Types of reactions and the reactivity series (A)

In a chemical reaction the bonds between the particles of the reactants break and new bonds form, so the products are new substances with new properties. Signs that a reaction has happened are a change of colour or smell, a gas given off, a solid forming in a liquid, or light and heat. Chemists sort reactions by how the atoms or ions are rearranged. In direct combination, A + B gives AB. In decomposition, AB splits into A and B, usually when heated or lit. There are two more types, single and double replacement, which come in later lessons. When heated in air, most metals join with oxygen, though some do so far faster than others. Magnesium reacts fastest, iron more slowly and copper slowest, so we say that the reactivity falls in the order Mg > Fe > Cu. The three oxides are different: magnesium oxide is a white powder, the iron oxide is magnetic, and copper gives a black layer. A decomposition example is calcium carbonate, which breaks into calcium oxide and carbon dioxide when strongly heated. Never repeat these reactions at home: the book’s versions use flames, and the light of burning magnesium can damage the eyes.