Changes of state (melting, freezing, boiling, evaporating, condensing and sublimation) explained by the particle model, heating and cooling curves, conservation of mass, and the difference between physical and chemical changes.

What this topic is asking

OCR wants you to describe and explain the changes of state using the particle model, interpret heating and cooling curves, apply conservation of mass, and distinguish physical changes from chemical changes. This is topic P1.2 of the OCR Gateway Physics A (J249) specification, examined on the Paper 1 or Paper 3 side.

The changes of state

In terms of the particle model, heating a substance gives its particles more energy. When the particles have enough energy to overcome the forces holding them in place, the substance changes state:

• Melting and boiling require energy to be supplied to separate the particles further apart, so they happen on heating.
• Freezing and condensing release energy as the particles move closer and bonds form, so they happen on cooling.
• Evaporation happens at the surface of a liquid below its boiling point, when the fastest particles escape; this is why a puddle dries up and why evaporation cools what is left behind.

Heating and cooling curves

So a heating curve for a solid being heated all the way to a gas has two flat sections: one at the melting point while it melts, and one at the boiling point while it boils. Reading these flat sections off a curve gives the melting and boiling points, a common OCR skill. The substance with stronger forces between its particles has higher melting and boiling points.

Conservation of mass

This is why a sealed flask of melting ice does not change mass, and why the apparent loss of mass when water boils in an open pan is just the vapour escaping into the room, not mass disappearing.

Physical changes versus chemical changes

A change of state is a physical change: the particles are rearranged but no new substance is made, and the change can be reversed (water can be frozen back to ice). A chemical change, such as burning or rusting, makes one or more new substances and is usually not easily reversible. Recognising changes of state as physical and reversible is a frequent OCR test point.

Try this

Q1. Name the change of state when a gas turns directly into a solid. [1 mark]

• Cue. Sublimation (deposition is also accepted for the gas-to-solid direction).

Q2. Explain why the temperature stays constant while a substance is boiling. [2 marks]

• Cue. The energy supplied increases the potential energy of the particles, separating them into a gas, rather than increasing their kinetic energy, so the temperature does not rise.