Conservation of Mass — Drills · Grade 8 Science

Chapter: Chemistry

What these drills covers

By the end of Conservation of Mass you should be able to:

8 questions, numeric, multiple choice and written answer, each with a worked explanation.

Vocabulary used: law of conservation of mass, closed system, model, evidence, system boundary, assumption.

Problem 1

In a sealed container, 15 g of reactant A combines fully with 25 g of reactant B. What is the total mass of products, in grams?
Answer and explanation

40 g

15 + 25 = 40 g. Mass is conserved in a chemical reaction.

Problem 2

Conservation of mass holds because during a reaction atoms are
  1. created
  2. destroyed
  3. rearranged, not created or destroyed
  4. converted to energy
Answer and explanation

rearranged, not created or destroyed

The same atoms are present before and after, bonded differently.

Problem 3

A candle and the air around it are sealed in a jar and weighed. The candle burns for a minute. By how many grams does the total mass change?
Answer and explanation

0 g

Nothing enters or leaves a sealed jar, so the total mass is unchanged even though the candle itself is lighter.

Problem 4

An open flask of 50.0 g of reactants loses 3.2 g of gas to the air during a reaction. What mass remains in the flask, in grams?
Answer and explanation

46.8 g

50.0 − 3.2 = 46.8 g. The missing mass left as gas; it was not destroyed.

Problem 5

Burning wood in an open fire appears to lose mass because
  1. atoms are destroyed
  2. gases and ash carry mass away from the pile
  3. mass becomes heat
  4. the wood was never massive
Answer and explanation

gases and ash carry mass away from the pile

Carbon dioxide, water vapour and smoke leave the fire; weighing everything including the gases would show no loss.

Problem 6

Iron wool is heated in open air and its mass rises from 4.0 g to 5.6 g. What mass of oxygen combined with it, in grams?
Answer and explanation

1.6 g

5.6 − 4.0 = 1.6 g of oxygen from the air joined the iron.

Problem 7

Explain why a reaction that appears to gain mass and one that appears to lose mass are both consistent with conservation of mass.
Answer and explanation

Both are open systems where part of the matter is not being weighed. Burning wood loses mass from the pile because gases escape into the air; heating iron wool gains mass because oxygen from the air joins the solid. In each case the total mass of everything involved, including the gases, is unchanged — only the amount sitting on the balance has shifted.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 8

Describe how you would demonstrate conservation of mass with a reaction that produces a gas.
Answer and explanation

Carry it out in a sealed container on a balance. Weigh the container with both reactants inside before mixing, mix them without opening it, and weigh again afterwards. The reading should be the same within the balance's precision. Opening the container afterwards and reweighing shows the mass fall as the gas escapes, which demonstrates where the apparent loss in an open reaction comes from.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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