Specific Latent Heat
Brook Edgar & Hannah Shuter
Teachers
Explainer Video
Specific Latent Heat
When a substance melts from a solid to a liquid, energy is required to overcome the forces of attraction holding its particles in fixed positions. The amount of energy required depends on both the type of substance and its mass. The energy required to change the state of the substance is called the specific latent heat.
It’s important to remember that as this energy is used to overcome the forces of attraction holding the particles close together, allowing the particles to move further apart, the energy increases the potential energy of the particles. The particles' kinetic energy does not change because there is no change in temperature. Changes in state occur at fixed temperatures -> the change in state of ice to liquid water occurs at .
The energy absorbed to overcome the forces of attraction during melting is exactly the same as the energy released when those forces of attraction re-form during freezing. For example, this means the energy required to change of aluminium from a solid to a liquid is the same as the energy released when the aluminium is cooled and turns back from a liquid to a solid.
The energy required to change the state of of a substance without changing its temperature is called the specific latent heat.
Formula:
There are two different types of specific latent heat; the specific latent heat of fusion and the specific latent heat of vaporisation.
Fusion - change of state from solid liquid
Vaporisation - change of state from liquid gas (vapour)
We can use this equation to calculate the amount of energy required to change state of of aluminium from solid to liquid. The specific latent heat of fusion of aluminium is .
Note: if asked how much energy would be released when of liquid aluminium is cooled and turned back into a solid, it would be the same amount - . The energy required to change state from solid to liquid is the same as the energy released when a liquid changes back to a solid.
Worked Example:
of ice cubes are used to cool a drink. Calculate the amount of energy absorbed by the ice to melt completely.
The latent heat of fusion of ice is .
Answer:
First we must convert by dividing by , so becomes . Then we use the equation:
Worked Example:
A chocolate bar of mass is left in the sun and absorbs of thermal energy each second.
It takes minutes to melt completely. Estimate the specific latent heat of fusion of chocolate.
Answer:
We must first work out the total amount of energy transferred to the chocolate bar. We need to find out what minutes are in seconds:
As of energy is supplied each second, over the total energy supplied is:
Next, we need to convert by dividing by :
Calculating the specific latent heat of chocolate:
Practice Questions
A student melts of ice at . The specific latent heat of fusion of water is .
Define specific latent heat of fusion.
Calculate the energy needed to melt the ice
-> Check out Hannah's video explanation for more help.
The amount of energy required to change the state of of a substance from a solid to a liquid with a change in temperature.
A kettle is used to boil water at . It supplies of energy to of water. The specific latent heat of vaporisation of water is .
Calculate the mass of water that turns to steam.
Explain why the temperature of the boiling water does not rise above even though energy is still being supplied.
-> Check out Hannah's video explanation for more help.
Energy is used to overcome attractive forces between particles and increase potential energy. Since kinetic energy does not increase, temperature stays constant.