How to Write Chemical Equations in Exams

Equations are the language of chemistry, and 0620 tests them heavily. Across Papers 2 and 4, equation marks add up to a significant fraction of the total. The technique for writing equations is entirely trainable: learn the method, practise it on common reactions, and the marks become reliable. The detailed balancing method is covered in balancing equations step by step; the ionic equation technique is in ionic equations and balancing.

Word equations

Word equations are the simplest type, but the exam still awards marks for them when they are asked for. The format:

reactant + reactant -> product + product

Rules:

• Use the correct chemical names (sodium hydroxide, not NaOH).
• Use an arrow, not an equals sign.
• Include all products (acid + carbonate gives salt + water + carbon dioxide — three products, not two).

Example: Magnesium + hydrochloric acid -> magnesium chloride + hydrogen

Word equations cannot be balanced with coefficients. If the question asks for a word equation, give a word equation. If it asks for a balanced symbol equation, give that instead.

Symbol equations: the five-step method

Step 1: Write the word equation

Even if the question asks for a symbol equation, start with the word equation in your head or in the margin. This ensures you have the correct reactants and products.

Step 2: Convert each name to its correct formula

This is where most errors occur. The common formulae you must know:

Substance	Formula	Common error
Hydrochloric acid	HCl	HCl2
Sulfuric acid	H2SO4	H2SO4 is correct, but check the subscripts
Nitric acid	HNO3
Sodium hydroxide	NaOH	NaOH2
Calcium hydroxide	Ca(OH)2	CaOH2 (missing brackets)
Water	H2O
Carbon dioxide	CO2
Ammonia	NH3
Hydrogen	H2	H (must be diatomic)
Oxygen	O2	O (must be diatomic)
Chlorine	Cl2	Cl (must be diatomic)

The diatomic elements (H2, O2, N2, Cl2, Br2, I2, F2) catch students every series. These elements exist as diatomic molecules and must be written with the subscript 2.

Step 3: Balance by adjusting coefficients only

Never change subscript numbers to balance. Changing H2O to H2O2 does not balance the equation; it creates a different substance (hydrogen peroxide).

Place coefficients (large numbers in front) to make the count of each element equal on both sides. The method is covered in detail in balancing equations step by step.

Step 4: Add state symbols if required

• (s) = solid
• (l) = liquid
• (g) = gas
• (aq) = aqueous (dissolved in water)

Example with state symbols: Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)

State symbols are only required when the question asks for them, but they never lose marks if included correctly.

Step 5: Check

Count every element on both sides. A balanced equation has equal numbers of each element on both sides of the arrow. Also check that charges balance if writing ionic equations.

Ionic equations

Ionic equations show only the species that change during the reaction. Spectator ions (ions present on both sides, unchanged) are removed.

Method

1. Write the full balanced equation.
2. Split all aqueous ionic compounds into their ions.
3. Cancel ions that appear identically on both sides (spectator ions).
4. Write what remains.

Example: The reaction of sodium hydroxide with hydrochloric acid.

Full equation: NaOH(aq) + HCl(aq) -> NaCl(aq) + H2O(l)

Ionic form: Na+ + OH- + H+ + Cl- -> Na+ + Cl- + H2O

Na+ and Cl- appear on both sides, so they cancel.

Ionic equation: OH-(aq) + H+(aq) -> H2O(l)

This is the ionic equation for all neutralisation reactions between a strong acid and a strong alkali.

Half-equations

Half-equations separate the oxidation and reduction parts of a redox reaction.

• Oxidation half-equation: Shows the species losing electrons. Electrons appear on the right.
• Reduction half-equation: Shows the species gaining electrons. Electrons appear on the left.

Example: Zinc displacing copper from copper sulfate.

Oxidation: Zn -> Zn2+ + 2e- Reduction: Cu2+ + 2e- -> Cu

The number of electrons must be the same in both half-equations (or balanced with a multiplier) so that when combined, the electrons cancel.

Common reaction types and their products

Knowing the products helps you write the correct equation:

Reaction type	General equation	Products
Metal + acid	Metal + acid -> salt + hydrogen	Salt + H2
Metal oxide + acid	Metal oxide + acid -> salt + water	Salt + H2O
Metal hydroxide + acid	Hydroxide + acid -> salt + water	Salt + H2O
Metal carbonate + acid	Carbonate + acid -> salt + water + CO2	Salt + H2O + CO2
Displacement	Reactive metal + less reactive metal’s salt ->	New salt + displaced metal
Combustion (complete)	Hydrocarbon + O2 -> CO2 + H2O	CO2 + H2O
Thermal decomposition	Metal carbonate -> metal oxide + CO2	Metal oxide + CO2

Worked exam question

Q (Paper 4): Write a balanced equation, including state symbols, for the reaction of calcium carbonate with dilute hydrochloric acid. (3 marks)

Model answer: CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g) (3)

Mark allocation: correct formulae (1), correctly balanced (1), correct state symbols (1).

Common errors: writing CaCl instead of CaCl2 (1 mark lost), forgetting CO2 as a product (1 mark lost), omitting state symbols when asked (1 mark lost).

Equation writing is one of the most trainable skills in 0620. If equations are a persistent weak point, systematic practice with the five-step method usually fixes the issue within a few sessions. A trial lesson can identify whether the gap is in formula writing, balancing, or knowing the products.