Chapter section
1.1.2 Balanced Chemical Equations
Section 1.1.2
Balanced Chemical Equations
The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. Therefore, every element must have the same number of atoms before and after the reaction.
Activity 1.3 can be written as:
Zinc + Sulphuric acid → Zinc sulphate + HydrogenZn + H₂SO₄ → ZnSO₄ + H₂ (1.3)
Equation (1.3) is already balanced: Zn = 1, H = 2, S = 1 and O = 4 on each side.
Now consider the skeletal equation:
Fe + H₂O → Fe₃O₄ + H₂ (1.4)
Balancing by the hit-and-trial method
Step I: Draw boxes around every formula. Do not change anything inside a box.
Fe + H₂O → Fe₃O₄ + H₂ (1.5)
Step II: Count the atoms.
| Element | Reactants | Products |
|---|---|---|
| Fe |
❓ Study questions
Questions and answers
Question 1
Why should a magnesium ribbon be cleaned before burning in air?
Try first, then reveal the answer
It is cleaned to remove the thin magnesium oxide layer from its surface. This exposes fresh magnesium so that it burns readily.
Question 2
Write the balanced equation for the following chemical reactions.
Try first, then reveal the answer
(i) Hydrogen + Chlorine → Hydrogen chloride Answer: H₂ + Cl₂ → 2HCl
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride Answer: 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
(iii) Sodium + Water → Sodium hydroxide + Hydrogen Answer: 2Na + 2H₂O → 2NaOH + H₂
Question 3
Write a balanced chemical equation with state symbols for the following reactions.
Try first, then reveal the answer
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
⚠️ Common mistake
Common mistakes
- Changing subscripts instead of coefficients.
- Forgetting the unnumbered atom check before adding states.
- Writing liquid water in equation (1.10); NCERT uses steam,
H₂O(g). - Omitting
340 atm, sunlight or chlorophyll from condition-based equations.
💡 Board tip
Board tips
- Balance formulae first; add states and conditions only after the atom count is correct.
- Use the smallest whole-number coefficients.
- Check every element once more before finalising the answer.
✏️ Quick practice
Quick practice
Balance Al + O₂ → Al₂O₃ without changing any formula.
🔍 Practice feedback
Answer check
Try first, then reveal the answer
4Al + 3O₂ → 2Al₂O₃⏱️ 1-Minute revision
One-minute revision
- Conservation of mass requires equal atom counts.
- Coefficients may change; subscripts may not.
(aq)means dissolved in water.- Conditions belong above or below the reaction arrow.
🎭 Memory strip
Comic recap
Quick scene, quick smile, quick memory. Read this once and the idea sticks better.