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GCSE Physics Revision

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GCSE Physics revision

Changes of state and specific latent heat

Internal energy and energy transfers

AQA 4.3.2.3
Your specification

AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Interpret heating and cooling graphs that .5 include changes of state.

MOST 🎯🎯

Apply the specified scientific knowledge of changes of state and specific latent heat to a relevant example.

SOME 🎯🎯🎯

Analyse an unfamiliar example of changes of state and specific latent heat using the specified scientific ideas.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

Revisiting Change of State

  • Matter can change state from solid to liquid (melting) and from liquid to gas (boiling) when energy is supplied.
  • A change of state graph shows temperature against energy supplied, with flat sections where state changes occur.
  • During a change of state, energy is being transferred to the substance but the temperature does not rise.
  • What is Latent Heat?
  • Latent heat is the energy supplied to a substance that changes its state without changing its temperature.
  • The flat sections on a change of state graph represent where latent heat is being absorbed or released.
  • There are two flat sections on the graph: one for solid to liquid, and one for liquid to gas.

Defining Specific Latent Heat

  • Specific latent heat is the amount of energy needed to change the state of one kilogram of a substance with no change in temperature.
  • In an exam, a full definition must include both key parts: changing state and no change in temperature, which could be worth two marks.
  • Specific latent heat is a property of the material itself, not just the amount of it.

Specific Latent Heat of Fusion

  • The specific latent heat of fusion refers to the energy needed to change one kilogram of a substance from solid to liquid (or vice versa).
  • This corresponds to the lower flat section on the change of state graph, where melting or freezing occurs.
  • Fusion is associated with the solid-liquid boundary, for example when ice melts into water.

Specific Latent Heat of Vaporisation

  • The specific latent heat of vaporisation refers to the energy needed to change one kilogram of a substance from liquid to gas (or vice versa).
  • This corresponds to the upper flat section on the change of state graph, where boiling or condensing occurs.
  • Vaporisation is associated with the liquid-gas boundary, for example when water boils into steam.

The Specific Latent Heat Equation

  • The equation linking energy, mass, and specific latent heat is E = mL, where E is energy in joules (J), m is mass in kilograms (kg), and L is specific latent heat in joules per kilogram (J/kg).
  • This equation does not need to be memorised for GCSE as it is provided on the equation sheet in every exam paper.
  • A formula triangle can be used to rearrange the equation: L = E / m or m = E / L.

Worked Example: Specific Latent Heat of Fusion

  • To find the energy released when 0.5 kg of stearic acid freezes, use E = mL with m = 0.5 kg and L = 199,000 J/kg.
  • Substituting the values gives E = 0.5 × 199{,}000 = 79{,}500 J, which can also be written as 79.5 kJ.
  • This is an example of the specific latent heat of fusion because the substance is changing from liquid to solid.

Worked Example: Specific Latent Heat of Vaporisation

  • To find the specific latent heat of vaporisation of water, rearrange the equation to L = E / m.
  • Always convert energy values into joules before substituting into the equation, e.g.
  • 41 kJ becomes 41,000 J.
  • Substituting gives L = frac{41{,}000}{0.018} approx 2{,}300{,}000 J/kg when rounded to two significant figures.

Significant Figures and Units

  • When rounding to two significant figures, identify the first two non-zero digits and look at the next digit to decide whether to round up or down.
  • For example, 2,277,777.78 J/kg rounded to two significant figures becomes 2,300,000 J/kg (or 2.3 × 10⁶ J/kg).
  • Always include the correct unit for specific latent heat, which is joules per kilogram (J/kg), especially when exam questions specifically ask for it.

Key Takeaways on Specific Latent Heat

  • Specific latent heat explains why temperature stays constant during a change of state even though energy is continuously being supplied.
  • There are two types: specific latent heat of fusion (solid liquid) and specific latent heat of vaporisation (liquid gas).
  • The equation E = mL can be used to calculate energy, mass, or specific latent heat depending on which quantities are given.