Melting, boiling and evaporation | IGCSE Physics Revision Notes

1. Overview

This topic explores how substances transition between solid, liquid, and gaseous states through the transfer of thermal energy. Understanding these processes is vital for explaining everything from how our bodies stay cool by sweating to the industrial processes used to generate power.

Key Definitions

Melting: The process where a solid turns into a liquid at a constant temperature.
Boiling: The process where a liquid turns into a gas at a specific temperature (the boiling point) throughout the bulk of the liquid.
Condensation: The process where a gas turns into a liquid due to a loss of thermal energy.
Solidification (Freezing): The process where a liquid turns into a solid.
Evaporation: The process where a liquid turns into a gas at the surface of the liquid, occurring at temperatures below the boiling point.
Internal Energy: The sum of the total kinetic energy (related to temperature) and total potential energy (related to state) of the particles in a system.

Core Content

Melting and Boiling (Energy vs. Temperature)

When a substance reaches its melting or boiling point, it continues to absorb energy even though its temperature does not increase.

Energy Input: The thermal energy supplied is used to break the bonds (intermolecular forces) between particles rather than increasing their kinetic energy.
Temperature Plateau: On a heating curve, these changes of state are represented by horizontal (flat) lines.

A heating curve graph showing Temperature ($^\circ\text{C}$) on the y-axis and Time or Energy Input — A heating curve graph showing Temperature ($^\circ\text{C}$) on the y-axis and T...

Water at Standard Atmospheric Pressure

You must memorize the following for pure water at sea level:

Melting Point: $0^\circ\text{C}$
Boiling Point: $100^\circ\text{C}$

Condensation and Solidification (Particle Model)

Condensation: Gas particles lose kinetic energy and move slower. As they get closer together, the attractive forces pull them into a liquid structure.
Solidification: Liquid particles lose more energy, moving so slowly that they settle into a fixed, regular lattice structure.

Evaporation

Evaporation occurs because particles in a liquid have a range of energies.

Process: Some "more-energetic" particles near the surface have enough kinetic energy to overcome the attractive forces of neighboring molecules and escape as a gas.
Cooling Effect: Because the fastest (hottest) particles escape, the average kinetic energy of the remaining particles decreases. Since temperature is a measure of average kinetic energy, the liquid cools down.

Extended Content (Extended Curriculum Only)

Differences Between Boiling and Evaporation

Feature	Boiling	Evaporation
Temperature	Occurs only at the fixed boiling point.	Occurs at any temperature below boiling.
Location	Happens throughout the entire liquid (bubbles form).	Happens only at the surface.
Speed	A fast process.	A slow process.
External Energy	Requires an external heat source.	Can occur using the internal energy of the liquid.

Factors Affecting the Rate of Evaporation

The rate of evaporation increases when:

Temperature increases: More particles have the required kinetic energy to escape.
Surface Area increases: More particles are positioned at the surface, providing more "exit points."
Air Movement (Wind) increases: Escaped vapor is blown away, preventing particles from falling back into the liquid and allowing more to escape.

Cooling of an Object in Contact with Evaporating Liquid

When a liquid evaporates from the surface of an object (e.g., sweat on skin):

The liquid absorbs the "Latent Heat" required for the phase change from the object it is touching.
As the liquid evaporates, it carries this thermal energy away, reducing the temperature of the object.

Key Equations

While this topic is mostly descriptive, it links to the calculation of energy during state changes:

$E = mL$

$E$: Thermal energy transferred (Joules, J)
$m$: Mass of the substance (kilograms, kg)
$L$: Specific Latent Heat (J/kg)

Temperature Conversion: $T (\text{in Kelvin}) = \theta (\text{in } ^\circ\text{C}) + 273$

Example: Boiling water = $100^\circ\text{C} + 273 = 373\text{ K}$.

Common Mistakes to Avoid

❌ Wrong: Thinking that the temperature of boiling water keeps rising as you add more heat.
✓ Right: The temperature stays exactly $100^\circ\text{C}$ until all the water has turned into steam; the extra energy is "Latent Heat."
❌ Wrong: Suggesting evaporation only happens when a liquid is hot.
✓ Right: Evaporation happens at all temperatures (e.g., a puddle drying on a cold day).
❌ Wrong: Using "normal body temperature" ($37^\circ\text{C}$) as a fixed calibration point for thermometers in an exam.
✓ Right: Only use the fixed points of water ($0^\circ\text{C}$ and $100^\circ\text{C}$) as standard physical constants.
❌ Wrong: Forgetting to add 273 when converting Celsius to Kelvin.
✓ Right: Always check if the question requires the answer in Kelvin (K).

Exam Tips

Look for "Flat Lines": If an exam question provides a cooling or heating graph, any horizontal section is where a change of state is occurring. If the graph is sloped, the temperature is changing.
Explain the "Why": When asked why evaporation causes cooling, always mention that the "most energetic particles" escape, which "lowers the average kinetic energy" of the remaining particles.
State Change vs. Temperature: In "describe" questions, clarify that during melting/boiling, energy is used to increase potential energy (breaking bonds) rather than kinetic energy (increasing temperature).