National 5PhysicsSQARevision NotesProperties Of MatterSpecific Latent HeatCalculating Specific Latent Heat

Calculating Specific Latent Heat (SQA National 5 Physics): Revision Note

Exam code: X857 75

Leander Oates

Written by: Leander Oates

Reviewed by: Katie M

Updated on

Calculating specific latent heat

  • Specific latent heat is defined as:

The amount of energy required to change the state of 1 kg of a substance with no change in temperature

  • The specific latent heat of a substance can be calculated using the following relationship:

E subscript h space equals space m l

  • Where:

    • E subscript h = heat energy required for a change in state, in joules (J)

    • m = mass, in kilograms (kg)

    • l = specific latent heat, in joules per kilogram (J kg-1)

  • This relationship can be rearranged with the help of a triangle

Heat energy, mass, and Specific Latent Heat relationship triangle
Specific latent heat relationship triangle

  • The same relationship is used for both the specific latent heat of fusion and the specific latent heat of vaporisation

  • However, the numerical values of l are very different, so be sure to use the correct values from the data sheet in calculations

  • For example, the values of latent heat for water are:

    • Specific latent heat of fusion = 3.34 cross times 10 to the power of 5 space straight J space kg to the power of negative 1 end exponent

    • Specific latent heat of vaporisation = 22.6 cross times 10 to the power of 5 space straight J space kg to the power of negative 1 end exponent

  • Therefore, evaporating 1 kg of water requires roughly seven times more energy than melting the same amount of ice to form water

Worked Example

Water ice melts and freezes at its melting point.

Specific latent heat of fusion of water = 3.34 cross times 10 to the power of 5 space straight J space kg to the power of negative 1 end exponent

Specific latent heat of vaporisation of 22.6 cross times 10 to the power of 5 space straight J space kg to the power of negative 1 end exponent

(i) Calculate the minimum amount of energy required to melt 1.3 kg of water ice at its melting point.

(ii) State the amount of heat energy given out as 1.3 kg of water freezes into ice at its melting point.

Answer (i):

Step 1: Write down the relevant relationship

E subscript h space equals space m l

Step 2: Substitute in the known values to calculate

  • Melting ice is changing state from a solid to a liquid

  • Therefore, the value for specific latent heat of fusion is used

E subscript h space equals space 1.3 space cross times space open parentheses 3.34 cross times 10 to the power of 5 close parentheses

E subscript h space equals space 4.342 cross times 10 to the power of 5 space straight J

Step 3: Round to an appropriate number of significant figures

  • The least precise data point is 2 s.f. (1.3 kg)

  • Therefore, the answer must be given to the same precision

E subscript h space equals space 4.3 cross times 10 to the power of 5 space straight J space open parentheses 2 space straight s. straight f. close parentheses

Answer (ii):

  • If 4.3 cross times 10 to the power of 5 space straight J of heat energy is required to melt 1.3 kg of ice into liquid water, then the same amount of heat energy is given out when 1.3 kg of liquid water freezes into ice at its melting point

  • Therefore, 4.3 cross times 10 to the power of 5 space straight J is given out

Examiner Tips and Tricks

Both the relationship and the latent heat values will be provided to you in the exam. You do not need to remember any of the specific values.

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    Author
    Reviewer
    Leander Oates

    Author: Leander Oates

    Expertise: Physics Content Creator

    Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.

    Katie M

    Reviewer: Katie M

    Expertise: Physics Content Creator

    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.