Chemical Reactions and Equations Class 10: Balancing Made Easy

Balancing is the skill that unlocks the whole of Chemical Reactions and Equations in Class 10. In this post you will learn why equations must be balanced, a step-by-step method that works every time, six worked examples from easy to tricky, the mistakes to avoid, and quick questions to check yourself.

Why equations must balance

A chemical reaction is a rearrangement of atoms that makes new substances. Think of building blocks: you take a model apart and rebuild a different model. The blocks are all still there; only their arrangement has changed.

In an ordinary chemical reaction, atoms are rearranged rather than created or destroyed. So a balanced equation must show the same number of atoms of each element on both sides. This is how an equation respects conservation of mass.

A skeletal equation has the correct formulae but may not yet have equal atom counts. For example, Mg + O₂ → MgO is skeletal: there are 2 oxygen atoms on the left but only 1 on the right.

Coefficients versus subscripts

This is the single most important rule:

• A coefficient is the number in front of a formula. It counts complete formula units. In 3H₂O there are 3 × 2 = 6 hydrogen atoms and 3 oxygen atoms.
• A subscript is the small number inside a formula. It belongs to the substance's identity.

You balance by changing coefficients only. Changing a subscript changes the substance. 2H₂O means two water molecules, but H₂O₂ is hydrogen peroxide, a different substance.

The step-by-step method

1. Write the word equation, then the skeletal equation with correct formulae.
2. Make an atom-count table: list each element, and count its atoms on the reactant side and the product side.
3. Pick an element that is unbalanced and change a coefficient to fix it. A formula with many atoms is often a helpful place to start.
4. Recount everything after each change, because fixing one element can unbalance another.
5. Check the final equation: every element balanced, smallest whole-number coefficients.
6. Add state symbols and conditions: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water, and heat, light or electricity written above the arrow.

Worked examples: easy to tricky

Example 1: magnesium burning. Skeletal: Mg + O₂ → MgO. Oxygen is 2 on the left, 1 on the right, so put 2 before MgO: Mg + O₂ → 2MgO. Now magnesium is 1 on the left, 2 on the right, so put 2 before Mg.

Balanced: 2Mg(s) + O₂(g) → 2MgO(s). Mg: 2 = 2. O: 2 = 2.

Example 2: forming water. Skeletal: H₂ + O₂ → H₂O. Put 2 before H₂O to fix oxygen, then 2 before H₂ to fix hydrogen.

Balanced: 2H₂(g) + O₂(g) → 2H₂O(l). H: 4 = 4. O: 2 = 2.

Example 3: iron and steam. Skeletal: Fe + H₂O → Fe₃O₄ + H₂.

• Fe₃O₄ has 4 oxygen atoms, so write 4H₂O: Fe + 4H₂O → Fe₃O₄ + H₂.
• 4H₂O has 8 hydrogen atoms, so write 4H₂: Fe + 4H₂O → Fe₃O₄ + 4H₂.
• Fe₃O₄ has 3 iron atoms, so write 3Fe.

Balanced, with states (the water is steam): 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g).

Worked examples: the tricky ones

Example 4: aluminium and oxygen. Skeletal: Al + O₂ → Al₂O₃. Oxygen is 2 on the left and 3 on the right. The smallest number both 2 and 3 divide into is 6, so write 3O₂ and 2Al₂O₃. Now aluminium on the right is 2 × 2 = 4, so write 4Al.

Balanced: 4Al + 3O₂ → 2Al₂O₃. Al: 4 = 4. O: 3 × 2 = 6 and 2 × 3 = 6.

Example 5: respiration. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Check it: C: 6 = 6. H: 12 = 6 × 2 = 12. O: 6 + (6 × 2) = 18 on the left; (6 × 2) + 6 = 18 on the right. Balanced.

Example 6: heating lead nitrate. 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g). Brackets multiply: one Pb(NO₃)₂ has 2 nitrogen and 6 oxygen atoms. Check: Pb: 2 = 2. N: 2 × 2 = 4 on the left; 4 on the right. O: 2 × 6 = 12 on the left; 2 + (4 × 2) + 2 = 12 on the right. Balanced.

Common mistakes to avoid

• Changing subscripts. Writing H₂O₂ instead of 2H₂O changes the substance.
• Forgetting brackets. In Pb(NO₃)₂, everything inside the bracket is doubled.
• Not recounting. After every new coefficient, recount all elements.
• Leaving fractions or non-simplest numbers. Use the smallest whole numbers.
• Skipping states. When the question asks, add (s), (l), (g) or (aq), and any condition such as heat.

Balancing and reaction types

Once an equation is balanced, you can classify it. The chapter groups reactions like this:

One equation can fit more than one label. For example, quicklime with water is both a combination and an exothermic reaction.

Quick check questions

1. How many hydrogen atoms are in 3H₂O? Answer: 3 × 2 = 6.

2. Balance H₂O → H₂ + O₂. Answer: 2H₂O → 2H₂ + O₂. H: 4 = 4. O: 2 = 2.

3. Balance Cu + O₂ → CuO. Answer: 2Cu + O₂ → 2CuO. Cu: 2 = 2. O: 2 = 2.

4. Balance CH₄ + O₂ → CO₂ + H₂O. Answer: CH₄ + 2O₂ → CO₂ + 2H₂O. C: 1 = 1. H: 4 = 4. O: 4 = 2 + 2.

5. Balance Pb(NO₃)₂ + KI → PbI₂ + KNO₃. Answer: Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃. K: 2 = 2. I: 2 = 2. N: 2 = 2.

6. Balance AgCl → Ag + Cl₂. Answer: 2AgCl → 2Ag + Cl₂. Ag: 2 = 2. Cl: 2 = 2.

With a little daily practice, balancing becomes quick and almost automatic. To work through more equations with guidance, study this chapter with Joy.