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States of matter: kinetic theory and latent heat
The kinetic-molecular model, temperature and kinetic energy, phase transitions and latent heat, and the strange density of water. With a heating-curve experiment on melting ice.
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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.[2]
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.[4]
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.[3]
The energy needed is the latent heat. For water, the heat of fusion is 6.01 kJ/mol at 0 °C and the heat of vaporisation is 40.67 kJ/mol at the normal boiling point (100 °C at 1 atm). With a molar mass of 18.0 g/mol, that is about 334 J to melt one gram of ice, and about 2260 J to boil away one gram of water: almost seven times as much.[3]
Water behaves unusually: it is densest at about 4 °C, and on freezing its density drops by about 9%, so ice floats and water expands as it freezes. A gas-like state with many charged particles, plasma, exists in stars and lightning.[5][1]
Try it: The heating curve of melting ice
You need
- A 250 ml beaker of crushed ice (from a freezer at about −18 °C)
- A digital thermometer or temperature probe (−20 to 100 °C)
- A stirring rod and a stopwatch
- Optional: a beaker of warm tap water (about 40 °C) to stand the ice beaker in
- Graph paper or a spreadsheet
Safety
Use warm tap water only (no kettles or Bunsen burners). Handle freezer ice with a scoop or gloves. Wipe up spills.
What to do
- 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.
What happens, and why?
The temperature rises to 0 °C, then stays flat at 0 °C while the ice melts, because the heat taken in is used to overcome the attractions between the water molecules (the latent heat of fusion), not to increase their kinetic energy. When all the ice has melted, the temperature rises again. A probe touching the warm beaker wall, or poor stirring, makes the plateau less flat.[3][2]
Play: Phase change detective
Which phase change is it? Six possible changes, including sublimation and deposition.
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Quiz
Other ages: Ages 6–7 · Ages 10–11