Squash a gas into a smaller volume and its particles hit the walls more often: the pressure goes up. Solids and liquids hardly change.
Particles in the solid are drawn paler, so you can tell the two states apart at the melting point. The yellow particle leaves a trail: compare how far it gets in each state.
50 g of ice from −30 °C to steam at 130 °C at normal air pressure. Data: specific heat of ice 2.1, water 4.18, steam 2.0 J/(g·°C); latent heat of fusion 334 J/g, of vaporisation 2260 J/g. More power makes every part faster, but the shape stays the same.
Heating gives the particles more energy (red arrows); cooling takes energy away (blue arrows). Tap a change, or an arrow on the diagram, to see what the particles do and where you meet it in everyday life.
There are six: melting and freezing (solid ↔ liquid), evaporation or boiling and condensation (liquid ↔ gas), and sublimation and deposition (solid ↔ gas, skipping the liquid).
This is the diagram you draw and label in a test. Switch between the labeled diagram, a blank one to test your memory, and a quiz where you place each label yourself. You can also download both versions or print a label worksheet.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back. On a phone, swipe the diagram sideways to see every number.
Tap an item, then tap the box it belongs in. Tap Check when all nine are placed.
In a solid the particles are packed tightly in a regular pattern and only vibrate, so a solid keeps its shape. In a liquid they are still touching but can slide past each other, so a liquid flows and takes the shape of its container. In a gas they are far apart and move quickly in all directions, so a gas spreads out to fill any container and is easy to squash.
The hotter something is, the faster its particles move or vibrate: temperature measures their average kinetic energy. Heating a solid makes its particles vibrate harder until, at the melting point, they break free of the pattern. Heating the liquid makes them move faster until, at the boiling point, they escape completely and form a gas. Cooling reverses each step: condensation and freezing.
While ice melts, the temperature stays at $0\,^\circ\text{C}$ even though you keep heating. The energy is used to overcome the forces between the particles, not to make them move faster. This hidden energy is called latent heat: $Q = mL$. Boiling needs far more energy than melting ($2260$ J/g against $334$ J/g for water) because the particles must be separated completely, which is why the boiling plateau on a heating curve is so long.
Set water to $-30\,^\circ\text{C}$, then press Melting point and Boiling point and watch the particles. Switch to oxygen or iron: the same three states happen, only at very different temperatures. Try dry ice: at normal pressure it never becomes a liquid. Then open the Heating curve lab, press Heat and compare the lengths of the two flat parts.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | regular pattern, packed closely | random, close together | random, far apart |
| Movement | vibrate about fixed positions | slide past each other | move quickly in all directions |
| Forces between particles | strong | weaker | very weak |
| Shape | fixed | takes the shape of the container | fills the container |
| Volume | fixed | fixed | not fixed |
| Can be compressed? | no | hardly | yes, easily |
| Energy of particles | lowest | more | most |
Heat 50 g of ice from $-30\,^\circ\text{C}$ at a steady rate. The graph of temperature against time has five parts: A ice warms, B ice melts at $0\,^\circ\text{C}$ (flat), C water warms, D water boils at $100\,^\circ\text{C}$ (flat), E steam warms. On the sloping parts the energy speeds the particles up; on the flat parts it changes the state. A cooling curve is the same shape backwards, with flat parts for condensing and freezing.
| Step | Working (50 g of water) |
|---|---|
| Warm the ice from −30 to 0 °C | $Q = mc\Delta T = 50 \times 2.1 \times 30 = 3150$ J |
| Melt the ice | $Q = mL_f = 50 \times 334 = 16\,700$ J |
| Warm the water from 0 to 100 °C | $Q = 50 \times 4.18 \times 100 = 20\,900$ J |
| Boil the water | $Q = mL_v = 50 \times 2260 = 113\,000$ J |
| Warm the steam from 100 to 130 °C | $Q = 50 \times 2.0 \times 30 = 3000$ J |
| Total | $156\,750$ J $\approx 157$ kJ. Boiling alone is about 72% of it. |
Both turn a liquid into a gas. Evaporation happens only at the surface, at any temperature, and is faster when it is warm, windy and dry; because the fastest particles leave, it cools what is left (that is how sweating cools you). Boiling happens all through the liquid, with bubbles, at one temperature, the boiling point, which depends on the air pressure: water boils at about $70\,^\circ\text{C}$ on top of Mount Everest.
| Substance | Melting point | Boiling point | State at 20 °C |
|---|---|---|---|
| Oxygen | −219 °C | −183 °C | gas |
| Ethanol | −114 °C | 78 °C | liquid |
| Water | 0 °C | 100 °C | liquid |
| Iron | 1538 °C | 2862 °C | solid |
| Carbon dioxide | sublimes at −78.5 °C (no liquid at normal pressure) | gas | |
A substance is a solid below its melting point, a liquid between its melting and boiling points, and a gas above its boiling point.
Solid, liquid and gas. In a solid the particles are packed in a regular pattern and vibrate; in a liquid they are close together but move past each other; in a gas they are far apart and move quickly in all directions. (Plasma, found in stars and lightning, is often called the fourth state.)
Key takeaway: the same particles, arranged and moving differently.Melting (solid to liquid), freezing (liquid to solid), evaporation or boiling (liquid to gas), condensation (gas to liquid), sublimation (solid to gas) and deposition (gas to solid). Melting, evaporation and sublimation take energy in; the other three give energy out.
Key takeaway: three take energy in, three give it out.The energy being added is used to break the forces holding the particles in their fixed pattern (latent heat), not to make them move faster. Only when all the ice has melted does the temperature start rising again. The same happens at the boiling point.
Key takeaway: during a change of state energy changes the state, not the temperature.Evaporation happens only at the surface of a liquid and at any temperature. Boiling happens throughout the liquid, with bubbles of gas, and only at the boiling point.
Key takeaway: evaporation: surface, any temperature; boiling: everywhere, at the boiling point.The particles in a gas are far apart, so pushing them closer together is easy. In solids and liquids the particles are already touching, so there is almost no space to squash. Try the volume slider in the 3D lab.
Key takeaway: gases have lots of empty space between particles.Sublimation is a solid turning straight into a gas without melting. Dry ice (solid carbon dioxide) does this at −78.5 °C at normal pressure, which is why it gives off cold gas but never leaves a puddle. The reverse, gas to solid, is deposition, such as frost forming on a window.
Key takeaway: solid straight to gas; the reverse is deposition.Latent (hidden) heat is the energy needed to change the state of a substance without changing its temperature: Q = mL. For water the latent heat of fusion (melting) is 334 J/g and the latent heat of vaporisation (boiling) is 2260 J/g.
Key takeaway: energy for a change of state: Q = mL.Yes. Lower air pressure means the particles escape more easily, so water boils at a lower temperature (about 70 °C on top of Mount Everest). A pressure cooker raises the pressure, so the water boils at about 120 °C and food cooks faster. This simulation uses normal air pressure.
Key takeaway: lower pressure, lower boiling point.