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)
1 · Read
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.
More to read
Phase changes
Everything around you is made of tiny particles. In a solid, like ice, the particles are packed tight and just vibrate in place. Heat them up and they start to wiggle more until they break free, that's melting. Keep heating and the particles zoom around as a liquid. Heat even more and they fly apart as a gas. During melting or boiling, the temperature stays flat even though you're adding heat. That energy goes into pulling particles apart, not making them move faster. When you graph temperature versus time, you get flat lines at the melting point and boiling point.
Physical changes
A physical change is like a costume change, the substance looks different but it's still the same stuff inside. Ice melts to water, water boils to steam, but it's all H2O. You can bend a copper wire or crush salt crystals, and they're still copper and salt. Physical changes don't make new substances. That's why melting point and boiling point are like ID cards for a pure substance, they're always the same. If your sample melts over a range of temperatures instead of at one sharp point, you know it's a mixture, not a pure substance.
10.3 Phase Transitions
Phase transitions are the moments when matter changes costume, solid to liquid, liquid to gas, or even solid straight to gas (that's sublimation). Each substance has its own melting point and boiling point because the tug-of-war between particle attractions and kinetic energy plays out differently for each one. Heating curves show this beautifully, flat lines where temperature pauses during a phase change, slanted lines where it climbs within a single phase. The energy hidden in those flat lines is called latent heat, it's the energy cost of rearranging particles without changing their speed.
11.3 Phase Change and Latent Heat
When a substance changes phase, the energy you add doesn't make the particles move faster, it breaks the attractions holding them in place. That's why the temperature stays constant during melting or boiling. The energy needed to melt a solid is the latent heat of fusion; the energy to boil a liquid is the latent heat of vaporization. You can calculate exactly how much energy a phase change needs using the mass and the latent heat value. Cooling curves work the same way in reverse, flat lines at the freezing and condensation points where energy leaves the particles but temperature holds steady.
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2 · Watch
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Where it sits
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Optional connections
Where this leads
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