What are the states of matter?
Everything around you is in a state of matter, set by how its tiny particles are arranged and how fast they move. The three everyday states are solid, liquid, and gas, and a substance moves between them as you add or remove heat. The interactive states of matter diagram above is the heating curve of water: drag heat in and watch ice warm, melt, warm again, and boil, all from real numbers.
How the particles are arranged: solid, liquid, gas
The difference between the states is not the particles themselves (a water molecule is the same molecule in ice, water, and steam), but how they are arranged and moving:
| State | Particle arrangement | Motion | Shape and volume |
|---|---|---|---|
| Solid | Packed in a fixed, ordered lattice | Vibrate in place | Fixed shape, fixed volume |
| Liquid | Close together but disordered | Slide past each other (flow) | Fixed volume, takes container’s shape |
| Gas | Far apart, mostly empty space | Move fast and freely | Fills the whole container |
As you heat a substance, its particles gain energy and the arrangement loosens: a rigid solid becomes a flowing liquid, then a free gas. You can watch why this happens at the particle level, with real attractions and collisions, in the Particle Box.
How many states of matter are there?
Three are taught as the everyday states (solid, liquid, gas), but there are more:
- Plasma, a gas so hot its atoms lose electrons, is the most common state in the universe: stars, lightning, and neon signs are plasma.
- A Bose-Einstein condensate forms a hair above absolute zero, where atoms merge into a single quantum state.
So a fair answer is “three common states, four with plasma, and a few exotic ones under extreme conditions.”
The heating curve of water: watch the temperature stall
Add heat steadily to ice and plot its temperature, and you do not get a straight climb. You get five pieces with two flat shelves, the heating curve:
- The ice warms up (temperature rises).
- The ice melts at 0 °C: temperature holds flat.
- The liquid water warms up (temperature rises).
- The water boils at 100 °C: temperature holds flat.
- The steam warms up (temperature rises).
The two flat plateaus are the surprising part, and they are the heart of this lesson.
Reading the curve: what the slopes and the plateaus mean
There is one rule, and it answers most questions asked about this graph:
- Where the line slopes up, exactly one state is present, and it is warming up. Solid on the first rise, liquid on the second, gas on the third.
- Where the line is flat, two states are present at once, and one is turning into the other. Solid and liquid together at the melting point, liquid and gas together at the boiling point.
So a plateau corresponds to a change of state. The temperature holds steady the whole way across it, and only starts climbing again once the last of the ice has melted or the last of the water has boiled away.
| Piece of the curve | Slope | States present | Temperature | Energy goes into | Energy per gram |
|---|---|---|---|---|---|
| Warming the solid, -20 to 0 °C | positive | solid | rising | kinetic: particles speed up | 41 J |
| Melting at 0 °C | flat (zero) | solid and liquid | holds at 0 °C | potential: attractions break | 334 J |
| Warming the liquid, 0 to 100 °C | positive | liquid | rising | kinetic: particles speed up | 418 J |
| Boiling at 100 °C | flat (zero) | liquid and gas | holds at 100 °C | potential: attractions break | 2260 J |
| Warming the gas, 100 to 120 °C | positive | gas | rising | kinetic: particles speed up | 40 J |
Notice how uneven the pieces are. Boiling takes 2260 J against melting’s 334 J, about 6.8 times as much, which is why the upper plateau stretches so much further across the graph than the lower one.
What is the melting point, and what is the boiling point?
The melting point is the temperature at which a solid turns into a liquid, and it is exactly the same temperature at which that liquid freezes going the other way. The boiling point is where the liquid turns to gas throughout its bulk. Both are properties of the substance, and at both of them the two states sit side by side while the change happens.
For water at 1 atmosphere:
| Celsius | Fahrenheit | Kelvin | |
|---|---|---|---|
| Melting point (and freezing point) | 0 °C | 32 °F | 273.15 K |
| Boiling point (and condensation point) | 100 °C | 212 °F | 373.15 K |
Pressure matters for boiling: at altitude there is less air pressing on the liquid, so water boils below 100 °C, which is why cooking takes longer up a mountain. Melting points shift far less with pressure.
Why the temperature stops rising: latent heat
Temperature measures the average kinetic energy (the speed) of particles. On the sloped parts of the curve, added energy speeds the particles up, so the temperature rises. But during a phase change, the added energy does something else: it breaks the intermolecular forces holding particles together. That stored energy is potential energy, not motion, so the thermometer does not move. This hidden energy is called latent heat (“latent” means hidden).
The six changes of state
A change between states is a phase change (or change of state). There are six, in three reversible pairs. The forward direction absorbs energy (endothermic); the reverse releases it (exothermic):
| Change | Direction | Energy | Everyday example |
|---|---|---|---|
| Melting | solid → liquid | absorbed | An ice cube turning to water |
| Freezing | liquid → solid | released | Water turning to ice in a freezer |
| Vaporization | liquid → gas | absorbed | Water boiling into steam |
| Condensation | gas → liquid | released | Dew on cold grass at dawn |
| Sublimation | solid → gas | absorbed | Dry ice fogging without melting |
| Deposition | gas → solid | released | Frost forming on a cold window |
Sublimation and deposition skip the liquid entirely: dry ice (solid carbon dioxide) turns straight into gas, and frost forms straight from water vapor. These same phase changes, driven by the sun and the cold, power the water cycle.
The cooling curve: the same graph backwards
Take heat out instead of putting it in and you get the cooling curve. It is the heating curve mirrored: the same two plateaus at the same two temperatures, but now the changes run the other way and the energy is released rather than absorbed.
Steam cools, condenses at 100 °C, the liquid water cools, freezes at 0 °C, and the ice cools. The plateaus mean exactly what they meant before: two states present at once, temperature holding steady, energy moving into or out of the attractions between particles rather than into their speed. The only difference is the sign.
Specific heat vs latent heat: do the math
The heating curve is built from two kinds of energy, and you can calculate each:
- Sloped segments (specific heat): energy to warm a substance,
q = m·c·ΔT, wherecis the specific heat. For liquid waterc = 4.18 J/(g·°C). - Flat plateaus (latent heat): energy to change state with no temperature change,
q = m·L. For water the latent heat of fusion is 334 J/g and the latent heat of vaporization is 2260 J/g.
Worked example: how much energy turns 1 g of ice at −20 °C into steam at 120 °C? Add the five steps:
- Warm the ice: 1 × 2.06 × 20 = 41 J
- Melt it (0 °C): 1 × 334 = 334 J
- Warm the water: 1 × 4.18 × 100 = 418 J
- Boil it (100 °C): 1 × 2260 = 2260 J
- Warm the steam: 1 × 2.02 × 20 = 40 J
- Total ≈ 3093 J (about 3.1 kJ)
Boiling alone is 2260 J, far more than any other step. That is why the boiling plateau on the curve is so much longer than the melting plateau: it takes about 6.8 times more energy to boil water than to melt it.
Where states of matter connect
This lesson is the macroscopic energy view of matter: temperature, heat, and the phase changes between states. For the microscopic why (how attractions and particle speed decide the state), play with the Particle Box. For the gas phase in real depth (how pressure, volume, and temperature relate), see the gas laws. It’s free to embed on your own site or LMS.