Specific Latent Heat

Latent heat

• The energy needed for a substance to change state is called latent heat
• During a change of state the energy supplied changes the internal energy (the potential energy of the particles) but not the temperature
• The specific latent heat is the energy required to change the state of one kilogram of the substance with no change in temperature (on the equation sheet):

$$E = m L$$

• $E$ energy in J; $m$ mass in kg; $L$ specific latent heat in joules per kilogram, J/kg
• Specific latent heat of fusion: solid to liquid (and liquid to solid). For water, 334 000 J/kg
• Specific latent heat of vaporisation: liquid to vapour (and back). For water, 2 260 000 J/kg
• Worked example: energy to melt 0.2 kg of ice at 0 °C: $E = 0.2 \times 334\,000 = 66\,800$ J