Paper II · Part B — Chemistry

Chemical Equations and Laws of Chemical Combination

Exam-focused notes, visual explanation, traps, revision sheet and every mapped past NDA question.

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Core5 mapped NDA questions

Build the idea

Build exam-ready understanding by connecting the core idea, formulas or facts, worked example and previous NDA questions. Focus on why each rule works, then test it under timed conditions.

Concept

A chemical equation represents a reaction using symbols and formulae and must be balanced to obey the law of conservation of mass. The laws of chemical combination govern how elements combine.

Balancing ensures equal atoms of each element on both sides.

Key Points & Formulas

Law of conservation of mass: total mass of reactants = total mass of products.
Law of constant proportions: a compound always has the same elements in a fixed ratio by mass.
Balance by adjusting coefficients, never subscripts.
States: (s) solid, (l) liquid, (g) gas, (aq) aqueous.

Worked Example

Q. Balance: H₂ + O₂ → H₂O.

Oxygen is unbalanced (2 left, 1 right).

Balance: 2H₂ + O₂ → 2H₂O.

Now H: 4=4 and O: 2=2. Balanced.

Common traps

Reading the prompt too quickly and missing a qualifier.
Memorising the result without checking when it applies.
Skipping units, signs, chronology or the final reasonableness check.

NDA shortcuts

State the governing rule before substituting values.
Eliminate impossible options before doing full calculation.

One-screen revision sheet

Recall before you practise

Core definition
Most-used rule
One representative example

Past-paper bank

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5 NDA questions

Choose an answer before the solution appears. Option order is mixed automatically.

Question 1

NDA I 2026

1.7 g of silver nitrate is dissolved in 100 g of water, and 0.585 g of sodium chloride is dissolved in 100 g of water. Both are mixed and a chemical reaction occurs. 1.435 g of silver chloride and 0.85 g of sodium nitrate are formed. Which law of chemical combination is shown by the above reaction?

Question 2

NDA II 2025

Match List I with List II and select the answer using the code given below the Lists:

List I (Reaction) — List II (Type)

A. CaO (s) + H₂O (l) → Ca(OH)₂ (aq) — 1. Single displacement

B. CaCO₃ (s) → CaO (s) + CO₂ (g) — 2. Double displacement

C. Pb (s) + CuCl₂ (aq) → PbCl₂ (aq) + Cu (s) — 3. Combination

D. Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2NaCl (aq) — 4. Decomposition

Question 3

NDA II 2023

Match List I with List II and select the correct answer using the code given below the Lists:
List I (Chemical process): A. Electrolysis of water; B. Burning of coal; C. Iron nail immersed in copper sulphate solution; D. Addition of barium chloride solution to aluminium sulphate solution.
List II (Reaction): 1. Double displacement reaction; 2. Combination reaction; 3. Decomposition reaction; 4. Displacement reaction.

Question 4

NDA I 2023

An iron nail dipped in copper sulphate solution turns brown. This is due to which one of the following types of reactions?

Question 5

NDA I 2022

Which one of the following reactions is an example of decomposition reaction?

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