Specific latent heat of fusion – change of state from solid to liquid

The idea

The specific latent heat of fusion is the energy required to change 1 kg of a solid into a liquid at its melting point, without any change in temperature. It is a specific case of latent heat, measured in joules per kilogram, J/kg. For example, melting 2.0 kg of ice at 0 °C requires energy E = mL, where L is the specific latent heat of fusion of ice. If L = 334 000 J/kg, then E = 2.0 × 334 000 = 668 000 J. During melting, energy is transferred to the ice to break the forces holding the particles in a fixed lattice, allowing them to move more freely as a liquid. The temperature stays at 0 °C until all the ice has melted. The same amount of energy is released when the liquid freezes back to a solid.

What earns the marks

• Define specific latent heat of fusion as the energy needed to change 1 kg of a substance from solid to liquid at its melting point, with no temperature change.
• Use the equation E = mL with L as the specific latent heat of fusion.
• Describe the change from solid to liquid in terms of particles gaining energy to overcome forces between them.
• State that the temperature remains constant during melting and that the energy supplied does not increase the kinetic energy of the particles.
• Recognise that the same value of L applies to freezing, where energy is released.

Common mistakes and how to put them right

• Thinking that the temperature increases while a solid melts.

  Put it right: The temperature stays constant at the melting point until all the solid has changed to liquid.

• Using the specific latent heat of vaporisation instead of fusion.

  Put it right: Fusion is for solid to liquid (or liquid to solid); vaporisation is for liquid to gas (or gas to liquid).

• Forgetting to convert mass to kilograms.

  Put it right: L is in J/kg, so mass must be in kilograms before using E = mL.

Exam tips

• When asked to calculate energy for melting, identify the mass in kilograms and the specific latent heat of fusion for the substance.
• In explanations, link the constant temperature to energy being used to break intermolecular forces, not to increase particle speed.
• If a question gives energy and asks for mass, rearrange E = mL to m = E/L and substitute carefully.