Phase change, latent heat and the triple point of water
During melting or boiling, heat goes into changing the state, not the temperature. At the triple point (273.16 K, about 611 Pa) ice, water and vapour coexist.
Phases and latent heat
Ice, water and steam are three phases of water: solid, liquid and gas. Heat a solid and its temperature rises to its melting point. While it melts, you keep adding heat, but the temperature stays the same. The heat goes into freeing the molecules from their fixed places, not into speeding them up. This hidden heat is called latent heat. Latent heat of fusion is the heat that changes unit mass of a solid into liquid at its melting point. Latent heat of vaporisation is the heat that changes unit mass of a liquid into vapour at its boiling point. For a mass m, Q = mL.
Heating ice to steam
On a graph of temperature against time, the two flat parts are the melting and boiling stages.
How to draw the phase diagram of water
A **phase diagram** is a graph of pressure P against temperature T that shows which phase is stable.
- 1Draw axes: pressure P up the side and temperature T along the bottom.
- 2Mark a point near the lower left and label it triple point (273.16 K, 611 Pa).
- 3From it, draw a curve rising to the right. Label it vaporisation curve (steam line): it shows how the boiling point changes with pressure.
- 4From it, draw a nearly upright line leaning slightly to the left. Label it fusion curve (ice line): the melting point of ice falls a little as pressure rises.
- 5From it, draw a short curve down to the left. Label it sublimation curve (hoarfrost line): here ice turns straight into vapour.
- 6Write ice in the region on the left, water in the upper middle and steam (vapour) on the lower right.
The triple point of water
The three curves meet at one point, the triple point. Ice, water and vapour coexist in equilibrium only at one pair of values: 273.16 K (0.01 °C) and about 611 Pa. The triple point is exact and easy to reproduce. So it is the fixed point used to define the kelvin scale of temperature. Raising the pressure lowers the melting point of ice and raises the boiling point of water.
Worked example: heat to turn ice into steam
How much heat is needed to change 100 g of ice at 0 °C into steam at 100 °C? (Latent heat of fusion of ice = 3.35 × 10⁵ J kg⁻¹, specific heat of water = 4.2 × 10³ J kg⁻¹ K⁻¹, latent heat of vaporisation of water = 2.26 × 10⁶ J kg⁻¹)
- m = 100 g = 0.100 kg.
- Melting the ice: Q₁ = mL = 0.100 × 3.35 × 10⁵ J = 33500 J.
- Warming the water from 0 °C to 100 °C: Q₂ = mcΔT = 0.100 × 4.2 × 10³ × 100 J = 42000 J.
- Boiling the water: Q₃ = mL = 0.100 × 2.26 × 10⁶ J = 226000 J.
- Total Q = 33500 J + 42000 J + 226000 J = 301500 J.
About 3.0 × 10⁵ J. Three quarters of it goes into boiling, which is why steam carries so much heat
Steam burns worse than boiling water, because it gives out its large latent heat as it condenses on the skin. Food cooks faster in a pressure cooker because the higher pressure raises the boiling point of water above 100 °C.
2 marks: “What is the triple point of water? Why is it used as the fixed point of the kelvin scale?” It is the single temperature and pressure (273.16 K, about 611 Pa) at which ice, water and vapour coexist. It is exact and easy to reproduce, so it makes a reliable fixed point.
2 marks: “How much heat is needed to melt 2.0 kg of ice at 0 °C? (L = 3.35 × 10⁵ J kg⁻¹)” Q = mL = 2.0 kg × 3.35 × 10⁵ J kg⁻¹ = 6.7 × 10⁵ J.
1 marks: “Why does the temperature of melting ice stay constant even though heat is supplied?” The heat (latent heat) is used to change the state by freeing the molecules, not to raise their kinetic energy.
Latent means hidden: heat goes in, but the thermometer stands still. Triple point: three phases at 273.16 K.