Heating and cooling curves and latent heatIB MYP Physics: Revision notes

A heating curve is a graph of temperature against time as a substance is heated at a steady rate.

For ice heated from below 0 °C:

• the temperature rises as the solid warms
• it stays constant at 0 °C (the melting point) while the ice melts
• it rises again as the liquid water warms
• it stays constant at 100 °C (the boiling point) while the water boils
• if heating continues, the steam's temperature rises again.

In the sloping sections the energy increases the kinetic energy of the particles, so the temperature rises.

During a flat section the substance is changing state. The energy supplied is used to overcome the forces between the particles and change their arrangement, not to make them move faster. So the kinetic energy and the temperature stay constant.

This energy is called latent heat. The specific latent heat is the energy needed to change the state of 1 kg of a substance without a change in temperature.

At MYP level you can use Q = m L, where Q is the energy in J, m is the mass in kg and L is the specific latent heat in J/kg.

Example: melting 0.50 kg of ice, L = 334 000 J/kg: Q = 0.50 × 334 000 = 167 000 J.

A cooling curve shows temperature against time as a substance loses energy.

For steam cooling down:

• temperature falls until condensing at 100 °C, where it stays flat
• temperature falls as the liquid cools
• it stays flat at 0 °C while the water freezes, then falls as the ice cools further.

On flat sections the particles are getting closer together, and energy is released to the surroundings as new forces form between them. The latent heat is released, not absorbed.

Evaporation is a liquid changing to a gas at the surface, at any temperature below the boiling point. The fastest particles escape. This leaves the remaining particles with lower average kinetic energy, so evaporation has a cooling effect (it is how sweat cools you).

Boiling happens throughout the liquid at a fixed temperature, the boiling point, when bubbles of vapour form inside the liquid. It needs a continuous energy supply.

Condensation is a gas changing to a liquid. When water vapour touches a cold surface, its particles lose energy, slow down and the forces between them pull them together. This is why a cold glass gets covered in droplets, and why windows mist up. Condensation releases energy to the surroundings.

Evaporation is faster when:

• the temperature is higher (more particles have enough energy to escape)
• the surface area is larger (more particles are at the surface)
• there is air movement, e.g. wind or a fan (carries vapour away so fewer particles return)
• the air is drier, with lower humidity (less vapour already in the air)

This is why washing dries quickly on a hot, dry, windy day.