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

2.2.3 Melting, Boiling and Evaporation

Learning Objective (AO1)

Describe melting and boiling as processes in which energy is absorbed **without a change in temperature**.

Key Facts (quick‑recall box)

How this fits the Cambridge IGCSE 0625 syllabus

• State the melting and boiling temperatures for water (0 °C and 100 °C at 1 atm).
• Describe melting, boiling, condensation and solidification in particle terms.
• Explain evaporation and why it produces cooling.
• Contrast boiling with evaporation.
• Discuss the effect of surface area, temperature, humidity and wind on the rate of evaporation.
• Outline a simple experiment to measure latent heat (AO3).

2.2.3.1 Key Concepts

• Phase change: transition between solid, liquid and gaseous states.
• Latent heat (heat of transformation): energy required to change phase at constant temperature.
  • \(L_f\) = latent heat of fusion (J kg⁻¹)
  • \(L_v\) = latent heat of vaporisation (J kg⁻¹)
• During a phase change the supplied energy is used to **overcome intermolecular forces**, not to increase the average kinetic energy of the particles; therefore the temperature remains constant.

2.2.3.2 Particle‑level view of each change

Insert a simple diagram showing particles (dots) before and after each change, with arrows indicating the type of intermolecular force (e.g., hydrogen bonds) that are broken or formed.

2.2.3.3 Energy & Temperature During a Phase Change

When a substance reaches its melting point (\(T_m\)) or boiling point (\(T_b\)), any heat supplied goes into overcoming intermolecular forces. The average kinetic energy – and therefore the temperature – stays constant until the whole mass has changed phase.

2.2.3.4 Melting (Fusion)

• Occurs at the melting point \(T_m\). For pure water at 1 atm, \(T_m = 0 °C\).
• Heat added: latent heat of fusion \(L_f\) (J kg⁻¹).
• Energy required: \[ Q_{\text{melt}} = mL_f \]
• Temperature remains at \(0 °C\) until all solid has become liquid.

2.2.3.5 Boiling (Vaporisation)

• Occurs at the boiling point \(T_b\). For pure water at 1 atm, \(T_b = 100 °C\).
• Heat added: latent heat of vaporisation \(L_v\) (J kg⁻¹).
• Energy required: \[ Q_{\text{boil}} = mL_v \]
• Temperature stays at \(100 °C\) while bubbles form throughout the bulk.

2.2.3.6 Condensation

• Reverse of boiling; occurs when a gas is cooled to its condensation point (same temperature as \(T_b\)).
• Particles lose kinetic energy, come together, and release the latent heat of vaporisation \(L_v\) to the surroundings.

2.2.3.7 Solidification (Freezing)

• Reverse of melting; occurs when a liquid is cooled to its freezing point (same temperature as \(T_m\)).
• Particles arrange into a fixed lattice and release the latent heat of fusion \(L_f\).

2.2.3.8 Evaporation & Evaporative Cooling

• Only the surface molecules with the highest kinetic energy escape into the gas phase.
• These energetic molecules take away a disproportionate amount of kinetic energy, so the average kinetic energy – and thus the temperature – of the remaining liquid **decreases** (cooling).
• Real‑world example: sweat on skin absorbs heat from the body as it evaporates, keeping us cool.
• Other culturally diverse examples:
  • Drying clothes on a line in a tropical climate (high temperature, strong wind).
  • Ice‑cream melting on a hot day at a street market in Lagos.

2.2.3.9 Boiling vs Evaporation (comparison table)

2.2.3.10 Quantitative illustration – effect of surface area on evaporation rate

Two identical beakers each contain 200 g of water at 25 °C.

• Beaker A has a surface area of 50 cm².
• Beaker B has a surface area of 200 cm² (four times larger).

Assuming all other conditions (temperature, humidity, wind) are the same, the evaporation rate is roughly proportional to surface area. Therefore Beaker B will lose water about **four times faster** than Beaker A. This relationship is useful for AO2 questions that ask you to compare rates.

2.2.3.11 Experiment: Determining the latent heat of fusion of ice (calorimetry)

1. Weigh a dry calorimeter (mass \(m_c\)).
2. Add a known mass of warm water (\(m_w\), temperature \(T_i\)). Record \(T_i\).
3. Place a known mass of ice (\(m_i\), at 0 °C) into the calorimeter, quickly close the lid and stir gently.
4. Measure the final equilibrium temperature (\(T_f\)).
5. Use energy‑balance: \[ m_w c_w (T_i - T_f) = m_i L_f + m_c c_c (T_f - T_{\text{room}}) \] (neglect heat loss to surroundings for a basic AO3 task).
6. Re‑arrange to solve for \(L_f\). Compare your result with the accepted value \(3.34\times10^5\ \text{J kg}^{-1}\).

This activity develops planning, data collection and error‑analysis skills required for AO3.

2.2.3.12 Energy‑Time (Temperature‑Time) Graph

2.2.3.13 Key Points to Remember

• During melting and boiling the temperature stays constant because the supplied energy is used to break intermolecular forces (latent heat), not to increase kinetic energy.
• Energy required:
  • \(Q=mL_f\) for melting (fusion).
  • \(Q=mL_v\) for boiling (vaporisation).
• Condensation and solidification release the same amount of energy that was absorbed during boiling and melting.
• Evaporation removes the most energetic surface molecules, causing the remaining liquid to cool.
• Boiling needs a fixed temperature and occurs throughout the liquid; evaporation needs no fixed temperature and occurs only at the surface.

2.2.3.14 Exam‑style Tip & Common Misconception