Describe melting and boiling in terms of energy input without a change in temperature

Published by Patrick Mutisya · 14 days ago

Cambridge IGCSE Physics 0625 – Melting, Boiling and Evaporation

2.2.3 Melting, Boiling and Evaporation

Learning Objective

Describe melting and boiling in terms of energy input without a change in temperature.

Key Concepts

  • Phase change: Transition between solid, liquid and gaseous states.

  • Latent heat: Energy required to change phase at constant temperature.

  • Melting (fusion): Solid → Liquid.

  • Boiling (vaporisation): Liquid → Gas throughout the bulk.

  • Evaporation: Surface‑only liquid → gas, occurs at any temperature.

Energy and Temperature During Phase Changes

When a substance undergoes a phase change, energy is absorbed (or released) but the temperature remains constant until the entire mass has changed phase. This is because the energy is used to overcome intermolecular forces rather than to increase kinetic energy.

Melting (Fusion)

The temperature at which a solid begins to melt is its melting point. At this temperature:

  1. Heat energy is supplied to the solid.

  2. The temperature does not rise; all added energy goes into breaking the bonds that hold the particles in a fixed lattice.

  3. The amount of energy required to melt a mass m is given by

    \$Q{\text{melt}} = mLf\$

    where \$L_f\$ is the latent heat of fusion (J kg⁻¹).

Boiling (Vapourisation)

Boiling occurs when a liquid reaches its boiling point. At this temperature:

  1. Heat energy is supplied to the liquid.

  2. The temperature remains constant while the liquid turns into vapour throughout the bulk.

  3. The energy needed to convert a mass m of liquid to vapour is

    \$Q{\text{boil}} = mLv\$

    where \$L_v\$ is the latent heat of vapourisation (J kg⁻¹).

Evaporation

Evaporation is a surface phenomenon that can occur at any temperature below the boiling point. Unlike boiling, it does not require the whole liquid to reach a specific temperature. Energy is still required to overcome intermolecular forces, but the temperature of the remaining liquid may fall slightly because the most energetic molecules leave the surface.

Comparison of Melting, Boiling and Evaporation

AspectMeltingBoilingEvaporation
Phase changeSolid → LiquidLiquid → Gas (throughout)Liquid → Gas (surface only)
Temperature during changeConstant at melting pointConstant at boiling pointMay decrease slightly; no fixed temperature
Energy required\$Q = mL_f\$\$Q = mL_v\$Variable; related to \$L_v\$ but less than bulk boiling
Typical latent heat values (for water)\$L_f = 3.34 \times 10^5\ \text{J kg}^{-1}\$\$L_v = 2.26 \times 10^6\ \text{J kg}^{-1}\$≈ \$L_v\$ for molecules that escape

Energy Flow Diagram (Suggested)

Suggested diagram: Energy input during melting and boiling showing constant temperature plateaus on a temperature‑time graph.

Key Points to Remember

  • During melting and boiling, temperature does not rise because the supplied energy is used to overcome intermolecular forces.

  • The amount of energy required depends on the mass of the substance and its specific latent heat.

  • Evaporation can occur at any temperature; it is slower than boiling because only surface molecules escape.

  • Latent heat values are characteristic of each substance and are essential for quantitative calculations.

Sample Calculation

How much energy is needed to melt 250 g of ice at 0 °C?

Given \$L_f = 3.34 \times 10^5\ \text{J kg}^{-1}\$:

\$Q = mL_f = 0.250\ \text{kg} \times 3.34 \times 10^5\ \text{J kg}^{-1} = 8.35 \times 10^4\ \text{J}\$

The temperature of the ice remains at 0 °C until the entire 250 g has melted.