CHEMISTRY · Stage 8 · Chapter 9: Chemical Reactions 2
Chemistry 9.2 Word equations — Writing Reactants and Products
5 min read 5 parts Lesson 2 of 12
Lesson summary
9.2 Word equations — Writing Reactants and Products — Alhussan International Academy — Chemistry (Stages 7–9) notes: Write and interpret word equations that show the reactants and products of a chemical reaction.Word equation = a simple way of representing a chemical reaction usi…
- Precipitation
- Combustion
- Acid reaction
- Gases in air
- Metal and acid
- Naming salts
Part 1 of 5What a word equation shows
- Word equation a simple way of representing a chemical reaction using the names of the substances.
- Reactantsthe starting substances, written on the left.
- Productsthe substances that are made, written on the right.
- Arrowit means "reacts to make", and it is not an equals sign because the reactants and products are different substances.
- Plus signit joins substances on the same side.
- Layoutwrite all reactants and products on the same line, or stack long names above each other with the plus signs between them.
- Examplesodium + chlorine → sodium chloride, where sodium and chlorine are the reactants and sodium chloride (the white fumes, common salt) is the product.
Part 2 of 5Examples of word equations
1Precipitation1 point
— potassium iodide + lead nitrate → lead iodide + potassium nitrate.
- Lead iodide is the yellow precipitate.
2Combustion1 point
— magnesium + oxygen → magnesium oxide.
- There is one product, magnesium oxide.
3Corrosion
— iron + oxygen + water → hydrated iron oxide (rust).
4Acid reaction1 point
— calcium carbonate + nitric acid → calcium nitrate + carbon dioxide + water.
- Nitric acid is one of the acids in acid rain.
5Gases in air
— nitrogen + oxygen → nitrogen monoxide, and nitrogen monoxide + oxygen → nitrogen dioxide.
6Metal and acid
— magnesium + hydrochloric acid → magnesium chloride + hydrogen.
Part 3 of 5Writing a word equation step by step
- Identify the reactants from the description, as words such as "burns in", "is added to" and "is heated with" point to them.
- Identify the products from observations: a gas that relights a glowing splint is oxygen, a gas that pops with a flame is hydrogen, and limewater turning cloudy shows carbon dioxide.
- Write the full names of the substances, not the observations.
- Put the reactants on the left, then an arrow, then the products.
- Check that every element on the right was present on the left, because atoms cannot appear from nowhere.
Part 4 of 5Patterns that save time
| Type | Pattern | Example |
|---|---|---|
| Combustion in oxygen | substance + oxygen → oxide(s) | methane + oxygen → carbon dioxide + water |
| Metal + acid | metal + acid → salt + hydrogen | zinc + hydrochloric acid → zinc chloride + hydrogen |
| Metal + oxygen | metal + oxygen → metal oxide | calcium + oxygen → calcium oxide |
| Thermal decomposition | compound → simpler substances | copper carbonate → copper oxide + carbon dioxide |
| Neutralisation | acid + alkali → salt + water | hydrochloric acid + sodium hydroxide → sodium chloride + water |
| Displacement | metal + salt solution → new salt + displaced metal | zinc + copper sulfate → zinc sulfate + copper |
- Naming salts — the name comes from the metal in the base or metal and from the acid.
- Hydrochloric acid gives chlorides, sulfuric acid gives sulfates and nitric acid gives nitrates.
- Decomposition — there is one reactant, because the energy comes from heating.
Part 5 of 5Uses and limits of word equations
- What they show — which substances react and form, supporting the idea that atoms are rearranged, not created or destroyed.
- What they do not show — the number of atoms or molecules, the state of each substance, or the conditions needed.
- These details appear in balanced symbol equations used at higher stages.
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