How Materials Change State
Explain melting, freezing, boiling, condensing, and sublimation using the particle model, interpreting heating and cooling curves to identify melting and boiling points
What a learner can do afterwards
- Describes what happens to particles during each change of state
- Reads a heating/cooling curve and identifies the melting point and boiling point from the flat regions
- Explains why temperature stays constant during a change of state
- Distinguishes between evaporation (from surface, any temperature) and boiling (throughout liquid, at boiling point)
The lesson
You already know particles move faster when heated and slower when cooled. A change of state is what happens when particles gain or lose enough energy to rearrange completely. Melting turns a solid into a liquid. Freezing turns a liquid into a solid. Boiling turns a liquid into a gas. Condensing turns a gas into a liquid. Sublimation skips the liquid step entirely: a solid turns straight into a gas.
During a change of state, the temperature stops rising even though you're still adding heat. The energy isn't speeding the particles up anymore. Instead, it's being used to break the bonds holding the particles in place. Once every particle has broken free and the change is finished, the temperature starts climbing again.
Priya heats a block of ice in a pan and records the temperature every minute. Her graph rises, flattens at 0°C while the ice melts, rises again, then flattens at 100°C while the water boils. She reads the two flat sections straight off the graph: the melting point is 0°C and the boiling point is 100°C.
Don't mix up evaporation and boiling. Evaporation happens only at the surface of a liquid and can happen at any temperature, like a puddle drying on a cool day. Boiling happens throughout the whole liquid, with bubbles forming down at the bottom, and only happens once the liquid reaches its boiling point.
The flat parts of a heating or cooling graph aren't the heating stopping, they're the melting point and boiling point, where energy is busy rearranging particles instead of raising the temperature.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.