Heat transfer to changes of state
A heating curve is a graph showing the temperature of a substance plotted against the amount of energy it has absorbed. You may also see a cooling curve, which is obtained when a substance cools down and changes state.
The temperature stays the same when a solid is melting or a liquid is boiling during a change of state, even though heat energy is being absorbed.
The temperature also stays the same while a liquid freezes, even though Heat energy is the product of the movement of particles such as, atoms, ions and molecules. It is also known as thermal energy. is still being released to the surroundings.
The specific latent heat of a substance is a measure of how much heat energy is needed to melt or boil it. It is the energy needed to melt or boil 1 kg of the substance.
Different substances have different specific latent heats. The specific latent heat of a given substance is different for boiling than it is for melting. Here are some examples.
| Substance | Melting (kJ/kg) | Boiling (kJ/kg) |
| Water | 334 | 2260 |
| Lead | 23 | 871 |
| Oxygen | 13.9 | 213 |
| Substance | Water |
|---|---|
| Melting (kJ/kg) | 334 |
| Boiling (kJ/kg) | 2260 |
| Substance | Lead |
|---|---|
| Melting (kJ/kg) | 23 |
| Boiling (kJ/kg) | 871 |
| Substance | Oxygen |
|---|---|
| Melting (kJ/kg) | 13.9 |
| Boiling (kJ/kg) | 213 |
Investigating latent heat
The latent heat of water can be determined like this.
- An electrical heater is used to melt ice for several minutes.
- The electrical energy used is calculated by multiplying the power of the heater by time.
- The mass of water melted is recorded and the relationship between energy input and The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). is calculated.
- The amount of heat needed to change one kilogram of ice (the latent heat) is then determined.
Here is the equation that relates energy to specific latent heat.
Q = m × L
You will probably find your values close, but not identical, to those in the table above. Differences between your value and those in the table are likely to result from energy loss to the room you worked in.
The latent heat of steam is determined using a calorimeter and steam generator.
The latent heat of melting is sometimes called the latent heat of fusion, while the latent heat of boiling is sometimes called the latent heat of vaporisation.