Chemical Reactions and Equations • Topic 1 of 3

Chemical Equations & Balancing

A chemical reaction is a change in which one or more substances (the reactants) are converted into one or more new substances (the products) with different properties. You can usually tell a chemical reaction is happening by looking for tell-tale signs: a change in colour, a change in state, the formation of a gas (bubbles), the formation of an insoluble solid called a precipitate, or a change in temperature. For example, when magnesium ribbon burns in air it gives a dazzling white flame and leaves a white powder of magnesium oxide — that powder is the product, not the metal you started with.

Writing a chemical equation

A chemical equation is a short-hand way of describing a reaction using symbols and formulae instead of words. Reactants are written on the left, products on the right, and an arrow (→) pointing from reactants to products shows the direction of change. The word equation 'magnesium + oxygen → magnesium oxide' becomes the symbol equation Mg + O2 → MgO.

Why an equation must be balanced

The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. This means the total mass of the reactants must equal the total mass of the products, so the number of atoms of each element must be the same on both sides of the equation. An equation that does not satisfy this is called a skeletal or unbalanced equation. The equation Mg + O2 → MgO is unbalanced, because there are two oxygen atoms on the left but only one on the right.

Steps to balance an equation

  • Write the correct formulae of all reactants and products — never change a formula to balance.
  • Count the atoms of each element on both sides.
  • Use coefficients (the big numbers written in front of a formula) to make the atom counts equal, starting with the element that appears in the fewest places.
  • Balance hydrogen and oxygen last, as they often appear in many compounds.
  • Check every element again, and reduce coefficients to the simplest whole-number ratio.

Balancing Mg + O2 → MgO gives 2Mg + O2 → 2MgO: two Mg and two O on each side. This trial-and-error approach is sometimes called the hit-and-trial method.

Making equations more informative

We add the physical states in brackets: (s) for solid, (l) for liquid, (g) for gas and (aq) for an aqueous (dissolved in water) solution. Conditions such as heat, pressure or a catalyst are written above or below the arrow. For instance, 6CO2(g) + 6H2O(l) → C6H12O6(aq) + 6O2(g) describes photosynthesis with states shown.

Conservation of mass: 2H2 + O2 gives 2H2O has equal atoms on both sidesAtoms balance across the arrowReactants2H₂ + O₂4 H atoms, 2 O atomsProducts2H₂O4 H atoms, 2 O atomsTotal mass of reactants = total mass of products
Solved Examples
1
Worked Example
Balance the equation Fe + H2O → Fe3O4 + H2.
Solution
  1. Count atoms: left Fe = 1, O = 1, H = 2; right Fe = 3, O = 4, H = 2.
  2. Balance Fe by placing 3 before Fe: 3Fe + H2O → Fe3O4 + H2.
  3. Balance O by placing 4 before H2O: 3Fe + 4H2O → Fe3O4 + H2.
  4. Now H on left = 8, so place 4 before H2 on the right.

Answer: 3Fe + 4H2O → Fe3O4 + 4H2.

2
Worked Example
Balance the combustion of methane: CH4 + O2 → CO2 + H2O.
Solution
  1. Carbon is already balanced (1 each).
  2. Hydrogen: 4 on left, so place 2 before H2O: CH4 + O2 → CO2 + 2H2O.
  3. Now oxygen on right = 2 (from CO2) + 2 (from 2H2O) = 4.
  4. Place 2 before O2 to give 4 O atoms on the left.

Answer: CH4 + 2O2 → CO2 + 2H2O.

3
Worked Example
Write a balanced equation with states for the reaction of zinc with dilute sulphuric acid.
Solution
  1. Word equation: zinc + sulphuric acid → zinc sulphate + hydrogen.
  2. Skeletal: Zn + H2SO4 → ZnSO4 + H2.
  3. Check atoms: Zn = 1, H = 2, S = 1, O = 4 on both sides — already balanced.
  4. Add states: solid Zn, aqueous acid and salt, gaseous hydrogen.

Answer: Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g).

4
Worked Example
Balance the equation NaOH + H2SO4 → Na2SO4 + H2O.
Solution
  1. Sodium: 1 on left, 2 on right — place 2 before NaOH.
  2. 2NaOH + H2SO4 → Na2SO4 + H2O.
  3. Check H: left = 2 + 2 = 4; right = 2, so place 2 before H2O.
  4. Recheck O: left 2 + 4 = 6, right 4 + 2 = 6 — balanced.

Answer: 2NaOH + H2SO4 → Na2SO4 + 2H2O.

5
Worked Example
Why can we not balance the equation H2 + O2 → H2O by changing the formula of water to H2O2?
Solution
  1. The formula of a compound is fixed by its actual composition; water is always H2O.
  2. Writing H2O2 would describe hydrogen peroxide, a different substance.
  3. We balance only by adjusting coefficients, never by altering subscripts.
  4. The correct balanced form is 2H2 + O2 → 2H2O.

Answer: Changing the subscript changes the substance; balance with coefficients, giving 2H2 + O2 → 2H2O.

6
Worked Example
Balance the equation Al + Cl2 → AlCl3.
Solution
  1. Chlorine: 2 on left, 3 on right; the LCM of 2 and 3 is 6.
  2. Place 3 before Cl2 (6 Cl) and 2 before AlCl3 (6 Cl).
  3. Al + 3Cl2 → 2AlCl3 gives 2 Al on the right.
  4. Place 2 before Al on the left to balance aluminium.

Answer: 2Al + 3Cl2 → 2AlCl3.

Key Points

  • A chemical reaction turns reactants into new products; signs include colour change, gas evolution, precipitate, change of state or temperature change.
  • A chemical equation shows reactants on the left and products on the right, joined by an arrow (→).
  • By the law of conservation of mass, atoms of each element must be equal on both sides — a balanced equation.
  • Balance using coefficients (the front numbers); never change a correct chemical formula or its subscripts.
  • Add physical states (s), (l), (g), (aq) and write conditions like heat or catalyst over the arrow to make an equation more informative.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.Which of the following is NOT a sign that a chemical reaction has occurred?
Explanation: A reaction must produce a new substance; merely changing shape (a physical change) is not a chemical reaction.
Q2.A balanced equation must obey the law of:
Explanation: Atoms are neither created nor destroyed, so total mass is conserved and atom counts match on both sides.
Q3.The balanced form of Fe + H2O → Fe3O4 + H2 is:
Explanation: 3 Fe, 8 H and 4 O appear on each side in 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
Q4.The symbol (aq) written after a formula means the substance is:
Explanation: (aq) stands for aqueous — the substance is dissolved in water.
Practice more MCQs → 📝 Assignment · 30 marks