IGCSE Physics / Solids, liquids and gases

Changes of state: melting, evaporating and boiling

Edexcel International GCSE Physics (4PH1), chapter 19: Solids, liquids and gases, specification point 5.8P, 5.9P. Free lesson.

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Heating can raise a substance’s temperature or change its state. The particle model explains melting, boiling and evaporation, including why evaporation cools the liquid left behind.

What heating does to a substance

Heating transfers energy into a substance, so the energy stored in it increases. That energy has one of two effects at a time: it raises the temperature or changes the state. It does not do both at once. When the temperature rises, particles move faster and gain kinetic energy (energy of movement). For example, water warming from 20 °C to 60 °C gets hotter because its particles move faster. During a state change, energy breaks the bonds between particles. The temperature stays constant while the change happens, as it does when ice melts at 0 °C.

How melting changes a solid into a liquid

Melting happens when energy is transferred to a solid at its melting point. The particles gain energy, first vibrating more strongly around their fixed positions. They then break free and can slide past each other, so the substance becomes a liquid. Freezing is the reverse of melting: energy is transferred away and the particles settle into fixed positions.

How boiling differs from evaporation

Boiling and evaporation both change a liquid into a gas, but they happen in different places and at different temperatures. Condensing is the reverse: a gas cools and turns back into a liquid. Boiling happens throughout the liquid, so bubbles form. It happens only at the boiling point. Evaporation happens only at the surface and can happen at any temperature. Only the fastest particles near the surface escape.

Why evaporation cools a liquid

Particles in a liquid have a range of speeds. When the fastest particles escape from the surface, slower particles are left behind. This lowers the average kinetic energy of the liquid, so its temperature falls. Evaporation can therefore cool your skin after a swim. Evaporation is faster when the liquid is warmer, when it has a larger surface area, or when air moves across it.

What happens when boiling water is heated more

At normal atmospheric pressure, boiling water stays at 100 °C while it boils. The extra energy goes into breaking bonds between particles, rather than making them move faster. More heating makes the water boil away faster, not get hotter. To compare boiling and evaporation for two marks, state where each happens and the temperature at which it happens.

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