AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Changes of state and specific latent heat: energy, E, in joules, J
Energy is a measure of the capacity to do work or transfer heat, and in the context of changes of state it is the quantity transferred when a substance melts, boils, condenses or freezes without…
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 6 of 12
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Energy is a measure of the capacity to do work or transfer heat, and in the context of changes of state it is the quantity transferred when a substance melts, boils, condenses or freezes without changing temperature. The symbol E stands for energy and the unit is the joule, J. One joule is the energy transferred when a force of one newton moves an object one metre, and it is also the energy needed to raise about 0.24 g of water by 1 °C. In specific latent heat calculations you use E = mL, where E is in joules, m is mass in kilograms and L is specific latent heat in J/kg.
For example: Melting 0.50 kg of ice with L = 3.34 × 10⁵ J/kg requires E = 0.50 × 3.34 × 10⁵ = 1.67 × 10⁵ J. Always convert kJ to J by multiplying by 1000 before substituting.
What earns the marks
- States that E represents energy and that its SI unit is the joule, J.
- Recognises that in latent heat contexts E is the energy transferred during a change of state at constant temperature.
- Uses E = mL correctly with E in joules, m in kilograms and L in J/kg.
- Converts energy values between joules and kilojoules correctly, for example 250 kJ = 2.5 × 10⁵ J.
- Interprets a calculated E as the energy needed to melt, boil, condense or freeze a stated mass.
- Checks that the final answer is given with the correct unit, J, and a sensible order of magnitude.
Common mistakes and how to put them right
Writing the unit as kJ when the calculation used J/kg and kg.
Put it right: Multiply or divide by 1000 to convert and label the answer J.
Substituting mass in grams into E = mL.
Put it right: Convert grams to kilograms by dividing by 1000 first.
Confusing energy E with specific latent heat L.
Put it right: E is the total energy in J, while L is energy per kilogram in J/kg.
Exam tips
- Write the equation E = mL, then substitute values with units before calculating.
- Convert all masses to kg and all energies to J before doing arithmetic.
- Give the final answer in joules unless the question explicitly asks for kJ, and show the unit.
Try the lesson
The start of the Changes of state and specific latent heat: energy, E, in joules, J lesson, made from checked teaching notes for this specification point.
Have a go first
A student writes down the unit used for energy, E, in the equation for specific latent heat. Which unit should the student write?
Listen
Energy is a measure of the capacity to do work or transfer heat, and in the context of changes of state it is the quantity transferred when a substance melts, boils, condenses or freezes without changing temperature. The symbol E stands for energy and the unit is the joule, J. One joule is the energy transferred when a force of one newton moves an object one metre, and it is also the energy needed to raise about 0.24 g of water by 1 °C. In specific latent heat calculations you use E = mL, where E is in joules, m is mass in kilograms and L is specific latent heat in J/kg. For example, melting 0.50 kg of ice with L = 3.34 × 10⁵ J/kg requires E = 0.50 × 3.34 × 10⁵ = 1.67 × 10⁵ J. Always convert kJ to J by multiplying by 1000 before substituting. A strong answer earns marks for these points. States that E represents energy and that its SI unit is the joule, J. Recognises that in latent heat contexts E is the energy transferred during a change of state at constant temperature. Uses E = mL correctly with E in joules, m in kilograms and L in J/kg. Converts energy values between joules and kilojoules correctly, for example 250 kJ = 2.5 × 10⁵ J. A common slip: Writing the unit as kJ when the calculation used J/kg and kg. Instead, multiply or divide by 1000 to convert and label the answer J. Exam tip: Write the equation E = mL, then substitute values with units before calculating.
See it
Key words
Say what each word means before you turn the card.
Energy
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Joule
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E = mL
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See every definition
- Energy
- A measure of the capacity to do work or transfer heat, with symbol E and the unit joule, J.
- Joule
- The energy transferred when a force of one newton moves an object one metre, and also the energy needed to raise about 0.24 g of water by 1 °C.
- E = mL
- The equation used in specific latent heat calculations, with E in joules, m in kilograms and L in J/kg.
Label the diagram
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