Indicator

A compound that changes its chemical structure and its colour in response to changes in its environment e.g. litmus is a pH indicator; it is red in acid solution and blue in alkali solution.

Litmus

A dye obtained from lichens, it changes colour in the presence of acid or alkali.

Aqueous

An aqueous solution is a solution in which the solvent is water.

pH

A scale for measuring acidity or alkalinity. A pH below 7 is acid; the lower the pH the greater the concentration of hydrogen ions in the solution. A pH above 7 is alkaline; the higher the pH, the lower the concentration of hydrogen ions and the higher the concentration of hydroxide ions in the solution. A solution with a pH of 7 is said to be neutral.

Mineral acid

Usually a strong acid obtained from inorganic materials e.g. sulphuric acid.

Organic acid

Usually a weak acid obtained from living materials e.g. citric acid.

Hydrogen

A colourless gas that is less dense than air. It can be prepared in the laboratory by the reaction of a metal and a dilute acid. Hydrogen burns in air to form water. If a lighted splint is brought to a test tube containing hydrogen mixed with air the mixture ignites with a squeaky pop.

Alkali

A base that is soluble in water. It forms hydroxide ions in aqueous solution e.g. sodium hydroxide.

Neutralisation

The reaction between an acid and a base to produce a neutral salt solution.

Carbon dioxide

A colourless gas that is denser than air. It can be prepared in the laboratory by the reaction of a carbonate and a dilute acid. When bubbled into limewater the gas forms a milky precipitate of calcium carbonate. Carbon dioxide dissolves in water to form carbonic acid.

Exothermic reaction

A reaction that gives out heat energy.