A student burns 10 g of ethanol in an open beaker. After the reaction, no residue is left and the mass appears to have decreased. Which statement correctly explains the observation?

Some of the products escaped as gases into the surroundings, so the observed mass decreased.

The Law of Conservation of Mass is violated because mass disappeared.

Ethanol atoms were destroyed during burning.

Some of the products escaped as gases into the surroundings, so the observed mass decreased.

The beaker absorbed the missing mass.

Assertion (A): When vinegar and baking soda are mixed in an open beaker, the reading on a digital weighing balance shows a decrease in mass after the reaction. Reason (R): The carbon dioxide gas produced escapes into the atmosphere, violating the law of conservation of mass.

Both A and R are true and R is the correct explanation of A.

Both A and R are true but R is not the correct explanation of A.

Oxygen is represented by the symbols 'O' and 'O₂'. Explain the difference between these two symbols.

The symbol 'O' represents a single atom of oxygen, while 'O₂' represents a molecule of oxygen which consists of two oxygen atoms chemically bonded together.

Design an experiment to verify the Law of Conservation of Mass for the reaction between zinc metal and dilute hydrochloric acid. Mention the materials required, the procedure, and the precaution taken to obtain correct results.

Materials: Conical flask, dilute HCl, zinc granules, balloon, thread, digital weighing balance. Procedure: Place about 2 g of zinc granules in a balloon and tie it securely. Pour about 20 mL of dilute HCl into a conical flask. Fix the balloon over the mouth of the flask with thread, keeping the zinc inside the balloon. Weigh the entire set-up (flask + acid + balloon + zinc) on the balance. Lift the balloon to drop the zinc into the acid without losing any gas. Allow the reaction to complete. Weigh again. Observation: The mass before and after the reaction remains the same. Precaution: The balloon must be tightly fixed so that no gas escapes; the balloon inflates due to the hydrogen gas produced, but the total mass inside the system is constant.

A student burns a magnesium ribbon in a crucible and finds that the mass after burning is greater than the original magnesium. Explain this observation using the Law of Conservation of Mass.

The increase in mass occurs because magnesium combines with oxygen from the air to form magnesium oxide. The total mass of magnesium and oxygen that reacted equals the mass of magnesium oxide, which is consistent with the Law of Conservation of Mass.

State the Law of Conservation of Mass as proposed by Antoine Lavoisier. Give one example of a chemical reaction where this law can be demonstrated.

The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction. Example: When solutions of silver nitrate and sodium chloride are mixed, a white precipitate of silver chloride forms; the total mass of the reactants equals the total mass of the products.

State the Law of Constant Proportions. A student analyses a sample of water from a pond and finds the mass ratio of hydrogen to oxygen is 1:8. Another student analyses water from a borewell and gets a ratio of 1:10. Suggest a possible reason for the difference and explain how the correct ratio can be verified.

The borewell water sample is not pure; it likely contains dissolved salts or impurities. To verify the correct ratio, the water should be purified by distillation and then analysed.

In an activity to verify the Law of Conservation of Mass, a student mixes barium chloride and sodium sulfate solutions in an open conical flask placed on a balance. The reading on the balance after mixing remains unchanged. However, when vinegar and baking soda are mixed in a similar open flask, the balance reading decreases. Explain both observations and suggest a simple modification to the second setup so that the law holds.

The first reaction produces no gas that escapes; all products remain in the flask. The second reaction produces carbon dioxide gas, which escapes, causing mass loss. Modification: cover the flask tightly with a balloon or use a stoppered flask to trap the gas.

A student performs an experiment where 5.3 g of sodium carbonate reacts with 6.0 g of ethanoic acid in a closed flask. After the reaction, the total mass of the contents is measured and found to be 11.3 g. The products formed are sodium ethanoate, water, and carbon dioxide. (1) Which law of chemical combination is illustrated by this experiment? (2) Calculate the total mass of the products formed. (3) If the student accidentally opened the flask during the reaction, what would happen to the reading on the balance? Explain.

The experiment illustrates the law of conservation of mass. The total mass of products is 11.3 g. Opening the flask would cause the balance reading to decrease because carbon dioxide gas escapes.

Class 9 Science Chapter 9: Atomic Foundations of Matter — Important Questions & Sample Paper

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Chapter 9, Atomic Foundations of Matter, in Class 9 Science unravels the fundamental nature of matter. It begins with the historical laws of chemical combination: the Law of Conservation of Mass, which states mass remains constant during a chemical reaction, and the Law of Constant Proportions, which asserts a compound always contains elements in fixed mass ratios. The chapter then introduces Dalton’s atomic theory, explaining how atoms of different elements combine to form molecules. Students learn to write chemical formulae using valency and the criss-cross method, calculate molecular masses, and distinguish between atoms and molecules. Key skills include applying the law of conservation of mass in both open and closed systems, verifying it experimentally, and solving numericals on constant proportions and molecular mass. Exam questions are diverse: multiple-choice items on correct formulae, short-answer reasoning on conductivity of solutions, experimental design problems on verifying mass conservation, and numericals requiring stoichiometric calculations. Teachers can use question banks to provide focused practice, ensuring students gain confidence in these essential topics for higher-level chemistry.

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Science — Atomic Foundations of Matter

Class 9Time: 3 hrsMax Marks: 80

SECTION A

1.
Hydrogen and oxygen combine in a mass ratio of 1:8 to form water. If 20 g of hydrogen reacts completely with 160 g of oxygen, the total mass of water produced is
(a) 160 g(b) 180 g(c) 200 g(d) 140 g
1
2.
The molecular mass of calcium carbonate (CaCO₃) is (Atomic masses: Ca = 40 u, C = 12 u, O = 16 u)
(a) 68 u(b) 84 u(c) 100 u(d) 116 u
1
3.
A student burns 10 g of ethanol in an open beaker. After the reaction, no residue is left and the mass appears to have decreased. Which statement correctly explains the observation?
(a) The Law of Conservation of Mass is violated because mass disappeared.(b) Ethanol atoms were destroyed during burning.(c) Some of the products escaped as gases into the surroundings, so the observed mass decreased.(d) The beaker absorbed the missing mass.
1

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Marks distribution & blueprint

In a CBSE exam, this chapter typically contributes questions across the following types. The last column shows how many original questions of each type we have ready in our bank for this chapter:

Question type	Marks each	In our bank
Multiple Choice (MCQ)	1 mark	13
Assertion–Reason	1 mark	6
Short Answer	2 marks	8
Short Answer	3 marks	6
Long Answer	5 marks	5
Case Study	4 marks	6

44 original, exam-style questions in our bank for this chapter — with answers.

Important & sample questions (with answers)

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Q1. Hydrogen and oxygen combine in a mass ratio of 1:8 to form water. If 20 g of hydrogen reacts completely with 160 g of oxygen, the total mass of water produced is
1 mark
Multiple Choice (MCQ)
(A) 160 g(B) 180 g(C) 200 g(D) 140 g
▸ Answer▾ Answer
180 g
Q2. The molecular mass of calcium carbonate (CaCO₃) is (Atomic masses: Ca = 40 u, C = 12 u, O = 16 u)
1 mark
Multiple Choice (MCQ)
(A) 68 u(B) 84 u(C) 100 u(D) 116 u
▸ Answer▾ Answer
100 u
Q3. A student burns 10 g of ethanol in an open beaker. After the reaction, no residue is left and the mass appears to have decreased. Which statement correctly explains the observation?
1 mark
Multiple Choice (MCQ)
(A) The Law of Conservation of Mass is violated because mass disappeared.(B) Ethanol atoms were destroyed during burning.(C) Some of the products escaped as gases into the surroundings, so the observed mass decreased.(D) The beaker absorbed the missing mass.
▸ Answer▾ Answer
Some of the products escaped as gases into the surroundings, so the observed mass decreased.
Q4. A compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass. Its empirical formula is
1 mark
Multiple Choice (MCQ)
(A) CHO(B) CH₂O(C) C₂H₄O₂(D) C₃H₆O₃
▸ Answer▾ Answer
CH₂O
Q5. Assertion (A): When vinegar and baking soda are mixed in an open beaker, the reading on a digital weighing balance shows a decrease in mass after the reaction. Reason (R): The carbon dioxide gas produced escapes into the atmosphere, violating the law of conservation of mass.
1 mark
Assertion–Reason
(A) Both A and R are true and R is the correct explanation of A.(B) Both A and R are true but R is not the correct explanation of A.(C) A is true but R is false.(D) A is false but R is true.
▸ Answer▾ Answer
A is true but R is false.
Q6. Oxygen is represented by the symbols 'O' and 'O₂'. Explain the difference between these two symbols.
2 marks
Short Answer
▸ Answer▾ Answer
The symbol 'O' represents a single atom of oxygen, while 'O₂' represents a molecule of oxygen which consists of two oxygen atoms chemically bonded together.
Q7. Design an experiment to verify the Law of Conservation of Mass for the reaction between zinc metal and dilute hydrochloric acid. Mention the materials required, the procedure, and the precaution taken to obtain correct results.
2 marks
Short Answer
▸ Answer▾ Answer
Materials: Conical flask, dilute HCl, zinc granules, balloon, thread, digital weighing balance. Procedure: Place about 2 g of zinc granules in a balloon and tie it securely. Pour about 20 mL of dilute HCl into a conical flask. Fix the balloon over the mouth of the flask with thread, keeping the zinc inside the balloon. Weigh the entire set-up (flask + acid + balloon + zinc) on the balance. Lift the balloon to drop the zinc into the acid without losing any gas. Allow the reaction to complete. Weigh again. Observation: The mass before and after the reaction remains the same. Precaution: The balloon must be tightly fixed so that no gas escapes; the balloon inflates due to the hydrogen gas produced, but the total mass inside the system is constant.
Q8. A student burns a magnesium ribbon in a crucible and finds that the mass after burning is greater than the original magnesium. Explain this observation using the Law of Conservation of Mass.
3 marks
Short Answer
▸ Answer▾ Answer
The increase in mass occurs because magnesium combines with oxygen from the air to form magnesium oxide. The total mass of magnesium and oxygen that reacted equals the mass of magnesium oxide, which is consistent with the Law of Conservation of Mass.
Q9. State the Law of Conservation of Mass as proposed by Antoine Lavoisier. Give one example of a chemical reaction where this law can be demonstrated.
3 marks
Short Answer
▸ Answer▾ Answer
The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction. Example: When solutions of silver nitrate and sodium chloride are mixed, a white precipitate of silver chloride forms; the total mass of the reactants equals the total mass of the products.
Q10. State the Law of Constant Proportions. A student analyses a sample of water from a pond and finds the mass ratio of hydrogen to oxygen is 1:8. Another student analyses water from a borewell and gets a ratio of 1:10. Suggest a possible reason for the difference and explain how the correct ratio can be verified.
5 marks
Long Answer
▸ Answer▾ Answer
The borewell water sample is not pure; it likely contains dissolved salts or impurities. To verify the correct ratio, the water should be purified by distillation and then analysed.
Q11. In an activity to verify the Law of Conservation of Mass, a student mixes barium chloride and sodium sulfate solutions in an open conical flask placed on a balance. The reading on the balance after mixing remains unchanged. However, when vinegar and baking soda are mixed in a similar open flask, the balance reading decreases. Explain both observations and suggest a simple modification to the second setup so that the law holds.
5 marks
Long Answer
▸ Answer▾ Answer
The first reaction produces no gas that escapes; all products remain in the flask. The second reaction produces carbon dioxide gas, which escapes, causing mass loss. Modification: cover the flask tightly with a balloon or use a stoppered flask to trap the gas.
Q12. A student performs an experiment where 5.3 g of sodium carbonate reacts with 6.0 g of ethanoic acid in a closed flask. After the reaction, the total mass of the contents is measured and found to be 11.3 g. The products formed are sodium ethanoate, water, and carbon dioxide.
4 marks
Case Study
1. (i) Which law of chemical combination is illustrated by this experiment?1 mark
2. (ii) Calculate the total mass of the products formed.1 mark
3. (iii) If the student accidentally opened the flask during the reaction, what would happen to the reading on the balance? Explain.2 marks
▸ Answer▾ Answer
The experiment illustrates the law of conservation of mass. The total mass of products is 11.3 g. Opening the flask would cause the balance reading to decrease because carbon dioxide gas escapes.

Frequently asked questions

Why is it important to use a closed system when verifying the Law of Conservation of Mass for reactions that produce a gas?

If a gas is produced and escapes into the atmosphere, the mass of the system appears to decrease, violating the law. Using a closed container (like a conical flask with a balloon or a stoppered flask) traps the gas, ensuring no mass escapes. The total mass before and after the reaction remains constant, correctly demonstrating the law.

How do I apply the Law of Constant Proportions to find the mass of a compound formed?

The Law of Constant Proportions states that a chemical compound always contains its constituent elements in a fixed mass ratio. If you know the combining masses (e.g., 2.0 g of X with 3.0 g of Y to form 5.0 g of compound), you can use this ratio to calculate how much compound forms from any mass of X or Y. Simply multiply the given mass by the compound-to-element mass ratio (e.g., 5.0 g X × (5.0 g compound / 2.0 g X) = 12.5 g compound).

Why does a common salt solution conduct electricity but a sugar solution does not?

Common salt (NaCl) dissociates into sodium and chloride ions in water. These charged ions are free to move and carry electric current. Sugar, however, dissolves as whole molecules without forming ions. Without mobile charges, the solution cannot conduct electricity. This distinction helps students understand ionic versus covalent bonding at the foundational level.

How do I write the chemical formula for compounds like aluminium sulphate?

Write the symbols of the ions: aluminium ion is Al³⁺, sulphate ion is SO₄²⁻. Using the criss-cross method, the charge of aluminium becomes the subscript for sulphate, and the charge of sulphate becomes the subscript for aluminium. Thus, Al₂(SO₄)₃. The parentheses are used because sulphate is a polyatomic ion, and the subscript 3 applies to the whole group.

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