Questions on Law of Conservation of Mass
Multiple-Choice Questions: Law of Conservation of Mass
1. What does the Law of Conservation of Mass state?
A) Mass can be created in a chemical reaction
B) Mass is always lost in physical changes
C) Matter can be created and destroyed
D) Mass is neither created nor destroyed in a chemical reaction
E) Energy and mass are completely unrelated
2. Who is credited with formulating the Law of Conservation of Mass?
A) John Dalton
B) Antoine Lavoisier
C) J.J. Thomson
D) Niels Bohr
E) Albert Einstein
3. In a closed system, if 50 grams of reactants are used, the total mass of products should be:
A) 25 grams
B) 50 grams
C) 100 grams
D) 75 grams
E) Depends on the reaction
4. The Law of Conservation of Mass applies to:
A) Only physical changes
B) Only nuclear reactions
C) Only open systems
D) All chemical reactions in a closed system
E) Combustion reactions only
5. If 10 g of hydrogen reacts with 80 g of oxygen, how much water is produced?
A) 80 g
B) 70 g
C) 90 g
D) 100 g
E) 60 g
6. What happens to mass during a chemical reaction in a sealed container?
A) It increases
B) It decreases
C) It remains the same
D) It depends on temperature
E) It doubles
7. Which best demonstrates the Law of Conservation of Mass?
A) Wood burning and disappearing
B) Ice melting into water
C) Water evaporating from a dish
D) Vinegar reacting with baking soda in a sealed flask
E) Gas escaping an open container
8. Why might mass appear to change during an experiment?
A) The Law of Mass is invalid
B) Atoms are destroyed
C) Gases escape in open systems
D) The scale is inaccurate
E) All of the above
9. In a reaction: A + B → AB, if A = 20g and B = 10g, what is the mass of AB?
A) 15g
B) 20g
C) 30g
D) 10g
E) 25g
10. What must be true for mass to be conserved in a reaction?
A) The reaction must be explosive
B) The system must be open
C) Products must be heavier than reactants
D) The system must be closed
E) Only liquids can be involved
11. Which process violates the Law of Conservation of Mass?
A) None; it’s a fundamental law
B) Melting of ice
C) Baking a cake
D) Iron rusting
E) Dissolving salt in water
12. If carbon and oxygen react to form carbon dioxide, what can be said about the mass?
A) It decreases
B) It increases
C) It stays the same
D) It is unpredictable
E) It evaporates
13. Which of the following is a chemical reaction where the law applies?
A) Freezing of water
B) Iron combining with sulfur to form iron sulfide
C) Water boiling
D) Dissolving sugar in tea
E) Cutting paper
14. When mass is not conserved in a lab experiment, the most likely reason is:
A) A new element was created
B) Atoms vanished
C) Mass was destroyed
D) Gas escaped
E) Lavoisier was wrong
15. Which device helps confirm the Law of Conservation of Mass in experiments?
A) Thermometer
B) Microscope
C) Balance scale
D) Bunsen burner
E) Spectroscope
16. When baking soda and vinegar react in a balloon-sealed flask, what happens to the mass?
A) It increases
B) It decreases
C) It stays the same
D) It is absorbed
E) The balloon breaks
17. If 25g of reactant produces 5g of product, what else must be true?
A) Law is invalid
B) 20g was lost
C) 20g of product was a gas
D) 5g was destroyed
E) The reaction stopped early
18. What role did Antoine Lavoisier play in chemistry?
A) Discovered electrons
B) Proposed atomic theory
C) Disproved conservation
D) Formulated the Law of Conservation of Mass
E) Invented the microscope
19. During a chemical reaction in an open beaker, mass appears to decrease. What happened?
A) Atoms split
B) Gas escaped
C) Law failed
D) Reaction ended
E) Energy was lost
20. Which of the following is an example of mass not being conserved due to experimental error?
A) Sealed system reaction
B) Reaction in a vacuum
C) Reaction with gas escape
D) Reaction in freezing water
E) Reaction in the sun
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Answer Key with Explanations
1. D – The law states that mass is conserved in chemical reactions.
2. B – Antoine Lavoisier is known as the father of modern chemistry.
3. B – Mass is conserved, so product = 50g.
4. D – The law applies to chemical reactions in closed systems.
5. C – 10g + 80g = 90g of water.
6. C – In a sealed system, no mass is lost or gained.
7. D – In a sealed system, gases can't escape, proving mass is conserved.
8. C – Escaping gases cause apparent mass loss in open systems.
9. C – Total mass of product equals sum of reactants: 20g + 10g = 30g.
10. D – Closed systems prevent mass loss through gas escape.
11. A – It’s a fundamental principle, always valid.
12. C – Mass of carbon + oxygen = mass of CO₂.
13. B – It’s a chemical change where mass is conserved.
14. D – Gas escape in open systems causes apparent mass loss.
15. C – A balance measures mass before and after reactions.
16. C – In a closed flask with a balloon, mass is conserved.
17. C – 20g must have escaped as gas to conserve mass.
18. D – He established the conservation principle.
19. B – Gas likely escaped the open beaker.
20. C – Gases can escape and cause apparent mass change.
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