Chemical reactions and word equations

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.

Worked examples

Example 1

Magnesium ribbon burns with a bright white light and leaves a white powder. Write the word equation and name the type of reaction.

  1. The magnesium reacts with oxygen from the air. These are the reactants.
  2. The white powder is the new substance, magnesium oxide: the product.
  3. Write reactants → products: magnesium + oxygen → magnesium oxide
  4. It burns with light and heat, so it is combustion; the magnesium gains oxygen, so it is also an oxidation.

Answer: magnesium + oxygen → magnesium oxide; combustion (an oxidation)

Example 2

Green copper carbonate is heated and turns black. The gas given off turns limewater milky. Explain what happens.

  1. The green solid is copper carbonate. Heating breaks it down: thermal decomposition.
  2. Limewater turning milky shows the gas is carbon dioxide.
  3. The black solid left is copper oxide. copper carbonate → copper oxide + carbon dioxide

Answer: thermal decomposition: copper carbonate → copper oxide (black) + carbon dioxide

Example 3

12 g of magnesium burns to make 20 g of magnesium oxide. What mass of oxygen reacted?

  1. No atoms are made or lost, so the mass of the magnesium oxide equals the mass of the magnesium plus the mass of the oxygen.
  2. Take the magnesium from the magnesium oxide. 20 − 12 = 8 g
  3. That is why the solid gets heavier when magnesium burns: oxygen from the air has joined it.

Answer: 8 g

Example 4

Zinc is added to hydrochloric acid and fizzes. Name the gas, say how to test for it and write the word equation.

  1. A metal and an acid make a salt and hydrogen. Hydrochloric acid makes chlorides.
  2. Test: hold a lit splint at the mouth of the tube. Hydrogen burns with a squeaky pop.
  3. The word equation: zinc + hydrochloric acid → zinc chloride + hydrogen

Answer: hydrogen; a lit splint goes out with a squeaky pop; zinc + hydrochloric acid → zinc chloride + hydrogen