Chemical Change – IGCSE Chemistry Definition

A chemical change, also called a chemical reaction, is a process in which one or more new substances are formed. The starting materials (reactants) are converted into products that have different chemical properties. During a chemical change, chemical bonds in the reactants are broken and new bonds are formed in the products. Most chemical changes are difficult or impossible to reverse by simple physical means.

Signs of a chemical change

Several observations can indicate that a chemical change has taken place. A colour change (for example, black copper oxide turning blue when dissolved in acid) suggests new substances are forming. Gas production (effervescence or bubbles) indicates a gaseous product is being released. A temperature change (the mixture getting hotter or cooler) shows energy is being transferred. A precipitate forming (an insoluble solid appearing in a solution) is another sign.

However, none of these signs alone proves a chemical change --- some physical changes also produce similar observations.

Compared with physical change

In a physical change, no new substances are formed. The same substance remains, just in a different state or form. Melting, boiling, dissolving, and changes of state are all physical changes. They are easily reversed. For example, melting ice gives water, and freezing water gives ice again --- no new substance is created.

In a chemical change, the products are chemically different from the reactants. Burning magnesium (Mg + O₂ → MgO) produces magnesium oxide, which is a white powder with completely different properties from the shiny metal.

Exam context

Paper 2 and Paper 4 questions may present a list of processes and ask candidates to classify each as a physical or chemical change. The key distinction is whether a new substance is formed. Dissolving sugar in water is physical (sugar can be recovered by evaporation). Burning sugar is chemical (carbon dioxide and water are produced, and sugar cannot be recovered).

Worked exam question

Classify each process as a physical or chemical change. (a) Rusting of iron. (b) Melting of wax. (c) Cooking an egg. (2 marks --- choose any two correct)

(a) Chemical change --- a new substance (hydrated iron(III) oxide) is formed [1]. (b) Physical change --- the wax changes state but no new substance is formed [1]. (c) Chemical change --- the proteins in the egg are permanently changed and new substances form [1].

Common mistakes

Candidates sometimes classify dissolving as a chemical change. At IGCSE level, dissolving (such as salt in water) is treated as a physical change because the salt can be recovered by evaporation. Another error is thinking that all colour changes indicate chemical changes --- some physical processes (like mixing paints) also produce colour changes without forming new substances.