echoesof.science · Ages 15–16
States of matter: kinetic theory and latent heat
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Whether a substance is a solid, liquid or gas depends on the balance between the attractive forces between its particles and their kinetic energy. In solids the particles vibrate about fixed positions; in liquids they stay in contact but move past each other; in gases they are far apart and move independently except when they collide.
The kinetic-molecular theory of gases: molecules are in continuous straight-line motion, tiny compared with the distances between them, collide elastically, and their average kinetic energy is proportional to the kelvin temperature.
Phase transitions: melting (s → l), freezing (l → s), vaporisation (l → g), condensation (g → l), sublimation (s → g) and deposition (g → s). During a change of state the temperature remains constant: the heat supplied changes the intermolecular attractions, not the temperature. Dry ice (solid CO₂) sublimes at normal pressure.
Experiment: The heating curve of melting ice
- Take the crushed ice straight from the freezer. Put the probe in the middle and record the starting temperature.
- Stand the beaker in the room (or in the warm-water bath to speed things up). Stir gently and record the temperature every 30 seconds.
- Write down when you first see liquid water and when the last ice disappears.
- Carry on for 10 minutes after all the ice has melted.
- Plot temperature (y) against time (x). Mark the start and end of melting.
- Explain each part of the graph using kinetic theory and latent heat.
| Time (min) | Temperature (°C) | Observations (ice, water, both) |
|---|---|---|
| 0 | ||
| 0.5 | ||
| 1 | ||
| 1.5 | ||
| 2 | ||
| … |
Questions
Explain, using particles and energy, why a substance's temperature stays constant while it melts.
Calculate the energy needed to turn 200 g of water at 100 °C into steam (use 2260 J/g).
Convert water's heat of fusion, 6.01 kJ/mol, into J/g (molar mass 18.0 g/mol). Show your working.
Sketch your heating curve and label: ice warming, melting, water warming. Why was the plateau not perfectly flat?
Sources for every fact on this sheet: echoesof.science/school/states-of-matter/sources/