Changes of state
When a substance is heated its particles gain energy. At the melting point a solid changes to a liquid, and at the boiling point a liquid changes to a gas. For a pure substance these changes happen at a fixed temperature. During melting or boiling the temperature stays constant even though energy is still being supplied, because the energy is used to break the forces between particles rather than to speed them up. This hidden energy is called latent heat.
Key idea
Energy to change state: Q = m L, where L is the specific latent heat (J/kg). For ice melting, L ≈ 3.34 × 10⁵ J/kg.
Worked example
How much energy melts 0.50 kg of ice at 0 °C? Take L = 3.34 × 10⁵ J/kg.
Q = m L = 0.50 × 334 000 = 167 000 J. The temperature stays at 0 °C during melting.
Boiling and evaporation
Both turn a liquid into a gas, but they are different:
| Boiling | Evaporation |
|---|---|
| At one fixed temperature | At any temperature below boiling |
| Throughout the liquid (bubbles) | Only at the surface |
| Fast | Slower |
Evaporation happens when the fastest particles escape from the surface. Because the more energetic particles leave, the average energy of those remaining falls, so the liquid cools down. This is why sweating cools the body.
Remember
- Evaporation is faster with higher temperature, larger surface area, and more air movement (wind); it is slower in humid air.
- Increasing the pressure raises the boiling point (as in a pressure cooker).