AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Changes of state and specific latent heat: E = m L
This is the shorthand form of the energy equation for a change of state.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 5 of 12
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This is the shorthand form of the energy equation for a change of state. E stands for the energy transferred in joules, m for the mass in kilograms and L for the specific latent heat in joules per kilogram. The equation applies only while the substance is changing state, so the temperature is constant and no temperature change term appears. Rearranged, L = E ÷ m gives the energy needed per kilogram, and m = E ÷ L gives the mass that a given energy can change. For example, if 6.0 × 10⁴ J vaporises 0.025 kg of a liquid, then L = 6.0 × 10⁴ ÷ 0.025 = 2.4 × 10⁶ J/kg. Always convert grams to kilograms and keep powers of ten consistent when substituting.
What earns the marks
- Identify each symbol correctly: E is energy in joules, m is mass in kilograms and L is specific latent heat in joules per kilogram.
- Rearrange the equation to find L or m when those are the unknowns.
- Convert masses to kilograms and express answers with the correct unit and a sensible power of ten.
- Substitute numerical values into E = m L and evaluate accurately, showing the substitution.
- Interpret L as the energy needed per kilogram for a stated change of state, not as a temperature change.
Common mistakes and how to put them right
Treating L as a temperature.
Put it right by noting L has units J/kg and no temperature change is involved.
Forgetting to convert grams to kilograms.
Put it right by dividing by 1000 before substituting.
Rearranging incorrectly, for example writing m = E L.
Put it right by using L = E ÷ m and m = E ÷ L.
Exam tips
- Write the full equation in words first, then substitute the symbols, so the meaning of each quantity is clear.
- Show the rearrangement step before numbers so an examiner can follow your method.
- Give the unit with every numerical answer and check it follows from kg × J/kg = J.
Try the lesson
The start of the Changes of state and specific latent heat: E = m L lesson, made from checked teaching notes for this specification point.
Have a go first
A block of ice melts at 0 °C. The specific latent heat of fusion of water is 334 000 J/kg. The mass of ice that melts is 0.50 kg. Use E = m L to calculate the energy transferred to melt the ice.
Listen
This is the shorthand form of the energy equation for a change of state. E stands for the energy transferred in joules, m for the mass in kilograms and L for the specific latent heat in joules per kilogram. The equation applies only while the substance is changing state, so the temperature is constant and no temperature change term appears. Rearranged, L = E ÷ m gives the energy needed per kilogram, and m = E ÷ L gives the mass that a given energy can change. For example, if 6.0 × 10⁴ J vaporises 0.025 kg of a liquid, then L = 6.0 × 10⁴ ÷ 0.025 = 2.4 × 10⁶ J/kg. Always convert grams to kilograms and keep powers of ten consistent when substituting. A strong answer earns marks for these points. Identify each symbol correctly: E is energy in joules, m is mass in kilograms and L is specific latent heat in joules per kilogram. Rearrange the equation to find L or m when those are the unknowns. Convert masses to kilograms and express answers with the correct unit and a sensible power of ten. Substitute numerical values into E = m L and evaluate accurately, showing the substitution. A common slip: Treating L as a temperature: the error confuses latent heat with specific heat capacity. Instead, noting L has units J/kg and no temperature change is involved. Exam tip: Write the full equation in words first, then substitute the symbols, so the meaning of each quantity is clear.
See it
Key words
Say what each word means before you turn the card.
E = m L
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Rearranging equations
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Unit consistency in calculations
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See every definition
- E = m L
- Shorthand form of the energy equation for a change of state, where E is energy transferred, m is mass and L is specific latent heat.
- Rearranging equations
- Writing the equation in another form, such as L = E ÷ m to give the energy needed per kilogram, or m = E ÷ L to give the mass.
- Unit consistency in calculations
- Converting grams to kilograms and keeping powers of ten consistent when substituting values into the equation.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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