IGCSE Chemistry (0620)

Balancing symbol equations

What this topic tests, the mistakes that cost marks, and one exam-style question worked through step by step.

Key ideas and common mistakes

To write a balanced equation, put the correct formulas down first, then add whole-number coefficients until each element has equal atoms on both sides, and use ⇌ for a reversible reaction.
Balancing combustion equations requires matching the number of each atom on both sides, especially adjusting oxygen last since it appears in multiple products.
Common mistake: Students often forget to count all oxygen atoms or balance hydrogen and carbon first.
Thermal decomposition of a metal carbonate produces the metal oxide and carbon dioxide gas.
Common mistake: Students often forget to include CO2 as a product or miswrite the products, especially for less familiar metals.
Writing a balanced symbol equation for a reaction requires correct formulas for all reactants and products and ensuring the number of each type of atom is equal on both sides.
Common mistake: Students often miswrite product formulas or fail to balance atoms.
Writing a balanced symbol equation for a metal–acid reaction requires knowing the metal’s ionic charge to determine the salt’s formula, then balancing all atoms.
Common mistake: Students often forget to balance hydrogen or miswrite the salt’s formula (e.g., using MgCl instead of MgCl2).
To write a symbol equation for neutralisation, identify the correct salt formed by combining the metal from the alkali with the acid's anion, not just swapping H for Na or mixing up the ions.

Write the symbol equation for this reversible reaction.

Try it first, then show the working

Answer: 2SO2 + O2 ⇌ 2SO3

23 exam-style questions on Balancing symbol equations, each marked to the scheme.