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Class 9 Science: Atomic Foundations of Matter, 8 practice questions with answers

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These are 8 practice questions on Chapter 9, Atomic Foundations of Matter, of the new NCERT Class 9 Science textbook, Exploration (First Edition, April 2026). Each has a worked answer and the textbook page it comes from. They cover the law of conservation of mass, the law of constant proportions, a single covalent bond, writing the formula of an ionic compound by criss-crossing, naming and finding the molecular mass of a covalent compound, the formula unit mass of an ionic compound, and why ionic compounds conduct only in solution. Try each one on paper before you open the answer.

These questions are our own, drafted with AI help. Every number was re-computed by a test script and none of the numbers is taken from the chapter, but no practising teacher has reviewed them yet. If you find a mistake, please tell us.

Atomic masses are the ones the textbook uses: H = 1 u, N = 14 u, O = 16 u, K = 39 u (pages 179 and 180).

What the chapter says you need

Facts from the NCERT textbook (Exploration: Textbook of Science for Grade 9, Chapter 9, printed pages 162 to 183):

Laws of chemical combination

Question 1. In a sealed flask, 17.5 g of one substance reacts completely with 6.3 g of another. No gas can escape. What is the total mass of the products?

Answer: 23.8 g. Mass is neither created nor destroyed in a chemical reaction, so the products have the same total mass as the reactants: 17.5 + 6.3 = 23.8 g. (Law of Conservation of Mass, page 165.)

Question 2. In a certain compound, elements X and Y always combine in the mass ratio 5 : 7. How much of Y combines completely with 45 g of X?

Answer: 63 g. The ratio is fixed, so Y = 45 × 7/5 = 63 g. Check: 45 : 63 = 5 : 7. (Law of Constant Proportions, page 167.)

Question 3. One student collects carbon dioxide from burning charcoal; another collects carbon dioxide from the reaction of baking soda with vinegar. Will the mass ratio of carbon to oxygen be the same in both pure samples? Which law says so?

Answer: yes, the same; the Law of Constant (Definite) Proportions. A pure compound always has its elements in the same fixed ratio by mass, whatever its source or the way it was made. (Page 167.)

Bonds, formulae and names

Question 4. Hydrogen bromide is HBr. Hydrogen needs one electron and bromine (seven valence electrons) needs one electron to be stable. How many electrons does each atom share, and what kind of bond forms?

Answer: each atom shares one electron, forming one shared pair: a single covalent bond, H—Br. This is the same pattern the textbook shows for hydrogen chloride. (Covalent bond in HCl, page 171.)

Question 5. Using Table 9.1, aluminium forms Al³⁺ and sulfur forms the sulfide ion S²⁻. Write the formula of aluminium sulfide.

Answer: Al₂S₃. Write the charges, 3+ for Al and 2− for S, and criss-cross them: Al gets 2 and S gets 3. The subscripts 2 and 3 have no common factor, so the formula is Al₂S₃. Check: 2 × (+3) = +6 and 3 × (−2) = −6, so the compound is neutral. (Table 9.1, page 174; criss-crossing, page 176.)

Question 6. A covalent compound has the formula N₂O₃. Name it and find its molecular mass.

Answer: dinitrogen trioxide; 76 u. Two nitrogen atoms (di) and three oxygen atoms (tri, ending in -oxide). Molecular mass = (14 u × 2) + (16 u × 3) = 28 u + 48 u = 76 u. (Naming covalent compounds, page 172; molecular mass, page 179.)

Masses and properties

Question 7. Find the formula unit mass of potassium nitrate, KNO₃.

Answer: 101 u. One formula unit has one K, one N and three O: (39 u × 1) + (14 u × 1) + (16 u × 3) = 39 + 14 + 48 = 101 u. (Formula unit mass, pages 179 and 180; the nitrate ion, Table 9.1, page 175.)

Question 8. A crystal of an ionic compound does not let a bulb glow in a conductivity test, but its solution in water does. Explain why.

Answer: In the solid crystal, the ions are held in fixed positions by strong forces, so they cannot move and carry a current. When the compound dissolves in water, the ions become free to move, so the solution conducts electricity. (Properties of ionic compounds, page 178.)

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Sources

All opened and read on 21 October 2026. Page numbers are the printed page numbers of the textbook; printed page = PDF page + 161.

The laws, rules and atomic masses are from the textbook pages above. The questions, worked answers and checks are our own, and every number was re-computed by script. This guide is general information; follow your teacher and the textbook.