Changing states
Matter can change from one state to another when it gains or loses energy. Ice turns to water, water turns to steam. These changes are physical changes: the substance is still the same substance.
The particle model
| State | Particles | Energy |
|---|---|---|
| Solid | Packed tightly in fixed positions; vibrate in place | Lowest |
| Liquid | Close together but can slide past each other | Medium |
| Gas | Far apart, moving fast in all directions | Highest |
Heating gives particles more energy, so they move faster. Cooling takes energy away, so they slow down.
Melting
Melting is the change from solid to liquid. As a solid is heated, its particles vibrate more and more. At the melting point, they vibrate so much they break free of their fixed positions and start to slide past each other.
| Substance | Melting point |
|---|---|
| Ice (water) | 0 °C |
| Chocolate | About 30–35 °C (that’s why it melts in your mouth!) |
| Candle wax | About 50–70 °C |
| Lead | 327 °C |
| Iron | 1538 °C |
| Tungsten | 3422 °C (highest of any metal) |
Freezing is the opposite of melting. For a pure substance, the freezing point and the melting point are the same temperature.
Boiling
Boiling is the rapid change from liquid to gas. At the boiling point, particles have enough energy to escape from all through the liquid, forming bubbles of gas.
| Substance | Boiling point (at sea level) |
|---|---|
| Liquid nitrogen | −196 °C |
| Rubbing alcohol | 82 °C |
| Water | 100 °C |
| Cooking oil | Above 300 °C (it smokes and burns before boiling) |
Boiling point and air pressure
Boiling point depends on air pressure. High in the mountains, the air pressure is lower, so water boils at a lower temperature. On top of Mount Everest, water boils at only about 70 °C, which makes it hard to cook pasta! A pressure cooker does the opposite: it raises the pressure so water boils above 100 °C, cooking food faster.
Evaporation vs. boiling
| Evaporation | Boiling |
|---|---|
| Happens at any temperature | Happens only at the boiling point |
| Only at the surface | Throughout the liquid |
| Slow | Fast |
| Cools the liquid left behind | Temperature stays constant |
Sweating cools you because evaporating sweat carries heat away from your skin.
The heating curve
If you heat ice from −20 °C until it becomes steam, the temperature doesn’t rise steadily.
| Stage | What happens to temperature |
|---|---|
| Ice warms | Rises from −20 °C to 0 °C |
| Ice melts | Stays at 0 °C until all ice is melted |
| Water warms | Rises from 0 °C to 100 °C |
| Water boils | Stays at 100 °C until all water is gas |
| Steam warms | Rises above 100 °C |
During melting and boiling, the energy goes into breaking particles apart, not into making them move faster. This hidden energy is called latent heat. That’s why a glass of ice water stays at about 0 °C until the ice is gone.
Other changes of state
| Change | From → To | Example |
|---|---|---|
| Condensation | Gas → liquid | Water drops on a cold glass |
| Sublimation | Solid → gas | Dry ice making fog |
| Deposition | Gas → solid | Frost forming on a window |
Impurities change melting and boiling points
Adding salt to water lowers its freezing point and raises its boiling point. That’s why salt melts ice on roads in winter (it works down to about −10 °C). Pure substances have sharp melting points, so scientists use melting points to test purity.
Common mistakes
- Thinking bubbles in boiling water are air. They’re water vapour.
- Thinking temperature keeps rising while ice melts.
- Thinking steam is visible. The white cloud is tiny water droplets; real steam is invisible.
Try it yourself
- Heat ice in a pot (with an adult) and record the temperature every 30 seconds until the water boils. Graph your results.
- Put one ice cube on a plate with salt and one without. Which melts faster?
- Wet two paper towels; hang one in the sun and one in the shade. Which dries first?
Quick recap
- Melting turns solids into liquids; freezing does the reverse.
- Boiling happens throughout a liquid at its boiling point; evaporation happens at the surface.
- Boiling points change with air pressure.
- Temperature stays the same during a change of state because of latent heat.
- Impurities like salt change melting and boiling points.