# 18.1.3 Specific Latent Heat Capacity

### Defining Latent Heat Capacity

• Energy is required to change the state of substance
• Examples of changes of state are:
• Melting = solid to liquid
• Evaporation/vaporisation/boiling = liquid to gas
• Sublimation = solid to gas
• Freezing = liquid to solid
• Condensation = gas to liquid
• When a substance changes state, there is no temperature change
• The energy supplied to change the state is called the latent heat and is defined as:

The thermal energy required to change the state of 1 kg of mass of a substance without any change of temperature

• There are two types of latent heat:
• Specific latent heat of fusion (melting)
• Specific latent heat of vaporisation (boiling)
• The specific latent heat of fusion is defined as:

The thermal energy required to convert 1 kg of solid to liquid with no change in temperature

• This is used when melting a solid or freezing a liquid
• The specific latent heat of vaporisation is defined as:

The thermal energy required to convert 1 kg of liquid to gas with no change in temperature

• This is used when vaporising a liquid or condensing a gas

#### Calculating Specific Latent Heat

• The amount of energy Q required to melt or vaporise a mass of m with latent heat L is:

Q = mL

• Where:
• Q = amount of thermal energy to change the state (J)
• m = mass of the substance changing state (kg)
• L = latent heat of fusion or vaporisation (J kg-1)
• The values of latent heat for water are:
• Specific latent heat of fusion = 330 kJ kg-1
• Specific latent heat of vaporisation = 2.26 MJ kg-1
• Therefore, evaporating 1 kg of water requires roughly seven times more energy than melting the same amount of ice to form water
• The reason for this is to do with intermolecular forces:
• When ice melts: energy is required to just increase the molecule separation until they can flow freely over each other
• When water boils: energy is required to completely separate the molecules until there are no longer forces of attraction between the molecules, hence this requires much more energy

Step 3:            Substitute in values

m = 530 g = 530 × 10-3 kg

Q = 0.6 MJ = 0.6 × 106 J L is the latent heat of vaporisation because the change in state is from liquid to gas (boiling)

#### Exam Tip

Use these reminders to help you remember which type of latent heat is being referred to:

• Latent heat of fusion = imagine ‘fusing’ the liquid molecules together to become a solid
• Latent heat of vaporisation = “water vapour” is steam, so imagine vaporising the liquid molecules into a gas ### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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