Use particle motion to distinguish melting, evaporation, boiling and condensation.
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Method and worked examples
Heating usually increases particle motion as temperature rises. Melting changes solid to liquid; freezing reverses it. Evaporation occurs at a liquid surface, including below boiling temperature. Boiling forms vapour bubbles throughout a liquid. Condensation changes gas to liquid. These changes preserve the substance: gaseous water remains water, not air.
For a pure substance at fixed pressure, temperature can remain approximately constant during melting or boiling while heating continues. Energy changes particle arrangement and overcomes attractions rather than raising temperature. On a temperature–time graph, time is horizontal and temperature is vertical. Read a plateau as an interval of steady temperature, not automatically as the heater being off.
Example 1
Water droplets appear outside a cold glass. Where did the water come from?
Water vapour in surrounding air cooled and condensed on the glass. Water did not pass through the glass.
Example 2
A sample warms from 20∘C to 40∘C and then stays at 40∘C while melting. Explain the plateau.
At fixed pressure, energy supplied during melting changes the arrangement of particles while the pure sample remains approximately at its melting temperature.
Watch out:Surface evaporation occurs below the boiling point. Boiling is a different process that forms bubbles throughout the liquid.
Heating, cooling and changes of state practice | Year 7 Science | Ascent Arcade