School › States of matter › Ages 10–11
States of matter and the particle model
Solids, liquids and gases explained with particles, the names of the changes of state, the boiling and melting points of water, and why ice floats. With a melting fair test.
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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.[2][1]
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.[3]
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.[3]
Water is unusual. When it freezes, it expands and becomes about 9% less dense, which is why ice floats on water. Liquid water is densest at about 4 °C.[5]
There is a fourth state too, plasma: a very hot gas full of electrically charged particles. Stars and lightning are made of plasma.[1]
Try it: Which melts first?
You need
- Four identical small plates or foil cases
- Four solids of the same size: an ice cube, a square of chocolate, a cube of butter, a piece of a wax candle
- A tray of warm tap water (not hot: about 40 °C, checked by the teacher) to float the cases on
- A stopwatch
Safety
Use only warm tap water (about 40 °C), never boiling water. No flames: do not light the candle. Don't eat the chocolate or butter afterwards.
What to do
- Predict the order in which the four solids will melt.
- Put each solid in its own case and float the cases on the warm water at the same moment. Start the stopwatch.
- Watch carefully. Write down the time when each one has completely melted (or after 20 minutes: how much has melted).
- Keep everything the same except the material: same size, same water, same starting time.
- Explain your results using the particle model.
What happens, and why?
Heat from the warm water gives the particles more energy, so they vibrate harder until they break out of their fixed positions and the solid melts. Each material has its own melting point. Ice melts at 0 °C; a material whose melting point is higher than the temperature of the warm water will not melt at all, however long you wait.[2][3]
Play: Name that change
What change of state is happening? Melting, freezing, evaporation or condensation?
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Quiz
Other ages: Ages 6–7 · Ages 15–16