Learning outcomes
By the end of this lesson, students should be able to:
- describe melting, solidification, boiling and condensation in energy terms
- explain constant-temperature state changes using particle potential energy
- state the melting and boiling temperatures of water at standard atmospheric pressure
- interpret heating and cooling curves
- describe latent heat qualitatively without confusing it with temperature rise
7.1 Heating within one state
When a solid, liquid or gas is heated without changing state, the average kinetic energy of its particles increases and temperature rises. The exact rate of temperature rise depends on mass, specific heat capacity and power input, as well as energy losses.
On a heating curve produced by a constant-power heater, a sloping section represents temperature rise within one state. A steeper gradient can indicate a lower total heat capacity, although the graph must be interpreted carefully if heat losses vary.
7.2 Melting and solidification
At the melting temperature, continued energy input does not necessarily raise temperature. Instead, energy weakens the structure and increases the potential energy associated with particle separation and arrangement. Solid and liquid can coexist while melting occurs.
During solidification, energy is transferred from the substance as particles form a more ordered structure. Temperature remains at the freezing temperature until the change is complete under constant pressure for a pure substance. For pure water at standard atmospheric pressure, the melting and freezing temperature is 0 °C.
Figure 15. Original KG2UNI diagram.
7.3 Boiling and condensation
Boiling occurs when vapour bubbles form throughout the liquid. At the boiling temperature, supplied energy separates particles into the gas state rather than increasing average kinetic energy, so temperature stays constant while boiling continues. For water at standard atmospheric pressure, the boiling temperature is 100 °C.
Boiling temperature depends on pressure. At lower atmospheric pressure, water boils below 100 °C. In a pressure cooker the pressure is higher, so water boils above 100 °C and food cooks faster. Unless the question states otherwise, the syllabus value 100 °C assumes standard atmospheric pressure.
Condensation is the reverse change. Gas particles lose energy, move closer and form a liquid. Energy is released to the surroundings. Steam can cause severe burns because it transfers energy both while cooling and while condensing.
7.4 Latent heat
Latent heat is the energy required to change the state of a substance without changing its temperature. The word latent means hidden: energy enters or leaves, but the thermometer may show no temperature change. The energy changes particle potential energy and arrangement rather than average kinetic energy.
The current 5054 syllabus requires a qualitative description of latent heat and its particle explanation. The equation Q = mL may be useful enrichment, but students should not replace the required explanation with an equation when the question asks about particle behaviour.
Figure 16. Original KG2UNI diagram.
7.5 Heating and cooling curves
Flat sections of a heating curve correspond to state changes for a pure substance at constant pressure. The length of a flat section depends on the power and the latent energy required. A longer plateau does not mean the substance is “stuck”; energy is still being transferred.
On a cooling curve, plateaus occur during condensation or solidification because energy is released while temperature remains constant. Supercooling can occur experimentally, so real graphs may not be perfectly ideal.
Worked examples
Reading a heating curve
A flat section at 0 °C while energy is supplied indicates melting. The average kinetic energy remains approximately constant, while particle potential energy increases.
Pressure cooker
Higher pressure raises the boiling temperature of water. Food is therefore heated at a temperature above 100 °C, increasing the cooking rate.
Condensing steam
Steam at 100 °C can transfer more energy than water at 100 °C because steam also releases latent energy as it condenses.
Practical focus
Investigation
Heat crushed ice gently while stirring and record temperature at regular intervals until the water is well above 0 °C. Plot temperature against time. Use a low-power heater and eye protection. The region near 0 °C should be much less steep while melting occurs, although heat loss and impurities may prevent a perfectly flat line.
Examination guidance
- When temperature is constant during a state change, say energy changes potential energy and separation.
- State 0 °C and 100 °C for pure water only at standard atmospheric pressure.
- Do not describe boiling as only surface escape; that is evaporation.
Check your understanding
- Why can temperature remain constant while a substance is melting?
- State the melting and boiling temperatures of water at standard atmospheric pressure.
- Where does boiling occur in a liquid?
- What energy change occurs during condensation?
- What is latent heat?
Answers
- Supplied energy increases particle potential energy and changes arrangement rather than average kinetic energy.
- 0 °C and 100 °C.
- Throughout the liquid, where vapour bubbles form.
- Energy is transferred from the substance to the surroundings.
- Energy required to change state without a temperature change.
