Specific Latent Heat

When you boil a kettle, the water reaches 100 °C and then continues to absorb energy — but the temperature stops rising. The energy is still going somewhere: it is breaking the intermolecular bonds that hold the liquid water together, converting it into steam. The energy required for this change of state is called latent heat, from the Latin word "latens" meaning hidden, because the energy is absorbed without any temperature change.

---

Definition and Equation

The specific latent heat (L) of a substance is the energy required to change the state of 1 kg of that substance without a change in temperature.

Q = mL

where:

• Q = energy transferred (J)
• m = mass of substance changing state (kg)
• L = specific latent heat (J kg⁻¹)

There are two types:

• Specific latent heat of fusion (L_f): the energy to change 1 kg of solid to liquid (or liquid to solid) at the melting point.
• Specific latent heat of vaporisation (L_v): the energy to change 1 kg of liquid to gas (or gas to liquid) at the boiling point.

---

Key Values