Practice Model of Water - 3.2.3 Changes of Heat and Specific Latent Heat (GCSE Physics AQA)

Changes of Heat and Specific Latent Heat

Changes of State

• Latent heat is needed to change states. When we want to change the state of a substance, we have to use energy called latent heat. This latent heat is used to describe the energy transferred into or out of the substance.

Below, we can see the energy changes between the states. This is the same diagram that we have seen previously. All  of these state changes will require latent heat.

Specific Latent Heat

Defining Specific Latent Heat

For AQA exams, you are expected to know the definition of specific latent heat:

The specific latent heat of a substance is the amount of energy required to change the state of one kilogram of the substance with no change in temperature.

Specific Latent Heat vs. Specific Heat Capacity

Although the definition of specific heat capacity seems very similar to the definition of specific latent heat, there is one key difference.

• Specific heat = energy needed to change one kilogram of a material by 1°C.
• Latent heat = energy needed to change the state (gas, liquid, solid) of one kilogram of material.

Types of Specific Latent Heat

There are two types of specific latent heat:

The specific latent heat of fusion is the energy when we convert from a solid to a liquid, or from a liquid to a solid.

The specific latent heat of vaporisation is the energy when we convert from a gas to a liquid, or from a liquid to a gas.

Calculating Specific Latent Heat

We can calculate specific latent heat by rearranging the following formula:

Where:

• energy, E, in joules, J
• mass, m, in kilograms, kg
• specific latent heat, L, in joules per kilogram, J/kg

Make sure you understand this equation rather than just memorising it. Specific latent heat is the energy needed per kg for a state change. So to work out the energy needed for a state change, we just need to multiply specific latent heat (energy per kg) by the number of kgs.

Question: Calculate the energy transferred to the surroundings as 0.40 kg of stearic acid changed state from liquid to solid. The specific latent heat of fusion of stearic acid is 199 000 J / kg.

1. Write out the equation.

E =mL

2. Substitute in the numbers.

E = 0.40 x 199 000
E = 79 600 J

Heating and Cooling Graph

For AQA Exams, you must be able to interpret heating and cooling graphs.

Heating Graph

We are going to discuss the heating graph step by step:

1. Solid Heating – as temperature and time increases, the solid is starting to heat up.
2. Melting – once the solid reaches its melting point (in this case it is 0 degrees), it starts to melt and turn into a liquid.
3. Liquid Heating – as temperature and time increases, the liquid is starting to heat up.
4. Boiling – once the liquid reaches its boiling point (in this case, it is 100 degrees), it starts to evaporate and turn into a gas.

The straight lines on the graph are very important. These are points where the substance is being heated, but instead of increasing in temperature, the substance is changing state.

Cooling Graph

We are going to discuss the cooling graph step by step:

1. Gas cooling – the gas is being cooled down as time goes on.
2. Gas to liquid – once the gas reaches its condensation point, the gas condenses into a liquid.
3. Liquid cooling – the liquid is being cooled down as time goes on.
4. Liquid to solid – once the liquid reaches its freezing point, the liquid freezes into a solid.

Again, the straight lines on the graph are very important. These are points where the substance is being cooled, but instead of decreasing in temperature, the substance is changing state.