Changes of state

Six changes, three pairs of opposites. Every change of state either adds heat (particles break free of each other) or removes heat (particles lock closer together), and each has an exact reverse:

• Melting (fusion): solid to liquid, at a fixed melting point. Freezing (solidification): the reverse — liquid to solid, at the same fixed temperature.
• Vaporisation (boiling): liquid to gas, throughout the bulk of the liquid, at a fixed boiling point — this is what makes bubbles form deep inside a boiling kettle, not only at the surface. Condensation (liquefaction): the reverse — gas to liquid, as when droplets form on a cold steel bottle.
• Sublimation: solid straight to gas, skipping the liquid stage entirely — camphor, naphthalene mothballs and dry ice all do this. Deposition: the reverse — gas straight to solid, as when frost forms directly from water vapour in very cold air.

Evaporation is not boiling. Both turn liquid into gas, but evaporation happens only at the exposed surface of a liquid, at any temperature, slowly and without visible bubbles — a puddle dries up on a warm day without ever reaching $100^\circ\text{C}$. Boiling happens throughout the whole liquid, only at the fixed boiling point, rapidly and with vigorous bubbling. Mixing these two up is the single most common error in this topic.

Why evaporation speeds up or slows down. Because evaporation is a surface phenomenon, anything that helps surface particles escape or helps escaped vapour get out of the way will speed it up: higher temperature (particles already have more kinetic energy, so fewer need a lucky extra push to escape), larger surface area (more particles are exposed to the air at once, as with clothes spread flat rather than bunched), lower humidity (drier air is farther from saturated, so it can absorb more vapour before slowing down), and higher wind speed (moving air sweeps escaped vapour away, stopping it from crowding back near the surface and re-condensing).

The reasoning behind every arrow in that diagram is the same: state depends only on how energetic the particles are compared to how strongly they attract each other. In a solid, attraction wins — particles vibrate about fixed positions but cannot move away. Heating them raises their vibration energy until, at the melting point, they finally break free of those fixed positions and slide past one another as a liquid. Heat further and, at the boiling point, particles gain enough energy to overcome attraction completely and fly apart as a gas, spreading to fill any space available. Removing heat reverses each step exactly, at the same fixed temperatures, because the same attractive forces are doing the pulling-back. Sublimation and deposition follow the identical logic, just skipping the middle stage: naphthalene's particles are attracted to each other weakly enough, and its vapour pressure is high enough even as a solid, that heating pushes particles straight from their fixed positions into free gas motion without ever pausing as a liquid. Evaporation is this same escape process happening only at the surface, at ordinary room temperature, one particle at a time — a small, lucky fraction of surface particles always have enough extra energy to break free, even though the liquid as a whole is nowhere near its boiling point. Anything that increases how many particles have that extra energy, or clears escaped particles away so fewer wander back in, evaporates the liquid faster.

Question: A wet handkerchief is hung out on a hot, dry, breezy afternoon (day 1) and an identical one on a cool, humid, still morning (day 2). (a) Explain, using the four evaporation factors, why day 1 dries the cloth much faster. (b) Separately, a cold steel water bottle left on the same humid morning gets covered in droplets — name and explain this change of state.

Step 1. Temperature: day 1 is hotter, so the water particles in the cloth already carry more kinetic energy, and a larger fraction of surface particles have enough extra energy to break away as vapour.

Step 2. Humidity: day 1's dry air is far from saturated with water vapour, so it keeps absorbing more; day 2's humid air is closer to saturated, so vapour leaves the cloth more slowly and some re-condenses back onto it.

Step 3. Wind speed: day 1's breeze continuously sweeps newly evaporated vapour away from the cloth's surface, keeping the air right next to it able to accept more; on the still day, vapour builds up locally and evaporation slows.

Step 4. Surface area is identical in both cases here (same handkerchief, same way of hanging), so it does not distinguish the two days — but spreading the cloth flatter would speed evaporation on either day. Combining Steps 1 to 3, higher temperature, lower humidity and higher wind speed all independently favour day 1, so the handkerchief dries noticeably faster.

Step 5 (part b). The droplets on the cold bottle are condensation, not evaporation reversed from inside the bottle. Water vapour already present in the humid surrounding air loses heat on contact with the cold steel surface; as those vapour particles slow down, inter-particle attraction pulls them together into the closer liquid arrangement, appearing as droplets on the outside.

Day 1 dries fastest because three of the four evaporation factors favour it, and the bottle's droplets are condensation of ambient vapour, not a leak.

Competitive stretch

If asked to also classify "mothballs shrinking without ever forming a puddle," the answer is sublimation: the solid naphthalene turns directly into gas, so no liquid stage — and no puddle — ever appears. The quick test for any such scenario: ask which two states are involved and whether heat is being added or removed.

• Treating evaporation and boiling as the same process — evaporation is surface-only, happens at any temperature, and is slow; boiling happens throughout the liquid, only at the fixed boiling point, and is fast with visible bubbles.
• Thinking sublimation is rare or exotic — camphor, naphthalene mothballs and dry ice are everyday Class 8 examples of it.
• Assuming droplets on a cold glass or bottle come from inside — they form from water vapour already present in the surrounding air, condensing on contact with the cold surface.
• Believing only temperature affects evaporation rate — surface area, humidity and wind speed each independently matter too.

Ask the tutor: Curious why dry ice "smokes" without ever forming a puddle, or want more real-life examples to practise classifying changes of state? Ask the AI tutor.