The particle model
All matter is made of tiny particles: atoms, molecules or ions. The particle model (also called the kinetic theory of matter) explains the states of matter with three ideas. The particles are alwaysmoving. They attract each other when close together, and repel strongly if pushed into each other. And the hotter the substance, the faster its particles move on average.
Whether a substance is a solid, liquid or gas depends on a contest between those attractions, which try to hold the particles together, and their motion, which tries to separate them.
Solids, liquids and gases
| State | Particles | Properties |
|---|---|---|
| Solid | Close together, usually in a regular pattern; vibrate about fixed places | Fixed shape and volume; hard to compress |
| Liquid | Close together, irregular; slide past each other | Flows and takes the shape of its container; fixed volume; hard to compress |
| Gas | Far apart, random; move quickly in straight lines between collisions | Fills its container; easy to compress |
Notice in the simulator that the gap between neighbouring particles barely changes between solid and liquid, but jumps in the gas. That is why solids and liquids have similar densities, while a gas is far less dense.
Temperature and particle motion
Temperature is a measure of the average kinetic energy of the particles. Not every particle moves at the same speed: at any temperature there is a range, from slow to fast, and collisions constantly swap energy between them. Click a few particles in the simulator to see their different speeds. Heating raises the average; cooling lowers it. At absolute zero (0 K, −273.15 °C) particle motion would be at its minimum.
Changes of state
- Melting (solid → liquid): particles vibrate so strongly that they break out of their fixed places.
- Freezing (liquid → solid): particles slow down until attractions lock them into place.
- Evaporation (liquid → gas at the surface): the fastest particles at the surface escape, at any temperature. Because the fastest particles leave, evaporation cools the liquid left behind.
- Boiling (liquid → gas throughout): at the boiling point, particles throughout the liquid can escape, forming bubbles of gas inside it.
- Condensation (gas → liquid): slow gas particles stick together when they meet, forming droplets.
- Sublimation (solid → gas) and deposition (gas → solid) skip the liquid state entirely.
These are physical changes: the particles themselves are not changed, and no new substance is made. Melting ice and freezing water are the same water molecules, just arranged differently.
Energy during a change of state
Heat a pure solid and its temperature rises until it starts to melt. Then something surprising happens: the temperature stays almost constant until all of it has melted. The energy supplied is not making the particles faster; it is being used to overcome the attractions between them. This energy is calledlatent heat. The same happens during boiling, which needs much more energy because the particles must be separated completely. Water needs about 334 joules to melt each gram of ice, but about 2,260 joules to boil each gram of water. When a gas condenses or a liquid freezes, the same energy is released again.
The two energy bars in the simulator show this: the red bar is the motion energy (temperature), and the green bar is the energy stored by pulling particles apart. Watch the green bar jump during melting and especially during boiling.
Pressure and phase diagrams
State depends on pressure as well as temperature. Higher pressure pushes particles together, so it favours the denser states. A phase diagram maps which state a substance is in at every temperature and pressure. The lines mark the conditions where two states can exist together. At the triple pointall three coexist; below that pressure, a solid cannot melt, and it sublimes instead. For water the triple point is 0.01 °C at about 0.6% of normal air pressure, so ice normally melts. For carbon dioxide it is −56.6 °C at about 5 times normal air pressure, so dry ice sublimes on a kitchen table. This is also why water boils at only about 70 °C on top of Mount Everest, where the air pressure is about a third of that at sea level.
Key terms
- Kinetic energy
- Energy of motion. The average kinetic energy of the particles sets the temperature.
- Melting point
- The temperature at which a solid turns into a liquid (and a liquid freezes) at a given pressure.
- Boiling point
- The temperature at which a liquid boils throughout at a given pressure.
- Latent heat
- Energy taken in or given out during a change of state without a change in temperature.
- Triple point
- The single temperature and pressure at which solid, liquid and gas can coexist.
Real-world applications
- Sweating: evaporation of sweat removes the fastest water molecules and cools your skin.
- Refrigerators pump a fluid that evaporates inside (absorbing heat) and condenses outside (releasing it).
- Pressure cookers raise the boiling point of water above 100 °C so food cooks faster.
- Freeze-drying foods and medicines removes ice by sublimation at low pressure.
- Metal casting: metals are melted, poured into moulds and frozen into shape.
Common misconceptions
- “Particles in a solid don’t move.” They vibrate constantly; they just stay in place.
- “Particles expand when heated.” The particles stay the same size; the spaces between them change.
- “Evaporation only happens at the boiling point.” It happens at any temperature, more slowly when cold.
- “The bubbles in boiling water are air.” They are mostly water vapour.
- “Changing state is a chemical reaction.” It is a physical change; no new substance forms.
What the model simplifies
The simulator is a small molecular dynamics model: 100 identical round particles in two dimensions, moving by Newton’s laws with a standard Lennard-Jones attraction and repulsion. A thermostat plays the role of the heater, and weak gravity keeps the liquid at the bottom. Temperatures are in model units, chosen so that this model melts at about 35 and boils at about 70; they are not degrees Celsius and do not describe a particular substance. With so few particles, changes of state are spread over a range of temperatures rather than happening at one sharp temperature, boiling does not form proper bubbles, and the temperature does not stay perfectly constant during a change. Sublimation is real but far too slow to see in a model this small, so it is shown on the schematic phase diagram instead. Real molecules have shapes, and some (like water) form hydrogen bonds, which is why ice is less dense than liquid water: something this model cannot show.
Key takeaways
- States of matter differ in particle arrangement and motion, not in the particles themselves.
- Temperature measures the average kinetic energy of the particles, and speeds vary around that average.
- A change of state happens when particle motion overcomes, or gives in to, the attractions between particles.
- During melting and boiling, energy goes into separating particles, so the temperature stays nearly constant.
- Pressure also controls state; below the triple point a solid sublimes instead of melting.