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Specific Latent Heat (Oxford AQA IGCSE Combined Science Double Award)

Revision Note

Ann Howell
Oxford AQA Combined Science Double Award: PhysicsOxford AQA Physics

Specific Latent Heat of Vaporisation

Extension Tier only

  • The specific latent heat of vaporisation is defined as:

The amount of energy required to change the state of one kilogram of the substance between a liquid and a gas with no change in temperature

  • This applies when vaporising a liquid or condensing a gas

  • A substance's temperature will remain constant until all of the substance has vaporised

  • For a vaporising liquid:

    • A liquid will vaporise at its boiling point

    • Its temperature will remain constant until all of the liquid substance has vaporised

    • Energy is transferred into the system

    • The latent heat of vaporisation is the amount of energy per kg needed for all the particles in the liquid to overcome the intermolecular forces of attraction holding them together in their liquid state

  • For a condensing gas:

    • A gas will condense at its boiling point

    • Its temperature will remain constant until all of the gaseous substance has condensed

    • Energy is transferred away from the system

    • The latent heat of vaporisation is the amount of energy per kg transferred away from the gas until all the particles have succumbed to the intermolecular forces of attraction holding them together in their liquid state

Specific latent heat of vaporisation equation

  • The amount of energy E required to vaporise or condense a mass of m with latent heat of vaporisation Lv is:

E space equals space m space cross times space L subscript v

  • Where:

    • E = thermal energy required for a change in state, in joules (J)

    • m = mass, in kilograms (kg)

    • Lv = specific latent heat, in joules per kilogram (J / kg)

  • The specific latent heat of vaporisation for water = 2.26 MJ/kg

Specific Latent Heat of Fusion

Extension Tier only

  • The specific latent heat of fusion of a substance is defined as:

The amount of energy required to change the state of one kilogram of the substance between a solid and a liquid with no change in temperature

  • This applies when melting a solid or freezing a liquid

  • When melting a solid:

    • A solid will melt at its melting point

    • Its temperature will remain constant until all of the solid substance has melted

    • Energy is transferred into the system

    • The latent heat of fusion is the amount of energy needed per kg for all the particles in the solid substance to overcome the intermolecular forces of attraction holding them together in their solid state

  • When freezing a liquid:

    • A liquid will freeze at its melting point

    • Its temperature will remain constant until all of the liquid substance has solidified

    • Energy is transferred away from the system

    • The latent heat of fusion is the amount of energy per kg transferred away from the liquid substance until all the particles have succumbed to the intermolecular forces of attraction that hold them together in their new solid structure

Specific latent heat of vaporisation equation

  • The amount of energy E required to melt or freeze a mass of m with a latent heat of fusion Lf is:

E space equals space m space cross times space L subscript f

  • Where:

    • E = thermal energy required for a change in state, in joules (J)

    • m = mass, in kilograms (kg)

    • Lf = specific latent heat of fusion, in joules per kilogram (J / kg)

The graph of specific latent heat of fusion and vaporisation

The graph of temperature against thermal energy supplied is a stepped line with an increasing gradient. The line is horizontal when the substance is changing state both during the latent heat of fusion and the latent heat of vaporisation
The graph shows that temperature remains constant when an object is undergoing a change of state

Worked Example

Calculate the energy transferred to the surroundings as 0.60 kg of stearic acid changes state from a liquid to a solid.

The specific latent heat of fusion of stearic acid is 199 000 J/kg.

Answer:

Step 1: List the known quantities

  • Mass, m = 0.60 kg

  • Specific latent heat of fusion, Lf = 199 000 J / kg

Step 2: Write down the relevant equation

E space equals space m space cross times space L subscript f

Step 3: Substitute in the values

E space equals space 0.60 space cross times space 199 space 000

E space equals space 119 space 400 space straight J

Examiner Tip

The specific latent heat of fusion and vaporisation values of all substances will be provided for you in the exam question, so you do not need to memorise these.

Make sure you include 'with no change in temperature' in your definition of specific latent heat to be awarded full marks.

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

But remember that the change of state can go in either direction!

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    Ann Howell

    Author: Ann Howell

    Expertise: Physics Content Creator

    Ann obtained her Maths and Physics degree from the University of Bath before completing her PGCE in Science and Maths teaching. She spent ten years teaching Maths and Physics to wonderful students from all around the world whilst living in China, Ethiopia and Nepal. Now based in beautiful Devon she is thrilled to be creating awesome Physics resources to make Physics more accessible and understandable for all students, no matter their schooling or background.

    Caroline Carroll

    Author: Caroline Carroll

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    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.