Chemical reactions and word equations · Grade 6
In a chemical reaction, the substances you start with (the reactants) change into new substances (the products). The atoms are rearranged, never made or destroyed, so the total mass stays the same. We write reactions as word equations.
Key facts
- Signs of a reaction: fizzing (a gas), a colour change, a temperature change, a precipitate (a new solid) or light.
- Physical changes (melting, dissolving, boiling) make no new substance and can be reversed. Chemical changes make new substances.
- A word equation: reactants → products, for example magnesium + oxygen → magnesium oxide.
- Combustion is burning in oxygen: methane + oxygen → carbon dioxide + water. Oxidation is gaining oxygen: metal + oxygen → metal oxide.
- Thermal decomposition: heat breaks one substance down, for example copper carbonate → copper oxide + carbon dioxide.
- Metal + acid → salt + hydrogen. Gas tests: hydrogen gives a squeaky pop; oxygen relights a glowing splint; carbon dioxide turns limewater milky.
- Conservation of mass: the total mass of the reactants equals the total mass of the products. If a gas escapes, the mass left goes down; if a gas from the air joins, it goes up.