2.2.3 Melting, Boiling and Evaporation
Learning objective
Describe the differences between boiling and evaporation, and explain the related phase‑change concepts required by the Cambridge IGCSE 0625 syllabus.
Key concepts required by the syllabus
- Phase changes occur at constant temperature; the energy required is the latent heat.
- Each phase change has a characteristic temperature at standard pressure (1 atm = 101 kPa):
- Melting point of water = 0 °C
- Boiling point of water = 100 °C
- Latent‑heat values (per kilogram of substance):
- Latent heat of fusion, \(L_f \approx 3.34\times10^{5}\;{\rm J\,kg^{-1}}\)
- Latent heat of vapour‑isation, \(L_v \approx 2.26\times10^{6}\;{\rm J\,kg^{-1}}\)
- Evaporation causes cooling because the most energetic surface molecules escape, taking the latent‑heat energy with them.
- Condensation is the reverse of evaporation (vapour → liquid) and forms an essential part of the water cycle.
Melting and solidification
When a solid reaches its melting point, energy is absorbed as the latent heat of fusion, \(L_f\), without a rise in temperature. The solid becomes a liquid. The reverse process, solidification, releases the same amount of energy.
Example: To melt 1 kg of ice at 0 °C requires
\[
Q = mL_f = 1 \times 3.34\times10^{5}=3.34\times10^{5}\ {\rm J}.
\]
Boiling
Boiling is a rapid, bulk formation of vapour that occurs when the liquid temperature reaches its **boiling point**. At this temperature the vapour pressure of the liquid equals the external (usually atmospheric) pressure.
- Where it occurs: Throughout the whole volume; bubbles form inside the liquid and rise to the surface.
- Temperature requirement: The entire liquid must be at its boiling point before vigorous bubbling begins.
- Visible signs: Continuous, noisy bubbling; a “rolling” motion of the surface.
- Energy input:
- First, heat raises the temperature to the boiling point.
- Then, further heat supplies the latent heat of vapour‑isation, \(L_v\), to each kilogram that changes to steam.
- Rate control: Primarily the amount of heat supplied (e.g., stove power).
Numerical illustration: Converting 1 kg of water at 100 °C to steam requires
\[
Q = mL_v = 1 \times 2.26\times10^{6}=2.26\times10^{6}\ {\rm J}.
\]
Evaporation
Evaporation is a slow surface phenomenon that can occur at any temperature below the boiling point.
- Where it occurs: Only molecules at the free surface with kinetic energy ≥ \(L_v\) escape into the air.
- Temperature requirement: No bulk heating is needed; the liquid may be well below its boiling point.
- Visible signs: The liquid level falls gradually; no bubbles are seen.
- Factors that increase the rate (exact syllabus wording):
- Temperature: Higher temperature → more molecules have enough kinetic energy.
- Surface area: A larger exposed area provides more molecules that can escape.
- Humidity of the surrounding air: Lower humidity increases the vapour‑pressure gradient.
- Air movement (wind): Moving air removes saturated vapour, allowing more molecules to leave.
- Cooling effect: Escaping molecules take the latent heat \(L_v\) with them; the remaining liquid loses energy and its temperature falls (e.g., sweating).
Condensation
Condensation is the reverse of evaporation: vapour molecules lose kinetic energy, give up the latent heat of vapour‑isation, and become liquid. It is a key step in the water cycle, producing clouds, dew and fog.
Comparison of boiling and evaporation
| Aspect |
Boiling |
Evaporation |
| Location |
Throughout the bulk; bubbles form inside the liquid. |
Only at the free surface. |
| Temperature requirement |
Liquid must reach its boiling point (100 °C for water at 1 atm). |
Can occur at any temperature below the boiling point. |
| Visible signs |
Rapid, continuous bubbling; audible “rolling” sound. |
Gradual lowering of level; no bubbles. |
| Energy input |
Heat first raises temperature to \(T_{\rm bp}\); then latent heat \(L_v\) is supplied as bubbles form. |
Only surface molecules need enough kinetic energy to overcome \(L_v\); bulk heating not required. |
| Rate control |
Amount of heat supplied (e.g., stove power). |
Temperature, surface area, humidity and airflow (wind). |
| Effect on liquid temperature |
Temperature remains constant at the boiling point while boiling continues. |
Temperature falls (cooling) because energy leaves with the escaping molecules. |
Typical IGCSE‑style exam questions
- Explain why water boils at a lower temperature on a mountain than at sea level.
- Describe how increasing the surface area of a puddle affects its rate of evaporation, and state any other factors that would change the rate.
- Why does a pot of water on a stove first heat up before it begins to boil?
- Explain why a wet shirt feels cooler as it dries.
- State the melting and boiling points of water at standard pressure and give the symbols for the two latent‑heat values.
Suggested diagrams
- Side‑view illustration showing (a) bubbling throughout a liquid during boiling and (b) surface molecules escaping during evaporation.
- Heating curve for water indicating the flat sections for melting (fusion) and boiling (vapourisation) together with the values of \(L_f\) and \(L_v\).
Summary
Both boiling and evaporation are phase changes from liquid to gas and both require the latent heat of vapour‑isation, \(L_v\). They differ in:
- Location (bulk vs. surface)
- Temperature requirement (boiling point vs. any lower temperature)
- Visual appearance (bubbling vs. gradual surface recession)
- Factors controlling the rate (heat supply vs. temperature, area, humidity, wind)
- Effect on the liquid’s temperature (constant at \(T_{\rm bp}\) vs. cooling).
Understanding these differences explains everyday phenomena such as why a kettle whistles when water boils, why a pond dries slowly on a calm night, and why sweating cools the body.