Adding or removing energy from a material can change the state. Heating a solid material will cause it to melt from a solid to a liquid. Continued heating will cause the liquid to boil or evaporate to form a gas. In some instances when heating the solid material, it can go straight to being a gas without being a liquid, this process is called sublimation.
Boiling is an active process. People actively apply energy to a liquid to turn it into a gas using a heater such as a kettle.
Throughout all of these changes the number of particles has not changed, just their spacing and arrangement. As a result the total mass has not changed. It does not matter if a substance melts, freezes, boils, evaporates, condenses or sublimates, the mass does not change.
These changes in state are called physical changes because the processes can be reversed (eg cooling instead of heating). This is different to the changes seen in a chemical reaction when the changes cannot be reversed so easily.
Changing the internal energy of a material will cause it to change temperature or change state:
As there are two boundaries, solid/liquid and liquid/gas, each material has two specific latent heats:
Some typical values for specific latent heat include:
|Substance||Specific latent heat of fusion (J/kg)||Specific latent heat of vaporisation (J/kg)|
An input of 334,000 joules (J) of energy is needed to change 1 kg of ice into 1 kg of water. The same amount of energy needs to be taken out of the liquid to freeze it.
The amount of thermal energy stored or released as the temperature of a system changes can be calculated using the equation:
energy to cause a change in state = mass × specific latent heat
This is when:
How much energy is needed to freeze 500 grams (g) of water from 0°C?
Latent heat can be measured from a heating or cooling curve line graph. If a heater of known power is used, such as a 60 W immersion heater that provides 60 J/s, the temperature of a known mass of ice can be monitored each second. This will generate a graph that looks like this.
The graph is horizontal at two places. These are the places where the energy is not being used to increase the speed of the particles, increasing temperature, but is being used to break the bonds between the particles to change the state.
The longer the horizontal line, the more energy has been used to cause the change of state. The amount of energy represented by these horizontal lines is equal to the latent heat.
If a horizontal line that shows boiling on a heating curve is one hour three minutes long, how much energy has a 60 watts (W) heater provided to the water?
If this energy had been applied to 100 g of water, what is the latent heat of vaporisation of water?
226,800 J for 100 g is equivalent to 2,268,000 J for 1 kg. The latent heat of vaporisation of water is 2,268,000 J/kg