Balanced chemical equation

An equation shows what goes in and what comes out of a reaction. The reactants go on the left and the products on the right. Mass is conserved, so each kind of atom must be counted the same on both sides.

You balance an equation by changing the numbers in front of the formulas. You never change a formula itself. H₂O stays H₂O. You can also add state symbols: (s) for solid, (l) for liquid, (g) for gas and (aq) for a solution in water.

A small example

Methane burns in oxygen: CH₄ + O₂ → CO₂ + H₂O. Count the atoms. Carbon is 1 and 1. Hydrogen is 4 on the left but 2 on the right, so put 2 before H₂O. Now oxygen is 2 on the left and 4 on the right, so put 2 before O₂. The result is CH₄ + 2O₂ → CO₂ + 2H₂O. Each side now has 1 C, 4 H and 4 O.

Where marks go

Question 29 of CBSE's 2026-27 Science sample paper gives two faulty equations to correct. One is Al + H₂SO₄ → Al₂SO₄ + H₂O. The 2026-27 Science marking scheme gives 1 mark for 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂.

Here the numbers in front were not the only fault. The product formula was wrong, and the gas given off is hydrogen, not water. So check each formula first, then balance. Our note on where the marks go in CBSE Science shows where the 2026 scheme took half a mark off an unbalanced equation.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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