Changes of state and specific latent heat
Changing state
Jonny Nelson introduces an animated explanation of latent heat
Adding or removing The capacity for doing work. from a material can change the state. Heating a solid material will cause it to The process that occurs when a solid turns into a liquid when it is heated. from a solid to a liquid. Continued heating will cause the liquid to boil or When liquid is vaporised and turns to a gas. 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 When a solid turns straight into a gas on heating, without becoming a liquid first - or when a gas turns straight into a solid, without becoming a liquid..
Cooling a gas will cause it to Condensation is a change of state in which gas becomes liquid by cooling. from a gas to a liquid and cooling it further will cause it to then A change of state in which liquid becomes solid by cooling. from a liquid to a solid.
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.
The process in which a liquid changes state and turns into a gas. on the other hand is a The opposite of active. Quiet and submissive. process. The liquid will slowly absorb energy from the surrounding area so that some of its particles will gain enough energy to escape the liquid.
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.
Specific latent heat
Changing the The total kinetic energy and potential energy of the particles in an object. of a material will cause it to change temperature or change state:
- the energy required for a particular change in temperature is given by the The amount of energy needed to raise the temperature of 1 kg of substance by 1°C.
- the energy required for a particular change in state is given by the The amount of energy needed to melt or vaporise 1 kg at its melting or boiling point..
As there are two boundaries, solid/liquid and liquid/gas, each material has two specific latent heats:
- latent heat of fusion - the amount of energy needed to The process that occurs when a solid turns into a liquid when it is heated. the material at its melting point
- latent heat of vaporisation - the amount of energy needed to When liquid is vaporised and turns to a gas. the material at its boiling point
Some typical values for specific latent heat include:
| Substance | Specific latent heat of fusion (J/kg) | Specific latent heat of vaporisation (J/kg) |
| Water | 334,000 | 2,260,000 |
| Lead | 2,400 | 855,000 |
| Oxygen | 13,900 | 213,000 |
| Substance | Water |
|---|---|
| Specific latent heat of fusion (J/kg) | 334,000 |
| Specific latent heat of vaporisation (J/kg) | 2,260,000 |
| Substance | Lead |
|---|---|
| Specific latent heat of fusion (J/kg) | 2,400 |
| Specific latent heat of vaporisation (J/kg) | 855,000 |
| Substance | Oxygen |
|---|---|
| Specific latent heat of fusion (J/kg) | 13,900 |
| Specific latent heat of vaporisation (J/kg) | 213,000 |
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.
Calculating thermal energy changes
The amount of A more formal term for heat 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
\(\Delta E_t = m \times l\)
This is when:
- energy to cause a change of state (\(\Delta E_t\)) is measured in joules (J)
- mass (m) is measured in kilograms (kg)
- specific latent heat (l) is measured in joules per kilogram (J/kg)
Question
How much energy is needed to freeze 500 grams (g) of water from 0°C?
\(E_t = m~l\)
\(E_t = 0.5 \times 334,000\)
\(E_t = 167,000~J\)
Measuring latent heat
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 A line with zero slope. 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.
Example
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?
- 63 minutes = 3,780 s
- 60 W means 60 J of energy is supplied every second
- energy = power × time
- energy = 60 × 3,780
- energy = 226,800 J
Example 2
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