Lesson T2.2.2

T2.2.2 Conservation of mass Quiz: KS3 Chemistry, Unit 2

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson T2.2.2, Conservation of mass: 20 multiple choice questions for the KS3 Chemistry (National Curriculum), Unit 2: Atoms, elements and compounds, written with Revision Ninja.

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The 20 questions

  1. What is the law of conservation of mass?

    • The mass of products is always greater than the mass of reactants because new atoms are made
    • In a closed system the total mass of reactants equals the total mass of products
    • Mass changes only when a reaction is heated, and it stays the same when it is cooled
    • Mass is destroyed when a gas is produced, so the products weigh less than the reactants

  2. A piece of magnesium ribbon is burned in air and the white powder weighs more. Why?

    • Magnesium atoms were created during the burning, which added new mass to the powder
    • Magnetic force pulled extra particles into the powder from the air around the dish
    • Oxygen from the air combined with the magnesium, adding mass to the product
    • The powder absorbed energy from the flame and this energy made it heavier as a solid

  3. Why does a wooden log lose mass when it is burned in an open fire?

    • The ash contains all the mass of the wood but it is too light to be weighed
    • The wood turns into pure energy with no products, so nothing is left to weigh
    • Some products such as carbon dioxide and water vapour escape into the air
    • The wood atoms are destroyed completely, so no atoms are left in the ash at all

  4. A closed flask holds 5 g of reactant that forms a product with no gas escaping. What is the mass of the product?

    • 5 g
    • More than 5 g
    • Less than 5 g
    • 0 g

  5. A student reacts 10 g of A with 8 g of B in a sealed flask. What is the total mass of the products?

    • 20 g
    • 18 g
    • 8 g
    • 10 g

  6. Why must a reaction be done in a closed container to show conservation of mass?

    • So that the reaction can produce more atoms during the time it takes to run
    • So that no gas can escape or enter and change the measured mass
    • So that the temperature stays exactly the same throughout the entire reaction
    • So that the reaction can be seen clearly by the eye of the person weighing it

  7. Which reaction would appear to lose mass in an open container?

    • Melting ice in a sealed bag, where the water stays inside the bag throughout
    • Dissolving sugar in water, which makes the sugar disappear into the liquid completely
    • Burning a candle, because carbon dioxide and water vapour escape into the air
    • Mixing two solutions in a closed flask, where all the mixture stays in the flask

  8. Which statement about atoms in a chemical reaction is correct?

    • Some atoms vanish as the reaction heats up and they cannot be found afterwards
    • The same atoms are present before and after, just arranged differently
    • Atoms of one element change into another element during the course of the reaction
    • New atoms are made during the reaction and they join the products as they form

  9. In an open beaker, the mass goes down after a reaction that releases gas. What explains this?

    • Mass was destroyed in the reaction, which is why the beaker now weighs less
    • The reaction stopped halfway through, so only part of the mass is left in the beaker
    • The gas has escaped to the air, so it is no longer in the beaker
    • The gas has become energy and left the beaker as heat rising into the room

  10. A student weighs a sealed bag holding reactants totalling 25 g. The reaction happens inside and the bag is weighed again. What should the reading be?

    • 26 g
    • 25 g
    • 24 g
    • 0 g

  11. Which measurement best shows conservation of mass in a reaction that produces a gas?

    • Weigh only the solid product in an open dish after the reaction has finished
    • Weigh the whole sealed container before and after the reaction
    • Measure the temperature before and after the reaction and compare the two readings
    • Count the bubbles of gas produced by the reaction in a timed one minute period

  12. Why does an iron nail gain mass as it rusts?

    • Iron gains mass from the water vapour that evaporates from the wet nail surface
    • Iron atoms split and become heavier when they are left in damp air for a long time
    • The nail absorbs energy from the sunlight and this energy makes the nail heavier
    • Iron combines with oxygen from the air to form iron oxide, which has more mass

  13. A student says mass is created when a gas is made. Which correction is best?

    • Mass is created only at high temperatures when the reactants are heated strongly
    • Mass is conserved; the gas atoms come from the reactants
    • Mass is created by the energy transferred to the gas as it is formed in the flask
    • Mass is created when the gas dissolves in water and becomes part of the liquid

  14. In a sealed flask 12 g of reactants give 9 g of solid product and a gas. What is the mass of gas formed?

    • 21 g
    • 12 g
    • 3 g
    • 9 g

  15. Why does conservation of mass support the idea that atoms are rearranged, not destroyed?

    • If atoms were destroyed the products would always be coloured in a bright shade
    • If atoms were destroyed the total mass after reaction would change
    • If atoms were destroyed the reaction would always be faster and need no energy
    • If atoms were destroyed the temperature would always fall as the reaction runs

  16. Which change shows no overall change of mass in a closed system?

    • Melting ice in a sealed bag
    • Burning magnesium in an open dish
    • Rusting iron in an open tray
    • Evaporating water from an open beaker

  17. In a closed container, iron and sulfur react and the total mass is 20 g before the reaction. What is it after?

    • 20 g
    • More than 20 g
    • It depends on the colour of the product
    • Less than 20 g

  18. Hydrogen peroxide decomposes into water and oxygen gas in an open flask, and the flask mass falls. What does this show?

    • Mass has been destroyed during decomposition, which is why the flask weighs less
    • The oxygen has escaped, so the mass in the flask is less
    • The decomposition released no atoms at all, so the flask should keep its mass
    • Mass has been created as the water formed, so the flask should gain weight

  19. A sealed reaction starts with a total mass of 50 g. Which statement is correct?

    • The mass is 50 g only if the reaction releases heat energy into the surroundings
    • The mass is always less than 50 g because some mass is lost as energy is given off
    • The mass is 50 g after reaction, as long as nothing enters or leaves
    • The mass is always greater than 50 g because the products are heavier than the reactants

  20. A student heats 4 g of copper carbonate in an open tube until the mass falls. What is the best explanation?

    • Copper carbonate turned into energy alone
    • The black solid absorbed extra mass from the air
    • Carbon dioxide gas escaped from the open tube
    • Copper atoms were destroyed by the heat

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