What Is a Word Equation?
A word equation is a written description of a chemical reaction using the names of the substances involved. It is the simplest way to record what happens in a chemical change.
The format is:
reactants → products
- Reactants are the substances that you start with (left of the arrow).
- Products are the new substances formed (right of the arrow).
- The arrow (→) means "reacts to produce" or "yields".
If there are two or more reactants or products, they are separated by + signs.
Examples of Word Equations
| Reaction | Word Equation |
|---|---|
| Burning hydrogen | hydrogen + oxygen → water |
| Iron rusting | iron + oxygen + water → iron(III) oxide |
| Burning methane | methane + oxygen → carbon dioxide + water |
| Making salt | sodium + chlorine → sodium chloride |
| Zinc and hydrochloric acid | zinc + hydrochloric acid → zinc chloride + hydrogen |
The arrow in a word equation represents a chemical change. The substances on the right are chemically different from those on the left — new bonds have formed and old ones have broken. The total mass, however, remains the same (conservation of mass).
Writing Word Equations Step by Step
Step 1: Identify the reactants (what you start with). Step 2: Identify the products (what is formed — look for new substances, gases, precipitates). Step 3: Write reactants on the left, products on the right, with an arrow in between. Step 4: Separate multiple substances with + signs.
Neutralisation reaction
Hydrochloric acid is added to sodium hydroxide solution. A neutral salt solution forms.
Reactants: hydrochloric acid, sodium hydroxide Products: sodium chloride, water
Word equation: hydrochloric acid + sodium hydroxide → sodium chloride + water
State Symbols
Sometimes state symbols are added after each substance name (or formula) to show the physical state:
- (s) = solid
- (l) = liquid
- (g) = gas
- (aq) = aqueous (dissolved in water)
For example: zinc(s) + sulfuric acid(aq) → zinc sulfate(aq) + hydrogen(g)
When you are asked to write a word equation, do not put formulas or symbols — just the names of the substances. Word equations use words only. Symbol equations (which use formulas like H₂O and Fe) are a more advanced form of the same information.
Combustion Reactions Pattern
All complete combustion reactions of hydrocarbons follow the same pattern:
hydrocarbon + oxygen → carbon dioxide + water
Knowing this pattern means you can write the word equation for burning almost any fuel in excess oxygen without memorising each one individually.
Real world: Baking powder making cakes rise
Baking powder contains sodium hydrogencarbonate (bicarbonate of soda). When heated in a cake, it decomposes: sodium hydrogencarbonate → sodium carbonate + water + carbon dioxide. The CO₂ gas produced creates bubbles in the batter, making the cake light and fluffy. The word equation describes exactly what is happening in your oven every time you bake.
Real world: Antacid tablets and stomach acid
Indigestion tablets (antacids) contain compounds such as calcium carbonate or magnesium hydroxide. When swallowed, they react with hydrochloric acid in the stomach. For example: calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide. The CO₂ is what causes the familiar "burp" after taking an antacid. The word equation makes the chemistry of digestion relief clear.
Real world: Car exhaust chemistry
Car exhausts produce several gases through combustion reactions that can be written as word equations: petrol (hydrocarbon) + oxygen → carbon dioxide + water (complete combustion). When combustion is incomplete: petrol + oxygen → carbon monoxide + water. Environmental regulations limit CO and unburned hydrocarbons in exhaust gases, directly targeting the products in these word equations.
Where do you see this in real life? Baking soda and vinegar volcanoes (acid + carbonate → salt + water + carbon dioxide), antacid tablets in stomachs, gas cooker combustion, metal rusting — every chemical reaction in your kitchen, car, and body can be described with a word equation.
Practice Problems
Write the word equation for the reaction between magnesium and oxygen when magnesium burns in air.
Show answer
Calcium carbonate reacts with hydrochloric acid. The products are calcium chloride, water, and carbon dioxide gas. Write the word equation.