Melting, boiling, condensing and evaporation
Read a heating curve, explain changing states and discover why wet skin can feel cool.
Core: melting, boiling, condensation, freezing, surface evaporation and its cooling effect. Supplement / Extended: comparing boiling with evaporation, rate factors and cooling an object. This is Cambridge IGCSE, separate from Egyptian Baccalaureate / Thanaweya Amma and not Year 3.
Core 2.2.3.1–5 · Supplement 2.2.3.6–8 · PDF pp.177–183 / printed pp.169–175.
A flat line can hide a big change
When pure ice melts at standard atmospheric pressure, energy enters but temperature stays at 0 °C while ice and water coexist. The energy changes the state rather than raising temperature. After the last ice melts, further energy can warm the liquid.
Worked example
Ice starts at −10 °C. Heating first raises its temperature to 0 °C, then melts it at 0 °C, then warms the resulting water. The pattern is rise → flat → rise, not a continuous rise.
Think first: Is the heater necessarily off during a flat section?
No. Energy can be entering steadily while the state changes. Temperature is not a meter of all energy stored in a sample.
Follow the five heating sections
What is present in this section?
Core. Follow an ideal pure-water sample at constant standard atmospheric pressure. The five sections are schematic: widths are not real durations or amounts of energy. Select a point inside a section, not its boundary.
Changing section clears your prediction. No lasting score is stored.
Worked explanation: what happens on the plateau?
Pure ice starts at −10 °C and receives energy until it becomes water at 20 °C, at standard atmospheric pressure. Describe the temperature changes.
- The solid warms from −10 °C to 0 °C. [1 mark]
- Temperature stays at 0 °C while ice and water coexist during melting. [1 mark]
- Energy still enters, but changes the state rather than raising temperature. [1 mark]
- After all ice melts, the liquid warms from 0 °C to 20 °C. [1 mark]
Each mark rewards a distinct physics idea. A final answer-check is useful but does not earn a separate mark.
| Time / min | Temperature / °C |
|---|---|
| 0 | -10 |
| 4 | 0 |
| 20 | 0 |
| 36 | 0 |
| 52 | 0 |
| 68 | 0 |
| 76 | 10 |
| 84 | 20 |
Worked data reading and a limit of real measurements
In this model, the 0 °C plateau lasts 68 − 4 = 64 minutes. At 36 minutes both ice and water are present. These durations are illustrative and deliberately give melting much longer than warming ice through 10 °C under steady net input. In a real experiment, sparse readings may only put the end of melting between two times; do not claim an exact endpoint without evidence.
Particles escape; the liquid does not have to boil
Predict a change in evaporation rate
Supplement / Extended. These are qualitative comparisons, not a numerical drying-time simulator. Water remains available and the air is not saturated unless stated otherwise.
Only the liquid temperature is higher; exposed area and air conditions are otherwise matched.
Changing comparison clears the answer. These predictions do not claim an exact final temperature or rate.
Practical comparison: a damp thermometer
Illustrative results: room 24 °C, damp bulb in still air 21 °C, damp bulb with fan 17 °C. The decreases are 3 °C and 7 °C. The extra decrease with the fan is 4 °C, not 17 °C. Match starting water amount, wetted area, air temperature, humidity and timing; repeat the comparison. Use room-temperature water, avoid spills and keep electrical equipment away from water.
More-energetic particles escape, tending to cool the liquid. Energy transfers from the bulb into the evaporating liquid. Moving air removes vapour and can increase evaporation; it does not prove the fan lowered the whole room temperature.
Practise, then explain
Four multiple-choice questions, four numerical data-reading responses and six written tasks. Basic state changes and evaporation are Core; comparison, evaporation-rate factors and cooling an object are Supplement / Extended. Numerical answers are checked automatically; written answers use a self-assessment guide. The suggested marks are for this original practice, not an official Cambridge mark scheme.
Practice stays only while this page is open. Refreshing or leaving clears these responses. Copy useful written work into your notes before leaving.
At standard atmospheric pressure, what is the boiling temperature of pure water?
Optional hint
Use the standard-pressure value; different pressures can change the boiling point.
Source clarifications and safe limits
The original PDF is unchanged. Its heating-ice table explicitly simplifies the times and its readings leave uncertainty in the end of melting. Our original table has clearly stated model boundaries and a longer melting interval; the separate evaluation question preserves the uncertainty lesson. The schematic five-stage curve is not a calibrated time graph.
The source sometimes says particles keep slowing during condensation/freezing. Distinguish cooling towards the transition from the ideal constant-temperature transition itself: average kinetic energy does not keep decreasing along a plateau. Particles still move in liquid and vibrate in solid. Water vapour is invisible; a visible white cloud is mainly droplets.
The source’s “fast boiling / slow evaporation” comparison is not a definition. Evaporation can occur below the boiling point and also at the surface while boiling occurs elsewhere. The wording “heading upwards” describes the picture, not a required direction of escape.
Source Q8 suggests higher temperature would make the jar cooler still. Higher temperature can increase evaporation rate, but a lower final temperature is not guaranteed because the energy balance also changes. Similarly, the sunny towel and windy towel cannot be ranked without more information. Source Q7’s extra mark for spreading towels is not part of the requested comparison; our marks reward only ideas each question asks for.
Do not heat sealed containers or put hands near steam: hot water, vapour and heaters can burn. These activities are screen models, not instructions to improvise boiling apparatus. This syllabus section does not require a specific-latent-heat calculation; the numerical questions here read data and temperature differences.
Source alignment: supplied Physics-0625 Volume A, PDF pp.177–183 / printed pp.169–175, Figures 25.1–25.2; official Cambridge 2026–2028 syllabus, section 2.2.3. Original teaching, models and questions; suggested marks are not an official Cambridge mark scheme. Existing original-PDF download access is unchanged.