Energy for a change of state = mass × specific latent heat

The idea

When a substance melts, freezes, boils or condenses, its temperature stays constant even though energy is being transferred. That energy does not raise the average kinetic energy of the particles; instead it breaks or forms the intermolecular bonds that hold the particles in a particular arrangement. The energy needed per kilogram is the specific latent heat, L, and it differs for each material and for each change. Melting or boiling uses the specific latent heat of fusion or vaporisation respectively; freezing and condensing release the same amount per kilogram. To calculate, multiply the mass in kilograms by the specific latent heat in joules per kilogram.

For example: Melting 0.50 kg of ice (L = 3.34 × 10⁵ J/kg) needs 0.50 × 3.34 × 10⁵ = 1.67 × 10⁵ J.

What earns the marks

• State that temperature remains constant during a change of state because energy is used to overcome intermolecular forces rather than to increase kinetic energy.
• Identify the correct latent heat: fusion for solid–liquid changes and vaporisation for liquid–gas changes.
• Convert mass to kilograms before substituting into the equation, for example 250 g becomes 0.250 kg.
• Substitute values correctly into energy = mass × specific latent heat and evaluate, keeping powers of ten consistent.
• Recognise that freezing and condensing release the same energy per kilogram as the reverse change absorbs.
• Use the correct unit, the joule (J), and show that kg × J/kg gives J.

Common mistakes and how to put them right

• Using the mass in grams instead of kilograms.

  Put it right by dividing the mass by 1000 first.

• Using specific heat capacity instead of specific latent heat.

  Put it right by checking that the question involves a change of state at constant temperature.

• Assuming the temperature rises while a substance is melting.

  Put it right by identifying the flat section as the change of state where energy breaks bonds.

Exam tips

• Underline the words melt, freeze, boil or condense to decide whether fusion or vaporisation applies.
• Write the equation, then substitute values with units before calculating so method marks are visible.
• Check the size and unit of your answer: latent heat energies are often in the range 10⁴–10⁶ J for laboratory masses.