Heat and Temperature: Specific Heat, Latent Heat and Calorimetry
This section covers the distinction between heat and temperature, specific heat capacity, latent heat and simple calorimetry.
Heat and temperature
Heat is energy transferred because of a temperature difference, measured in joules. Temperature measures the average kinetic energy of the particles and is measured in °C or K. Two bodies in contact reach thermal equilibrium when their temperatures are equal, and then heat flow stops.
Specific and latent heat
Q = mcΔT defines specific heat capacity c. For water c ≈ 4200 J kg⁻¹ K⁻¹, which is unusually high, so water is a good coolant and coastal climates stay moderate. During a change of state the temperature remains constant while heat Q = mL is supplied: latent heat of fusion of ice = 3.34 × 10⁵ J/kg and latent heat of vaporisation of water = 2.26 × 10⁶ J/kg. In calorimetry, heat lost by the hot body equals heat gained by the cold body when losses to the surroundings are negligible.
Worked example
How much heat converts 100 g of ice at 0 °C to water at 0 °C? The ice melts at constant temperature: Q = mL = 0.1 × 3.34 × 10⁵ = 3.34 × 10⁴ J. No mcΔT term appears, because the temperature does not change during melting. Warming the resulting water to 20 °C would then need a further 0.1 × 4200 × 20 = 8400 J.
Traps that cost marks
- Using Q = mcΔT across a phase change.
- Mixing grams with kilograms.
- Forgetting that a temperature difference in °C equals one in K, though absolute values differ.
- Ignoring heat absorbed by the calorimeter or lost to the surroundings.