Melting, Boiling and Evaporation
The change of state of matter can be explained using the kinetic particle theory. Heat energy changes the motion and spacing of particles, leading to melting, boiling or evaporation.
Heating Curve Explanation (Kinetic Theory)
Stages of Heating a Solid
- A–B: Temperature of the solid rises as particles vibrate faster with increased kinetic energy.
- B–C: Temperature remains constant while particles overcome forces of attraction — the solid melts.
- C–D: Temperature rises again as liquid particles gain more kinetic energy.
- D–E: Temperature remains constant; energy breaks remaining forces between particles — the liquid boils.
- E–F: Gas particles move freely, far apart, with rapidly increasing kinetic energy.
Equilibrium During Melting
Solid ⇌ Liquid (X(l))
Key Notes
- A pure substance shows sharp melting (B–C) and boiling (D–E) points.
- An impure substance melts at a lower and boils at a higher temperature range.
Comparison: Particles in Liquid vs Gas
| Stage | Separation between Particles | Movement of Particles | Can Fill Container? |
|---|---|---|---|
| C–D (Liquid) | Close and touching | Random and slow | Cannot move apart completely |
| E–F (Gas) | Far apart | Fast and random | Yes, fills entire container |
Evaporation
Evaporation is the process in which surface molecules of a liquid gain enough kinetic energy to escape into the air as vapour.
Explanation
- Only molecules with the highest kinetic energy escape from the surface.
- Energy is used to overcome intermolecular attraction, not to raise temperature.
- Hence, the liquid cools down after evaporation — evaporation causes cooling.
Factors Affecting Rate of Evaporation
- Temperature (higher → faster evaporation)
- Surface area (larger → faster evaporation)
- Draught or airflow (removes vapour, increasing rate)
Everyday Uses of Evaporation
- Making salt in salt pans by evaporating seawater.
- Drying clothes faster on a hot or windy day.
- Cooling of tea and perspiration from skin.
Difference Between Evaporation and Boiling
| No. | Evaporation | Boiling |
|---|---|---|
| 1 | Occurs only at the surface of the liquid. | Occurs throughout the liquid. |
| 2 | Can occur at any temperature. | Occurs only at the boiling point. |
| 3 | Causes cooling as high-energy particles escape. | Does not cause cooling; temperature remains constant. |
| 4 | No bubbling observed. | Vigorous bubbling occurs. |
Key Idea
During melting or boiling, temperature stays constant while latent heat is absorbed to break forces between particles. Kinetic energy does not increase until the phase change is complete.
Exam Tip
Remember: In both melting and boiling, energy is used to overcome forces of attraction — not to increase temperature. Cooling by evaporation relies on faster molecules escaping first.
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