Particle Model of Changes of State
The particle model of changes of state explains melting, freezing, evaporation, condensation and sublimation as changes in the energy, motion and arrangement of particles, without any change in the particles themselves. In brief: when a solid is heated, its particles vibrate more until they break free of fixed positions and the solid melts; the particles are the same before and after, but their arrangement and energy are different.
The macroscopic observations of changes of state, such as constant temperature during melting, have a simple particle explanation. This article describes what happens to the particles at each change, explains the flat sections of a heating curve and the role of energy, compares the changes in a table, works through a heating curve calculation and gives some history.
What happens to particles when a solid melts?
When a solid is heated, the particles gain kinetic energy and vibrate with greater amplitude. At the melting point, the vibrations are strong enough to overcome some of the attractions that hold the particles in a fixed pattern. The regular structure breaks down, and particles can move past each other. The substance is now a liquid.
During melting the heat supplied does not increase the average kinetic energy of the particles, but it increases their potential energy as they move apart slightly and the structure loosens.
What happens when a liquid boils or evaporates?
Liquid particles move at a range of speeds. Some near the surface have enough energy to overcome the attraction of their neighbors and escape into the air as gas. This is evaporation, and it can happen at any temperature. The fastest particles leave, so the average energy of the remaining liquid decreases and the liquid cools.
When the liquid is heated to the boiling point, particles throughout the liquid have enough energy to form bubbles of vapor. Almost all of the attractions between particles are overcome, and the particles are now far apart. Boiling needs far more energy than melting, since the particles must be separated almost completely.
What happens during condensation and freezing?
When gas particles cool, they slow down. Attractive forces can then pull them together into a liquid, which is condensation. The particles release energy as they come closer. In freezing, the liquid particles lose more energy and slow down until the attractions hold them in fixed positions, forming a regular solid. Energy is again released to the surroundings.
Why does temperature stay constant during a change of state?
Temperature measures the average kinetic energy of the particles. During a change of state, energy supplied goes into changing the potential energy of the particles by pulling them apart against the attractions between them. The kinetic energy and hence temperature do not change. For this reason, the heating curve is flat during melting and boiling. When cooling, the energy released as attractions form is exactly the energy that is removed, so the temperature stays constant until the change is complete.
Particle explanations of all changes
| Change | Particle explanation | Energy |
|---|---|---|
| Melting | Vibrations overcome fixed positions | Absorbed |
| Freezing | Particles settle into fixed positions | Released |
| Evaporation | Fast surface particles escape | Absorbed |
| Boiling | Particles throughout separate into vapor bubbles | Absorbed |
| Condensation | Slowing gas particles are pulled together | Released |
| Sublimation | Solid particles escape directly as gas | Absorbed |
| Deposition | Gas particles settle directly into a solid | Released |
Worked example: a heating curve
How much energy is needed to heat 10.0 g of ice from −10 °C to steam at 110 °C? Use these values: c(ice) = 2.09 J/(g·°C), Lf = 334 J/g, c(water) = 4.18 J/(g·°C), Lv = 2,260 J/g and c(steam) = 2.01 J/(g·°C).
| Stage | Calculation | Energy (J) |
|---|---|---|
| 1. Warm ice from −10 °C to 0 °C | 10.0 × 2.09 × 10 | 209 |
| 2. Melt ice at 0 °C | 10.0 × 334 | 3,340 |
| 3. Warm water from 0 °C to 100 °C | 10.0 × 4.18 × 100 | 4,180 |
| 4. Boil water at 100 °C | 10.0 × 2,260 | 22,600 |
| 5. Warm steam from 100 °C to 110 °C | 10.0 × 2.01 × 10 | 201 |
| Total | 209 + 3,340 + 4,180 + 22,600 + 201 | 30,530 |
The total is 30,530 J, or about 30.5 kJ. Boiling takes by far the largest share (74%), because the particles must be completely separated: 22,600 ÷ 30,530 × 100 = 74.0%. This large energy is why steam is so effective at transferring heat and why it can cause severe burns.
How did scientists explain changes of state?
In the 1760s Joseph Black noticed that melting ice absorbs heat without a rise in temperature, and he coined the idea of latent heat (“hidden heat”). The kinetic theory in the 19th century explained this by the energy needed to separate particles. In 1873 Johannes van der Waals proposed an equation for real gases that included attractions between particles, which provided a quantitative link between the behavior of gases and liquids and their condensation.
Which mistakes are common?
Mistake 1: Saying that particles expand or change in size during melting
Particles stay the same size. Their motion and spacing change.
Mistake 2: Thinking that the heat added during melting disappears
It is stored as potential energy in the liquid, and it is released again when the liquid freezes.
Mistake 3: Believing that bonds inside molecules break when water boils
Only the attractions between molecules are overcome. The molecules remain H2O.
Mistake 4: Forgetting that evaporation happens below the boiling point
Fast-moving particles at the surface can escape at any temperature.