Year 8 Science · Unit 3 · Lesson 8

Conservation of Mass in Changes

Challenge Worksheet

Name

Date

Class

Learning Goals

Evaluate a common misconception about burning and mass loss using particle-level reasoning.
Distinguish between open and closed systems and explain how each affects observable mass.
Construct a multi-part argument that addresses a specific counter-example.

Go Deeper

Evaluate the claim

Someone claims...

"When wood burns in a campfire, the ash left behind weighs a tiny fraction of the original log — sometimes less than 5% of the original mass. This is obvious proof that mass was destroyed during the burning process. The law of conservation of mass must be wrong, or at least doesn't apply to combustion reactions."

(a) What part of this claim is consistent with what you observe when wood burns? (You do NOT need to agree with the conclusion — just identify the accurate observation.)

Challenge 2 marks

(b) What is the flaw in the claim's reasoning? Use the ideas of open systems, gas production, and the particle model to explain where the "missing" mass actually went.

Challenge 3 marks

(c) Describe an experiment you could design that would demonstrate conservation of mass during combustion. What would you measure, and what system type would you need?

Challenge 3 marks

Show What You Know

1. When magnesium ribbon burns in air, the white powder produced (magnesium oxide) weighs MORE than the original ribbon. Does this contradict conservation of mass? Explain carefully using both the particle model and the idea of open systems.

Challenge 3 marks

2. At ANSTO (Australian Nuclear Science and Technology Organisation), scientists work with reactions where tiny amounts of mass convert to energy (as described by Einstein's famous equation). Does this disprove Lavoisier's law? Explain why conservation of mass is still a useful and valid law for chemistry students to apply.

Challenge 3 marks

Wrap Up

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