The specific latent heat of a substance is the amount of energy required…

The idea

Specific latent heat is a property of a material that tells you how much energy is needed to change the state of one kilogram of it without changing its temperature. It is measured in joules per kilogram (J/kg). For example, the specific latent heat of fusion of ice is 334 000 J/kg, meaning 334 000 J are needed to melt 1 kg of ice at 0°C into water at 0°C. The specific latent heat of vaporisation of water is 2 260 000 J/kg. To calculate the energy required for a given mass, use E = m × L, where E is energy in joules, m is mass in kilograms and L is specific latent heat in J/kg. This applies to both melting/freezing and boiling/condensing, using the appropriate latent heat value.

What earns the marks

• Specific latent heat is defined as the energy required to change the state of one kilogram of a substance with no temperature change.
• The unit of specific latent heat is joules per kilogram (J/kg).
• The energy transferred during a change of state is calculated using E = m × L.
• Different substances have different specific latent heat values, and the value depends on the change of state (fusion or vaporisation).
• For melting and freezing, the specific latent heat of fusion is used; for boiling and condensing, the specific latent heat of vaporisation is used.
• The temperature remains constant during the change of state, so the energy supplied does not increase the average kinetic energy of the particles.

Common mistakes and how to put them right

• Using the wrong unit for specific latent heat, such as J or J/kg°C.

  Put it right by stating it is J/kg because it is energy per kilogram at constant temperature.

• Forgetting to convert mass from grams to kilograms before using E = m × L.

  Put it right by dividing grams by 1000 to obtain kilograms.

• Mixing up specific latent heat with specific heat capacity.

  Put it right by noting that specific heat capacity involves a temperature change and uses J/kg°C, while specific latent heat involves no temperature change and uses J/kg.

Exam tips

• Always write the equation E = m × L and substitute values with correct units, converting grams to kilograms first.
• When asked to define specific latent heat, include 'one kilogram', 'change of state' and 'no change in temperature'.
• If a question gives a heating curve, use the flat section to identify the specific latent heat value from the energy supplied and mass.