What Are the States of Matter? Solid, Liquid, Gas
Ice, water and steam are all H₂O. Same molecule, same formula, wildly different behaviour — one you can stand on, one you can pour, one that fills the room. Nothing about the molecule changed. What changed is how the molecules are arranged.
The short answer: the states of matter are the physical forms a substance can take — solid, liquid and gas are the three you meet first, with plasma as the fourth. They differ only in how tightly the particles are packed and how freely they move, not in what the particles are.
What a "state" actually describes
Every state question comes down to a tug-of-war between two things:
- Attractive forces pulling particles together (the intermolecular forces between molecules, or the bonds holding a giant structure).
- Kinetic energy — the movement energy of the particles, which rises with temperature and pushes them apart.
Heat something up and you give the particles more kinetic energy. When that energy wins, the state changes. Cool it down and the attractions win again. That's the whole story, and it's why the same substance can be all three states at different temperatures.
The three states side by side
| Feature | Solid | Liquid | Gas |
|---|---|---|---|
| Particle arrangement | Fixed, regular, tightly packed | Close together but disordered | Far apart, random |
| Particle movement | Vibrate in place | Slide past each other | Fast, free, in all directions |
| Shape | Fixed | Takes the shape of its container | Fills the whole container |
| Volume | Fixed | Fixed | Changes to fill the space |
| Compressible? | Barely | Barely | Yes, easily |
| Density | Usually highest | High | Very low |
The volume rule is the one worth memorising: solids and liquids have a fixed volume; gases don't. That's because a gas is mostly empty space, so squeezing the particles together is easy. Squeezing a liquid is nearly impossible — the particles are already touching.
Solids: locked in place
In a solid, particles are held in fixed positions and can only vibrate. That's why solids hold their shape and resist being squashed.
What holds them there depends on the substance. In ice, hydrogen bonds between water molecules. In table salt, the electrostatic pull between Na⁺ and Cl⁻ ions. In copper, the sea of delocalised electrons that also makes metals conduct and bend — the reason a metal spoon and a salt crystal are both solids but behave nothing alike when you hit them with a hammer.
Liquids: close but mobile
In a liquid, particles still touch but have enough energy to slide past one another. So a liquid keeps its volume — 100 mL of water stays 100 mL — but flows into whatever shape the container gives it.
This is also why liquids have surface tension and can form droplets: the particles are still attracting each other, just not locked down.
Gases: far apart and fast
In a gas, particles have broken free of each other almost entirely. They travel in straight lines until they hit something, and there's a lot of empty space between them.
That empty space explains everything gases do. They spread out to fill a room (diffusion). They can be compressed into a cylinder. They push on the walls of their container — that's pressure. And they're roughly a thousand times less dense than the same substance as a liquid.
What about plasma?
Plasma is the fourth state: a gas heated so far that electrons are stripped from their atoms, leaving a mix of free electrons and positive ions. It conducts electricity and responds to magnetic fields.
It sounds exotic, but plasma is the most common state of ordinary matter in the universe — stars are made of it. Closer to home, it's what glows inside a neon sign, a fluorescent tube and a lightning bolt.
(Physicists also study a fifth state, the Bose–Einstein condensate, which forms within a hair of absolute zero. You won't be examined on it, but it's a good pub fact.)
Worked examples
Predict the state before you read the answer.
- Oxygen at room temperature: gas. Its molecules are nonpolar with only weak attractions, so ordinary room-temperature energy is more than enough to keep them apart.
- Water at −10 °C: solid. Below 0 °C the hydrogen bonds win and lock the molecules into ice.
- Mercury at 20 °C: liquid. It's a metal, but its metallic bonding is unusually weak, so it melts at −39 °C — the only metal that's liquid at standard room temperature (caesium and gallium melt just above it, at around 29 °C).
- Carbon dioxide at −100 °C and 1 atm: solid (dry ice). It never becomes a liquid at normal pressure.
- The inside of a fluorescent tube when it's lit: plasma.
Common mistakes to avoid
- Thinking a state change makes a new substance. Melting ice gives you water, not something new. The molecules are identical — only their arrangement changed. That makes it a physical change, not a chemical one.
- Saying gas particles "expand" when heated. The particles don't get bigger. They move faster and spread further apart, so the gas occupies more volume.
- Assuming solid always means dense and hard. Aerogel is a solid that's 99% air. Wax is a solid you can dent with a thumbnail. "Solid" describes fixed positions, not toughness.
FAQ
What are the 3 states of matter?
Solid, liquid and gas. They differ in how closely packed the particles are and how freely they move — solids hold shape and volume, liquids hold volume only, gases hold neither.
Is plasma a state of matter?
Yes — it's counted as the fourth state. It forms when a gas gets hot enough for electrons to break away from atoms, producing a mixture of ions and free electrons that conducts electricity.
Why do gases fill their container but liquids don't?
Gas particles have enough energy to overcome the attractions between them, so nothing stops them spreading out. Liquid particles are still held together by intermolecular forces, so they keep a fixed volume and just flow to the bottom.
Does changing state change the chemical formula?
No. Ice, water and steam are all H₂O. Only the arrangement and energy of the molecules change, which is why melting and boiling are physical changes.
The takeaway
The three states aren't three different kinds of stuff — they're three settings on the same dial. Turn the temperature up and particles gain enough energy to break free of one another: fixed positions become free-flowing, and free-flowing becomes far apart. Learn to picture the particles and you can predict the state, the density and the compressibility without memorising a single table.
Foundations → What Is an Atom? The Building Block of Everything and Intramolecular vs Intermolecular Forces, which explain the forces doing the holding. Why metals are the solids they are → What Is Metallic Bonding? The Sea of Electrons. Next up → [What Is a Phase Change?] — what happens at the moment one state becomes another.
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