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Everything is made of tiny particles. In a solid they are packed tightly, often in a regular pattern, and only vibrate on the spot, so a solid keeps its shape. In a liquid they are still close together but can slide past each other, so a liquid flows and takes the shape of its container while keeping the same volume. In a gas they are far apart and move freely, so a gas spreads out to fill its container.

When a substance changes from one state to another, we give the change a name: solid to liquid is melting, liquid to solid is freezing, liquid to gas is evaporation (or boiling), and gas to liquid is condensation. Some solids turn straight into a gas: this is sublimation. Dry ice (frozen carbon dioxide) does this, vanishing without ever becoming a liquid.

Water melts at 0 °C and, at normal air pressure, boils at 100 °C. While it is melting or boiling, its temperature stays the same: the energy goes into pulling the particles apart instead of making them hotter.

Experiment: Which melts first?

1. Predict the order in which the four solids will melt.
2. Put each solid in its own case and float the cases on the warm water at the same moment. Start the stopwatch.
3. Watch carefully. Write down the time when each one has completely melted (or after 20 minutes: how much has melted).
4. Keep everything the same except the material: same size, same water, same starting time.
5. Explain your results using the particle model.

Solid	Predicted order	Time to melt (min:s)	Fully melted? (yes/no)
Ice
Chocolate
Butter
Candle wax

Questions

1. Draw the particles in a solid, a liquid and a gas.

2. Name the change: liquid to gas, and gas to liquid.

3. Why does the temperature of melting ice stay at 0 °C until it has all melted?

4. Which solid melted first in your test, and which not at all? Explain.

Sources for every fact on this sheet: echoesof.science/school/states-of-matter/sources/

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