Specific latent heat

Picture a graph of temperature against time as ice at −20 °C is heated steadily:

1. The line slopes up as the ice warms to 0 °C.
2. It goes flat at 0 °C while the ice melts.
3. It slopes up again as the water warms to 100 °C.
4. It goes flat at 100 °C while the water boils.
5. It slopes up once more as the steam heats up.

On the flat parts, the substance is changing state. Energy is still going in, but it breaks the bonds between particles instead of making them move faster, so the temperature doesn't change.

A cooling curve is the same shape the other way round. The flat parts are where the substance condenses and freezes, giving out energy.

The specific latent heat of a substance is the energy needed to change the state of 1 kg of it with no change in temperature.

• Specific latent heat of fusion: for changing between solid and liquid.
• Specific latent heat of vaporisation: for changing between liquid and gas.

This equation is on the equation sheet:

energy for a change of state = mass × specific latent heat, or E = mL

For water, the latent heat of fusion is 334 000 J/kg and of vaporisation is 2 260 000 J/kg. Boiling needs far more energy because the particles must be pulled completely apart.

Example 1: how much energy does it take to melt 0.5 kg of ice at 0 °C? (L of fusion = 334 000 J/kg)

1. E = mL
2. E = 0.5 × 334 000
3. E = 167 000 J (167 kJ)

Example 2: 452 000 J boils away some water at 100 °C. What mass of water boiled? (L of vaporisation = 2 260 000 J/kg)

1. Rearrange: m = E ÷ L
2. m = 452 000 ÷ 2 260 000
3. m = 0.2 kg