Physics · Year 11 · Thermodynamics and Matter · Spring Term

Latent Heat of Fusion and Vaporisation

Students define and calculate latent heat, understanding the energy changes during melting, freezing, boiling, and condensation.

• National Curriculum Attainment Targets
• GCSE: Physics - Particle Model of Matter
• GCSE: Physics - Temperature and Changes of State

About This Topic

Latent heat of fusion is the thermal energy required to change one kilogram of a solid into a liquid at its melting point, with no temperature change. Latent heat of vaporisation works the same way for changing a liquid into a gas at its boiling point. Students calculate these values using the equation Q = mL, where Q is energy transferred, m is mass, and L is the specific latent heat. They explain why temperature stays constant during phase changes: the energy overcomes forces between particles rather than increasing kinetic energy.

This topic fits within the GCSE Physics Particle Model of Matter and builds on prior work with specific heat capacity. Students compare fusion and vaporisation values, noting vaporisation typically requires more energy due to greater separation of particles in gases. Graphing heating and cooling curves reinforces these ideas, while calculations develop quantitative skills essential for thermodynamics.

Active learning suits this topic well. Practical experiments, such as timing the melting of ice or boiling water while monitoring temperature, make the plateaus visible and memorable. Students then use their data for calculations, connecting observation to equations and correcting misconceptions through discussion.

Key Questions

1. Explain why temperature remains constant during a change of state.
2. Analyze the energy required for different phase transitions.
3. Compare the latent heat of fusion and vaporisation for a given substance.

Learning Objectives

• Calculate the energy required to change the state of a substance using the specific latent heat.
• Explain the energy transfer occurring at the particle level during melting, freezing, boiling, and condensation.
• Compare the specific latent heat of fusion and vaporisation for common substances.
• Analyze heating and cooling curves to identify phase transitions and calculate latent heat values.

Before You Start

Specific Heat Capacity

Why: Students need to understand that energy is required to change the temperature of a substance before they can grasp the concept of energy required for a phase change without temperature change.

States of Matter and Particle Theory

Why: A foundational understanding of how particles are arranged and move in solids, liquids, and gases is essential for explaining energy changes during phase transitions.

Key Vocabulary

Latent Heat	The energy absorbed or released by a substance during a change in its physical state, such as melting or vaporisation, without changing its temperature.
Specific Latent Heat of Fusion	The amount of energy required to change 1 kilogram of a substance from solid to liquid at its melting point, or vice versa, at constant temperature.
Specific Latent Heat of Vaporisation	The amount of energy required to change 1 kilogram of a substance from liquid to gas at its boiling point, or vice versa, at constant temperature.
Phase Transition	The physical process where a substance changes from one state (solid, liquid, gas) to another, occurring at a specific temperature and pressure.

Watch Out for These Misconceptions

Common Misconception

Temperature keeps rising during melting or boiling.

What to Teach Instead

Energy transfers to break particle bonds instead of raising kinetic energy. Hands-on heating curve experiments let students plot real data, spotting flat plateaus themselves. Class discussions then link graphs to particle model explanations.

Common Misconception

Latent heat of fusion equals latent heat of vaporisation.

What to Teach Instead

Vaporisation needs more energy since gas particles separate further than in liquids. Paired calculations from lab data for both processes reveal the difference clearly. Groups compare values to build accurate comparisons.

Common Misconception

Latent heat only applies to cooling processes.

What to Teach Instead

It works for both melting/boiling (absorbed) and freezing/condensing (released). Demonstrations of both directions in labs help students see symmetry. Recording energy signs in calculations reinforces this balance.

Active Learning Ideas

Demonstration: Heating Curve of Water

Start with ice in a beaker on a heater. Record temperature every minute until boiling, then plot a graph as a class. Pause at plateaus to discuss energy use. Extend by calculating latent heats from mass and time data.

35 min·Whole Class

Pairs Lab: Ice Melting Calorimeter

Pairs add known hot water mass to measured ice in an insulated cup. Record final equilibrium temperature and melted ice mass. Calculate latent heat of fusion using Q_lost = Q_gained equation. Compare results across pairs.

45 min·Pairs

Small Groups: Vaporisation Timing

Groups boil a fixed water mass and time the plateau phase with stopwatch. Measure energy input from power and time. Compute specific latent heat of vaporisation. Graph class data to compare with fusion values.

50 min·Small Groups

Individual: Curve Graphing Challenge

Provide temp-time data sets for different substances. Students graph curves, identify plateaus, and calculate latent heats from given masses and times. Peer review graphs for accuracy.

25 min·Individual

Real-World Connections

• Refrigeration and air conditioning systems rely on the latent heat of vaporisation of refrigerants. These fluids absorb heat from the inside of a space as they evaporate, cooling the air, and then release heat to the outside as they condense.
• Steam burns can be severe because steam at 100°C carries significantly more energy than water at 100°C due to the large latent heat of vaporisation released when steam condenses on skin.
• Geothermal power plants utilize the latent heat of underground water and steam to drive turbines and generate electricity, a process dependent on phase changes deep within the Earth.

Assessment Ideas

Quick Check

Provide students with a heating curve for water. Ask them to identify the sections representing solid, liquid, and gas phases, and the plateaus corresponding to melting and boiling. Then, ask them to calculate the energy needed to melt 0.5 kg of ice at 0°C, given Lf = 334,000 J/kg.

Discussion Prompt

Pose the question: 'Why does a puddle of water disappear on a warm day even if the temperature is below boiling point?' Guide students to discuss evaporation, energy absorption from the surroundings, and the role of latent heat in overcoming intermolecular forces.

Exit Ticket

Students are given the specific latent heat of fusion for aluminum (96.7 kJ/kg) and the specific latent heat of vaporisation (2340 kJ/kg). Ask them to write one sentence explaining which process requires more energy per kilogram and why, relating it to particle separation.

Frequently Asked Questions

What is the difference between latent heat of fusion and vaporisation?

Fusion changes solid to liquid at melting point; vaporisation changes liquid to gas at boiling point. Vaporisation values are larger, around 2260 kJ/kg for water versus 334 kJ/kg for fusion, due to greater intermolecular forces overcome. Students analyse data tables or lab results to compare these directly.

Why does temperature stay constant during a change of state?

Added heat increases potential energy to separate particles, not kinetic energy for temperature rise. This matches GCSE particle model requirements. Experiments graphing temp-time curves provide evidence, as plateaus appear when phase change occurs.

How do you calculate latent heat in phase changes?

Use Q = mL, where Q is energy (J), m is mass (kg), L is specific latent heat (J/kg). For example, melting 0.1 kg ice needs 33,400 J if L_fusion = 334,000 J/kg. Practice with varied data builds fluency for exam questions.

How can active learning help students grasp latent heat?

Labs like melting ice or boiling timed water let students observe temperature plateaus firsthand, making abstract energy transfers concrete. Group calculations from their data connect observation to Q = mL equation. Discussions of results address misconceptions, while graphing reinforces patterns across class datasets for deeper retention.