Year 8 Science · Unit 3 · Lesson 8

Conservation of Mass in Changes

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Learning Goals

Sequence the steps of Lavoisier's sealed flask experiment.
Interpret bar chart data comparing mass before and after reactions in open and closed systems.
Explain apparent mass loss or gain in terms of gas exchange with the surroundings.

Warm Up

Order the steps

Number the events from 1 to 7 to show the correct order of Lavoisier's sealed flask experiment demonstrating conservation of mass. Event 1 = what happens first.

Order	Event
	The flask is unsealed and the contents are examined — the same substances are present in rearranged form.
	A chemical reaction occurs inside the sealed flask (e.g. mercury oxide decomposes when heated).
	The flask and all its contents are placed on a balance and the total mass is recorded.
	The reactants are placed inside a flask, which is then sealed so no matter can enter or leave.
	Lavoisier concludes: mass is conserved — the total did not change during the reaction.
	The sealed flask is placed on a balance again and the total mass after the reaction is recorded.
	The mass before and the mass after are compared — they are identical.

Your Turn

Read the bar chart

The bar chart below shows the mass before and after four different reactions. Study the chart carefully, then answer the questions.

Data: simulated values based on standard Year 8 demonstration experiments.

(a) Which two reactions show an apparent decrease in mass after the reaction? Are these open or closed systems?

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(b) For the open burning reaction: the wood weighed approximately 100 g before, but only 8 g of ash remained after. Explain where the remaining ~92 g of mass went.

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Wrap Up

In one sentence, what was the main idea of this lesson?