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.
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Year 7
Heating, cooling and changes of state
Use particle motion to distinguish melting, evaporation, boiling and condensation.
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
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.