Word Equations and Symbol Equations: KS3 Chemistry

When particles collide with enough energy, bonds break and new substances form — that is a chemical reaction. A word equation names those substances in plain English, whilst a symbol equation uses chemical formulae and numbers to show exactly which atoms are involved and in what ratio, giving chemists a precise, universal shorthand understood in every language.

What is a word equation?

A word equation describes a chemical reaction using the full names of substances. Reactants (the starting materials) go on the left, products (the new substances formed) go on the right, and an arrow (→) shows the direction of the reaction.

General form: reactant + reactant → product + product

Example — combustion of magnesium:

magnesium + oxygen → magnesium oxide

The word equation tells you what reacted and what was made, but it says nothing about quantities or the exact numbers of particles involved. That is the job of the symbol equation.

What is a symbol equation?

A symbol equation replaces each substance's name with its chemical formula and includes numbers called coefficients to show how many formula units of each substance take part.

Example:

2Mg + O₂ → 2MgO

The large numbers in front of formulae (the coefficients) are what you adjust when balancing. The small subscript numbers inside formulae (like the ₂ in O₂) define the substance itself and must never be changed.

Symbol equations follow the Law of Conservation of Mass: atoms are neither created nor destroyed in a chemical reaction, so the count of each type of atom must be equal on both sides of the arrow.

How do you write a word equation step by step?

Identify the reactants. These are the substances you mix or heat together. Write them on the left of the arrow.

Identify the products. These are the new substances formed. Write them on the right.

Separate multiple substances with +. Use + between reactants, and again between products if more than one is produced.

Draw the arrow. Write → between the reactants and products. It means "reacts to form."

Read it aloud. Say it as a sentence: "Reactant A plus reactant B forms product C plus product D." If it makes chemical sense, you are done.

Worked example: Zinc reacts with hydrochloric acid to produce zinc chloride and hydrogen gas.

Reactants: zinc, hydrochloric acid

Products: zinc chloride, hydrogen

zinc + hydrochloric acid → zinc chloride + hydrogen

How do you convert a word equation to a symbol equation?

Step 1 — Replace each name with the correct chemical formula.

Substance | Formula
Zinc | Zn
Hydrochloric acid | HCl
Zinc chloride | ZnCl₂
Hydrogen | H₂

Step 2 — Write the unbalanced equation:

Zn + HCl → ZnCl₂ + H₂

Step 3 — Count atoms on each side:

Atom | Left | Right
Zn | 1 | 1 ✓
H | 1 | 2 ✗
Cl | 1 | 2 ✗

Hydrogen and chlorine are not balanced.

Step 4 — Add coefficients to balance:

Put a 2 in front of HCl:

Zn + 2HCl → ZnCl₂ + H₂

Step 5 — Recount:

Atom | Left | Right
Zn | 1 | 1 ✓
H | 2 | 2 ✓
Cl | 2 | 2 ✓

Balanced: Zn + 2HCl → ZnCl₂ + H₂

What are state symbols and when do you use them?

State symbols appear in brackets after each formula to show the physical state of each substance.

Symbol | State | Example
(s) | solid | Zn(s)
(l) | liquid | H₂O(l)
(g) | gas | H₂(g)
(aq) | aqueous (dissolved in water) | HCl(aq)

At KS3, include state symbols when the question asks for them. The full equation for zinc and hydrochloric acid with state symbols becomes:

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

What are some common chemical equations to know at KS3?

Reaction | Reactants | Products | Word equation
Combustion of magnesium | Magnesium, oxygen | Magnesium oxide | magnesium + oxygen → magnesium oxide
Combustion of hydrogen | Hydrogen, oxygen | Water | hydrogen + oxygen → water
Thermal decomposition of copper carbonate | Copper carbonate | Copper oxide, carbon dioxide | copper carbonate → copper oxide + carbon dioxide
Iron and sulfur | Iron, sulfur | Iron sulfide | iron + sulfur → iron sulfide
Neutralisation | Hydrochloric acid, sodium hydroxide | Sodium chloride, water | hydrochloric acid + sodium hydroxide → sodium chloride + water
Displacement | Iron, copper sulfate | Iron sulfate, copper | iron + copper sulfate → iron sulfate + copper
Photosynthesis | Carbon dioxide, water | Glucose, oxygen | carbon dioxide + water → glucose + oxygen

Frequently asked questions

Do word equations need to be balanced?

No. Word equations simply name the reactants and products — they carry no information about quantities or numbers of particles, so there is nothing to balance. Balancing only becomes necessary when you move to symbol equations, where the count of each atom must match on both sides to reflect the Law of Conservation of Mass.

What is the difference between a subscript and a coefficient?

A subscript is the small number written below and after a chemical symbol (for example, the 2 in H₂O). It is fixed — it defines the substance and must never be changed when balancing. A coefficient is the large number written in front of a formula (for example, the 2 in 2H₂O). It multiplies the whole formula and is the only number you adjust when balancing an equation.

How do I know which products are formed in a reaction?

At KS3 the question usually states the products, or you can predict them from the reaction type. In a combustion reaction, a fuel burns in oxygen to produce carbon dioxide and water. In a neutralisation reaction, an acid and an alkali always produce a salt and water. Learning the patterns for common reaction types — combustion, decomposition, displacement, neutralisation — lets you predict products confidently.

Can an arrow point in both directions?

Yes — a double arrow (⇌) indicates a reversible reaction, meaning products can re-form the reactants under the right conditions. The Haber process (making ammonia) is a key GCSE example. At KS3 you mostly encounter irreversible reactions shown with a single arrow (→), where the reaction goes to completion in one direction only.

Want to practise building chemical equations with particle-model thinking and instant Socratic feedback? Ask Professor Curie at aitutors.me — she builds the mental picture first, then guides you to the equation step by step.