Year 8 Science · Unit 3 · Lesson 3
Challenge Worksheet
Learning Goals
Evaluate the claim
A Year 8 student claims…
"Boiling water is definitely a physical change because the water molecules themselves don't change — H₂O stays H₂O whether it's liquid or gas. So the classification is obvious and doesn't need any more explanation. There's really nothing complex about it."
(a) What part of this claim is scientifically supported? Cite the specific particle-model evidence that backs up the student's core point.
(b) What is misleading or incomplete about the claim? What important ideas does the student leave out that a complete answer would include?
(c) What evidence or scenario would you use to show a sceptic that boiling is definitely not a chemical change, even though it produces bubbles and a dramatic visible transformation?
1. A scuba diver at the Great Barrier Reef compresses air (a gas mixture) into a metal cylinder before diving. Using the particle model, explain why gases can be compressed into a much smaller volume but the metal cylinder itself cannot be compressed. What does this tell us about the space between particles in each state?
2. The heating curve for water has two flat plateaus (at 0°C and 100°C). A student says these plateaus mean "the heater broke." Write a particle-model explanation for why the temperature stays constant during these plateaus, even though energy is still being added to the system.
Wrap Up
In one sentence, what was the main idea of this lesson?