How to write word and chemical equations

Before you can balance an equation, you have to write the right one, and that means knowing what the products actually are. In SPM Chemistry, questions often describe a reaction in words and ask you to produce a chemical equation. This guide takes you through both stages: writing a clear word equation, then converting it into a balanced chemical equation with state symbols. It sits alongside The mole concept, chemical formula and equation, where equations first appear.

What a word equation is

A word equation names the reactants and products without any formulae. The reactants go on the left, the products on the right, joined by an arrow that means “produces” or “yields”. A plus sign separates two substances on the same side. Reaction conditions, such as heat or a catalyst, are written above the arrow.

For example, when magnesium reacts with hydrochloric acid:

magnesium + hydrochloric acid → magnesium chloride + hydrogen

Read it aloud as “magnesium reacts with hydrochloric acid to produce magnesium chloride and hydrogen.” Word equations are worth marks on their own, and they force you to decide on the products before you worry about formulae.

Predicting the products

The hardest part is usually knowing what the products are. Fortunately, SPM reactions fall into a small number of predictable patterns. Learn these and you can write most word equations on sight:

• Reactive metal + acid → salt + hydrogen. (See acid and reactive metal.)
• Metal oxide + acid → salt + water.
• Metal carbonate + acid → salt + water + carbon dioxide.
• Acid + alkali (neutralisation) → salt + water.
• Thermal decomposition of a metal carbonate → metal oxide + carbon dioxide.
• Complete combustion of a hydrocarbon → carbon dioxide + water.

Naming the salt

In the first four patterns the product is a salt, and its name comes from two parts: the metal (from the base or metal) gives the first word, and the acid gives the second. The acid decides the ending:

• Hydrochloric acid (HCl) → chloride
• Nitric acid (HNO₃) → nitrate
• Sulfuric acid (H₂SO₄) → sulfate

So potassium hydroxide plus nitric acid gives potassium nitrate; zinc oxide plus sulfuric acid gives zinc sulfate. Get the salt name right and the word equation almost writes itself.

Converting a word equation into a chemical equation

Once the word equation is written, changing it into a chemical equation is a three-step routine:

1. Write the correct formula under each name. This is where you use ionic charges, magnesium chloride is MgCl₂ because Mg²⁺ needs two Cl⁻, not MgCl.
2. Balance by adjusting coefficients only, never subscripts, so each element has equal atoms on both sides.
3. Add state symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).

Take the magnesium example. The formulae are Mg, HCl, MgCl₂ and H₂. Balancing the chlorine and hydrogen gives:

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Worked example: carbonate and acid

Write the equation for calcium carbonate reacting with dilute hydrochloric acid.

• Word equation: calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide.
• Formulae: CaCO₃, HCl, CaCl₂, H₂O, CO₂.
• Balance: two HCl are needed for the two chlorides, CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g).

Worked example: neutralisation

Write the equation for sodium hydroxide neutralising sulfuric acid.

• Word equation: sodium hydroxide + sulfuric acid → sodium sulfate + water.
• Formulae: NaOH, H₂SO₄, Na₂SO₄, H₂O.
• Balance: two NaOH and two H₂O, 2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l).

Worked example: thermal decomposition

Write the equation for heating copper(II) carbonate.

• Word equation: copper(II) carbonate → copper(II) oxide + carbon dioxide (heat above the arrow).
• Formulae: CuCO₃, CuO, CO₂. Everything is already balanced: CuCO₃(s) → CuO(s) + CO₂(g).

Common mistakes to avoid

• Writing a formula instead of a name in a word equation, a word equation should have no formulae at all.
• Forgetting the water or gas product in carbonate and neutralisation reactions.
• Wrong salt formula because the ionic charges were not balanced first; check the charges before you balance atoms.
• Missing state symbols in Paper 2, which can cost a mark even when the equation is otherwise correct.

You can check the products for dozens of standard reactions in the reactions reference, then rewrite each one from memory as practice. If predicting products or writing formulae keeps letting you down, that is a fast thing for a teacher to fix one-to-one. Our lessons run online in English from RM50 an hour, with a paid one-hour trial, working through your own weak reactions until word-to-symbol conversion is automatic.