Topic D: Acids and Bases

Acids and bases are among the most useful classes of substances, from digestion to industry. This part defines acids and alkalis in terms of the ions they release, explains how indicators and the pH scale measure acidity, and shows how neutralisation produces salts. It finishes with the quantitative skills of titration and concentration that the DSE examines closely.

Acids, alkalis and their properties

An acid is a substance that releases hydrogen ions, H+, when dissolved in water; examples include hydrochloric acid, sulfuric acid and nitric acid. Acids taste sour, turn blue litmus red, and react with reactive metals, carbonates and bases. An alkali is a soluble base that releases hydroxide ions, OH-, in water, such as sodium hydroxide and aqueous ammonia. Alkalis feel soapy, turn red litmus blue, and neutralise acids. A base is any metal oxide or hydroxide that neutralises an acid, whether or not it dissolves. The characteristic reactions of acids come from the H+ ions they release, which is why a substance only shows acidic behaviour when dissolved in water.

Indicators and the pH scale

Indicators are substances that change colour according to acidity, allowing acids and alkalis to be distinguished. Litmus is red in acid and blue in alkali; universal indicator shows a range of colours and gives an approximate pH. The pH scale runs from below 0 to 14 and measures the concentration of hydrogen ions: pH = -log[H+]. A pH below 7 is acidic, exactly 7 is neutral, and above 7 is alkaline. The lower the pH, the higher the [H+] and the more acidic the solution; each whole pH unit represents a tenfold change in [H+]. A pH meter gives a more precise reading than indicators, which is important in careful work.

Strong and weak, dilute and concentrated

It is essential to separate two different ideas. Strength describes how completely an acid ionises in water: a strong acid such as hydrochloric acid ionises almost completely, while a weak acid such as ethanoic acid ionises only partially, existing mostly as molecules. Concentration describes how much acid is dissolved in a given volume, measured in mol/dm3. A strong acid can be dilute and a weak acid can be concentrated. At the same concentration, a strong acid has a lower pH, conducts better, and reacts faster than a weak acid, because it provides more free H+ ions. Confusing strength with concentration is one of the most common DSE errors.

Neutralisation and salt preparation

Neutralisation is the reaction of an acid with a base to form a salt and water, with the essential change being H+ + OH- -> H2O. The salt formed depends on the acid: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, and nitric acid gives nitrates. Soluble salts can be prepared by reacting an acid with an insoluble base, carbonate or metal, adding excess solid, filtering off what is left over, then evaporating and crystallising the filtrate. For example, CuO + H2SO4 -> CuSO4 + H2O. Insoluble salts are made by precipitation, mixing two soluble salts so the desired product comes out of solution and is filtered off, washed and dried.

Reactions of acids with metals and carbonates

Acids react with metals above hydrogen to give a salt and hydrogen, for example Mg + 2 HCl -> MgCl2 + H2, with the gas giving a squeaky pop with a lighted splint. Acids react with metal carbonates and hydrogencarbonates to give a salt, water and carbon dioxide, for example CaCO3 + 2 HCl -> CaCl2 + H2O + CO2; the gas turns limewater milky, which is the test for carbon dioxide. Acids react with metal oxides and hydroxides (bases) to give a salt and water only. These three patterns, with metals, carbonates and bases, let you predict the products of almost any simple acid reaction and identify an unknown by the gas evolved.

Titration and concentration calculations

Titration finds an unknown concentration by reacting measured volumes of acid and alkali to the end point, shown by an indicator colour change. A pipette delivers a fixed volume into a flask, and a burette adds the other solution until the indicator just changes. The calculation uses moles: concentration in mol/dm3 equals moles divided by volume in dm3, so n = c x V (with V in dm3). Worked example: 25.0 cm3 of sodium hydroxide is neutralised by 20.0 cm3 of 0.100 mol/dm3 hydrochloric acid. Moles of HCl = 0.100 x 0.0200 = 2.00 x 10^-3 mol; the 1:1 ratio gives the same moles of NaOH, so its concentration = 2.00 x 10^-3 / 0.0250 = 0.0800 mol/dm3.