Question
Using the particle model, explain the processes of melting, boiling, and evaporation. Why does evaporation cause cooling? How does the particle model explain differences in boiling and evaporation?
Solution — Step by Step
The particle model treats matter as collections of tiny particles (atoms, molecules, or ions) in constant motion. The key idea is that temperature is a measure of the average kinetic energy of these particles. Higher temperature = particles moving faster on average.
The state of matter depends on the balance between:
- Kinetic energy (tends to pull particles apart)
- Intermolecular forces (tend to hold particles together)
In a solid, particles vibrate about fixed positions in a regular lattice. Intermolecular forces are strong enough to keep them in place.
When we heat a solid, we supply energy as kinetic energy to the particles. At the melting point, particles have enough energy to overcome the lattice forces and break free of their fixed positions — they can now move past each other (liquid state).
During melting, temperature stays constant despite heating. The supplied energy goes into breaking intermolecular bonds (latent heat of fusion), not into increasing kinetic energy.
In a liquid, particles move freely but are still close together and attracted to each other. They have no fixed positions but have no permanent separation either.
Boiling is a bulk process: at the boiling point, the vapour pressure of the liquid equals atmospheric pressure. Bubbles of vapour form throughout the liquid (not just at the surface). Every particle in the liquid has enough energy to escape.
Again, temperature stays constant during boiling — all the energy goes into breaking intermolecular attractions (latent heat of vaporisation).
\text{Liquid} \xrightarrow{+\text{latent heat of vaporisation}} \text{Gas (vapour)}}Evaporation differs from boiling: it occurs only at the surface and at any temperature (not just the boiling point).
In a liquid, particle speeds follow a distribution (Maxwell–Boltzmann distribution). At any temperature, some surface particles have enough kinetic energy to overcome intermolecular forces and escape. These are the “fast” molecules — the upper tail of the speed distribution.
As these fast molecules leave, the average kinetic energy of the remaining liquid decreases — the liquid cools. This is why evaporation causes cooling.
This is worth emphasising because it’s a common exam question. When the fastest particles escape:
- The remaining pool loses its most energetic members
- Average kinetic energy of remaining particles falls
- Since temperature = average KE, temperature drops
This is why sweating cools us: water evaporates from skin, taking away high-energy molecules and lowering skin temperature. A wet cloth on a fever patient works the same way.
Boiling does not cool the liquid (temperature stays at boiling point throughout) because energy is being continuously supplied to maintain it.
Why This Works
The particle model is powerful because it connects macroscopic observations (temperature change, state changes) to microscopic behaviour (particle energy, intermolecular forces). The latent heat concept makes sense microscopically: breaking bonds requires energy input without a temperature change.
The Maxwell–Boltzmann speed distribution is the mathematical underpinning of evaporation — even at temperatures far below boiling, there’s always a fraction of molecules with sufficient energy to escape. The fraction is small at low temperatures, so evaporation is slow; it increases with temperature.
Alternative Method
For CBSE board answers, a simple table presentation works well:
| Process | Where? | When? | Temperature during process |
|---|---|---|---|
| Melting | Throughout solid | At melting point | Constant |
| Boiling | Throughout liquid | At boiling point | Constant |
| Evaporation | Surface of liquid | At any temperature | Decreases (cooling) |
Common Mistake
Students often say “evaporation and boiling are the same process.” They are not. Boiling requires the vapour pressure to equal atmospheric pressure — it happens at a fixed temperature throughout the liquid. Evaporation is a surface phenomenon that occurs at any temperature and causes cooling. In CBSE Class 9 and 11, distinguishing these correctly is worth full marks on a 3-mark question.
For a quick memory trick: Boiling = Bulk process at Boiling point; Evaporation = surface Escape at any temperature causing cEOLing (cooling). The B–B–B and E–E–E pattern helps.