Year 8 Science · Unit 3 · Lesson 3
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Learning Goals
Compare three states
Complete the table to compare how particles behave in each state of matter. Use what you know from the particle model.
| Feature | Solid | Liquid | Gas |
|---|---|---|---|
| Particle spacing | |||
| Type of particle movement | |||
| Does the substance keep its shape? | |||
| Can it be compressed easily? | |||
| Relative kinetic energy |
(a) When ice melts into liquid water, what happens to the particles? Describe the change in arrangement, spacing, and movement. Is this a physical or chemical change? Explain.
(b) Dry ice (solid CO₂) sublimes at room temperature — it turns from solid directly to gas. Using the particle model, explain what must happen to the particles for sublimation to occur.
Scenario
On a hot summer day at Bondi Beach, a swimmer emerges from the water and stands in the sun. Even though the air temperature is 35°C, the swimmer's skin feels cool as the water on their body evaporates. A friend watching says: "That doesn't make sense — if you're already hot, how can evaporation cool you down?"
(a) Predict: what does evaporation of water from skin require, and where does that energy come from?
(b) Justify your answer using the particle model. Explain which particles escape and why this leaves the skin cooler, not warmer.
1. Use the particle model to explain why you can compress (squash) a gas into a smaller container but you cannot compress a solid by the same amount.
2. A student says: "When I heat a metal rod, the particles get bigger — that's why the rod expands." Identify the error and write the correct particle-model explanation for why metals expand when heated.
Wrap Up
In one sentence, what was the main idea of this lesson?