How to write, balance and classify chemical reactions
A chemical reaction changes substances into new ones, and you can often tell by a change in colour, temperature or state, or by the release of gas. A chemical equation shows reactants and products using formulae. Because atoms are neither created nor destroyed, equations must be balanced so the same number of each atom appears on both sides. For example, H₂ + O₂ → H₂O becomes 2H₂ + O₂ → 2H₂O when balanced.
Reactions are grouped into types. In combination, two substances form one, as when calcium oxide reacts with water to make slaked lime. In decomposition, one substance breaks into simpler ones, often using heat, light or electricity. In displacement, a more reactive metal pushes a less reactive one out of its compound, like iron displacing copper from copper sulphate. Double displacement involves an exchange of ions, often forming a precipitate. Oxidation is the gain of oxygen or loss of hydrogen; reduction is the reverse, and together they are redox reactions.
Why it matters
Chemical reactions cook our food, digest it, rust iron and make medicines and fertilisers. Corrosion damages bridges and vehicles, so it is prevented by painting and galvanising. Rancidity spoils oily food, so manufacturers use antioxidants and nitrogen-filled packets. Balancing equations is a skill needed for every later chemistry calculation.
Key terms
- Balanced equation
- A chemical equation with the same number of each type of atom on both sides.
- Displacement reaction
- A reaction where a more reactive element replaces a less reactive one in a compound.
- Oxidation
- The gain of oxygen or loss of hydrogen by a substance during a reaction.
- Corrosion
- The slow destruction of metals by reaction with air, water or chemicals around them.