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Latent heat is the energy needed for a substance to change state without changing temperature, measured in joules (J). Free GCSE Physics notes and 14 FAQs.
Changes of State and Latent Heat is part of Internal energy and energy transfers in GCSE AQA Higher Physics.
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Latent heat is the energy needed for a substance to change state without changing temperature, measured in joules (J). Free GCSE Physics notes and 14 FAQs.
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Use this lesson to stay inside Internal energy and energy transfers while connecting the detail back to the wider Particle model of matter topic.
Read the core explanations from Changes of State and Latent Heat before testing yourself from memory.
Picture this: you heat ice cubes in a pan. The temperature rises steadily until it hits 0°C, then something strange happens. The ice keeps melting, but the thermometer stays stuck at 0°C. Where's all that energy going?
This puzzle reveals a crucial concept in physics. When substances change state, for example from solid to liquid or liquid to gas, the energy transferred does not disappear. It is used to overcome intermolecular forces between particles rather than to increase their average kinetic energy. That is why the temperature stays constant during the state change even though energy is still being supplied.
There are two specific types you need to know, each with different energy requirements:
Notice vaporisation needs much more energy than fusion. Breaking all the bonds between liquid particles (to make gas) requires more energy than just loosening them (to make liquid).
When you know the mass and the specific latent heat, you can calculate the total energy needed:
This equation works for both fusion and vaporisation - simply use the appropriate value of L for your process.
Worked example: Calculating Energy to Melt Ice
Calculate the energy needed to melt 2.5 kg of ice at 0°C. Specific latent heat of fusion for ice = 334,000 J/kg. [3 marks]
Answer: 835,000 J (or 8.35 × 10⁵ J)
Students often confuse these two concepts. Here's the key difference:
| Property | Specific Heat Capacity | Specific Latent Heat |
|---|---|---|
| What it measures | Energy to raise temperature | Energy to change state |
| Temperature change | Yes - temperature rises | No - temperature constant |
| Units | J/kg°C | J/kg |
| When used | Heating without state change | Melting, boiling, condensing |
Memory trick: Specific heat capacity has °C in the units because temperature changes. Latent heat doesn't because temperature stays constant.
Understanding latent heat explains many everyday phenomena:
The high specific latent heat of vaporisation for water (2.26 MJ/kg) makes sweating incredibly effective at cooling. Just 1 gram of sweat evaporating removes 2260 J of energy from your body.
Latent heat questions are mark-winners if you avoid the common traps. Here's how to get full marks:
Mark-winning phrases: 'Energy is used to break intermolecular forces', 'Temperature remains constant during state change', 'Internal energy increases without temperature change'.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Specific latent heat is the amount of energy required to change the state of one kilogram of a substance with no change in temperature, measured in joules per kilogram (J/kg). Latent heat is the energy needed for a substance to change state without changing temperature.
Latent heat is the energy needed for a substance to change state without changing temperature, measured in joules (J). The energy is used to overcome forces between particles rather than to increase their average kinetic energy.
The specific latent heat of fusion of water is 334,000 J/kg. This is the energy needed to change 1 kg of ice to liquid water, or liquid water to ice, at the melting point.
Sweating cools the body down because evaporation removes large amounts of energy. Water's high specific latent heat of vaporisation (2.26 MJ/kg) means just 1 gram of sweat evaporating removes 2260 J of energy from the body.
Ice is effective for cooling drinks because melting absorbs energy from the surroundings. The energy needed to melt the ice is latent heat, taken from the drink without the ice's temperature rising while it melts.
Specific latent heat of fusion is the energy to change 1 kg of solid to liquid at the melting point (334,000 J/kg for water), while specific latent heat of vaporisation is the energy to change 1 kg of liquid to vapour at the boiling point (2,260,000 J/kg for water).
Specific heat capacity is the energy to raise temperature, measured in J/kg°C, while specific latent heat is the energy to change state with no temperature change, measured in J/kg. Specific heat capacity has °C in its units because temperature changes.
Vaporisation needs much more energy than melting because breaking all the bonds between liquid particles to make a gas requires more energy than just loosening them to make a liquid. For water, the specific latent heat of vaporisation is 2,260,000 J/kg, while for fusion it is 334,000 J/kg.
Melting 2.5 kg of ice at 0°C takes 835,000 J, or 8.35 × 10⁵ J. Using E = m × L with the specific latent heat of fusion of ice, 334,000 J/kg, E = 2.5 × 334,000 = 835,000 J.
The specific latent heat of vaporisation of water is 2,260,000 J/kg, or 2.26 MJ/kg. This is the energy needed to change 1 kg of liquid water to vapour, or vapour to liquid, at the boiling point.
Specific latent heat is measured in joules per kilogram (J/kg). It has no °C in its units, unlike specific heat capacity (J/kg°C), because the temperature stays constant during a change of state.
Freezers work harder when ice forms because energy must be removed during freezing. Changing liquid water to ice at the melting point means removing the latent heat, 334,000 J for every kilogram.
During a change of state, internal energy increases without any change in temperature. The energy is used to break intermolecular forces, changing the potential energy of the particles rather than their average kinetic energy.
The thermometer stays at 0°C while ice melts because the energy supplied is used to overcome intermolecular forces between particles, not to increase their average kinetic energy. The energy does not disappear; it is latent heat.