Change Of State: Melting, Boiling & Evaporation

A change of state is a physical change from solid to liquid, liquid to gas, or the reverse, occurring at a definite temperature for a pure substance at a given pressure. Melting, boiling, and evaporation all involve rearranging how particles are held together, but they differ in where and how fast they happen.

These processes run our weather, our kitchens, and our refrigerators. Rain forms by condensation, sweat cools us by evaporation, and ice melts in drinks by absorbing heat. Understanding the differences between melting, boiling, and evaporation prevents common errors in the lab and at home.

Key Concepts

Melting (fusion) is solid to liquid at the melting point; freezing is the reverse. Boiling is rapid vaporisation throughout the liquid at the boiling point, where vapour pressure equals external pressure (100°C for water at standard pressure). Evaporation is slow vaporisation only at the surface at any temperature; fast surface molecules escape. During a change of state at constant pressure, temperature stays constant even though heat is supplied or removed; the energy changes particle arrangement, called latent heat.

How It Works

1. On heating a solid, particles vibrate more until bonds loosen at the melting point and the solid becomes liquid while temperature pauses.
2. Further heating raises liquid temperature until vapour pressure equals atmospheric pressure; bubbles form everywhere and boiling begins at a fixed temperature.
3. At any temperature, some surface molecules have enough energy to escape, so evaporation proceeds slowly and cools the remaining liquid.
4. Cooling reverses the sequence: condensation releases heat at the boiling point, and freezing releases heat at the melting point.

Worked Example

A pressure cooker raises pressure to about twice atmospheric, lifting water's boiling point to roughly 120°C. Food cooks faster because the liquid and steam are hotter, not because of pressure alone. Contrast with evaporation: a wet cloth at 30°C dries by evaporation without ever boiling. If 0.50 kg of water evaporates, it must take latent heat from its surroundings, approximately Q = mL = 0.50 × 2.26 × 10⁶ = 1.13 × 10⁶ J, which is why evaporation produces strong cooling on skin and in desert coolers.

Common Misconceptions

• Boiling and evaporation are the same. Boiling is bulk and at a fixed temperature; evaporation is surface-only and at all temperatures.
• Temperature keeps rising during melting. For a pure substance it stays constant until the change completes; all heat goes into latent heat.
• Steam is visible. True water vapour is invisible; the white cloud is tiny condensed droplets formed when vapour meets cool air.

Real-World Applications

Refrigerators and air conditioners cycle evaporation and condensation of a refrigerant to move heat. Distillation separates liquids by boiling points, and freeze-drying preserves food. Weather depends on evaporation from oceans and condensation into clouds, releasing latent heat that powers storms. Sweating and panting regulate body temperature through evaporative cooling.

Melting, boiling, and evaporation are changes of state governed by latent heat at characteristic temperatures. Distinguishingbulk boiling from surface evaporation and recognising constant temperature during change explains cooking, cooling, and the water cycle.