfate is blue; the anhydrous salt is white — a colour change used as a chemical test for water. Anhydrous cobalt(II) chloride changes from blue to pink when it absorbs water.