Ice, water, steam — the same stuff wearing three completely different personalities. What changes isn’t the water itself; it’s how the tiny particles are arranged and how hard they’re jiggling. Nail that one idea — the kinetic theory — and every property of solids, liquids and gases stops being a list to memorise and starts falling out of a single picture.
📚 What you need to know
The three states are solid, liquid and gas. They differ in how their particles are arranged, spaced, and moving, and in how much energy those particles have.
The kinetic theory of matter models every particle as a tiny sphere in constant motion.
Solid: particles locked in a fixed lattice, only vibrating on the spot. Low energy. Fixed shape & volume, very hard to compress, high density.
Liquid: particles still touching but randomly arranged, able to slide past each other. Medium energy. Fixed volume, but no fixed shape.
Gas: particles far apart (spacing about 10× that of a solid or liquid), random, whizzing in all directions. High energy. No fixed shape or volume, easily compressed, very low density (about 1000× less).
What decides the state is a tug-of-war between the particles’ energy and the intermolecular forces pulling them together.
One model, three states
Everything on this page comes from a single idea: the kinetic theory of matter. Picture every particle as a tiny sphere that is always moving — yes, even inside a rock-solid block of ice. What makes ice, water and steam behave so differently isn’t the particles themselves; it’s a tug-of-war between two things:
the energy of the particles (how much they jiggle and move), and
the intermolecular forces quietly pulling them together.
When the forces win, particles lock into place and you get a solid. Hand the particles more energy and they start to break free: first enough to slide over each other (liquid), then enough to fly apart completely (gas). That’s the whole story — everything else is just detail.
Here’s the mindset that saves you: don’t learn three separate lists of facts. Learn to track just two dials — how much energy the particles have, and how strong the forces between them are. Turn the energy dial up and you march solid → liquid → gas. Every property in the table below is a knock-on effect of those two dials.
The same particles in three arrangements: a fixed lattice that only vibrates (solid), close but disordered and free to flow (liquid), and far apart and fast-moving (gas).
Solids
In a solid, the forces win easily. The particles are pulled into a neat, repeating pattern called a lattice, packed tightly with no real gaps between them. They can’t travel — they’ve only got enough energy to vibrate on the spot, like people jammed shoulder-to-shoulder in a crowd, jiggling but going nowhere.
Because the particles are locked in place, a solid:
has a fixed shape (though a big enough force can still bend or break it)
has a fixed volume
is very hard to compress — there’s almost no empty space to squeeze out
has a high density — lots of mass packed into a small space.
Liquids
Warm a solid up and you hand its particles more energy. Eventually they can wriggle free of their fixed spots — but not escape each other entirely. In a liquid the particles are still close together (almost touching), yet now randomly arranged and able to slide and roll past one another. That sliding is exactly why liquids flow.
So a liquid:
has no fixed shape — it takes the shape of whatever holds it
still has a fixed volume — pour it between cups and the amount doesn’t change
is hard to compress — there’s still very little empty space
has a density between a solid and a gas (usually close to the solid).
Water is the famous rule-breaker: its solid (ice) is actually less dense than its liquid, which is why ice floats. For almost every other substance the solid is the densest form. Don’t let water trick you into thinking that’s the general rule — it isn’t.
Gases
Give the particles even more energy and they finally win the tug-of-war outright. In a gas the particles break away from each other and spread out. The average gap between them is roughly 10 times bigger than in a solid or liquid, so a gas is mostly empty space. The particles zoom around in all directions at a range of speeds, bouncing off each other and off the walls of their container.
That means a gas:
has no fixed shape and no fixed volume — it expands to fill whatever container it’s in
is easy to compress — all that empty space can be squeezed
has a very low density — roughly 1000× smaller than the solid or liquid it came from.
The three states side by side
Here’s the whole comparison on one grid. Notice how every column is really just a consequence of the arrangement and energy in the top two rows.
Every property below the top rows follows from how the particles are arranged and how much energy they carry.
Why a gas is about 1000× less dense
That “1000×” isn’t a random fact to memorise — it drops straight out of the spacing. Density is just how much mass you cram into a chunk of space. The particles in a gas weigh the same as they did in the liquid; they’ve simply spread out. If each particle now sits in a “box” whose edges are about 10 times longer, that box is 10³ = 1000 times bigger — so the same mass is smeared over 1000× the volume, and the density drops by the same factor.
Stretch the spacing between particles by 10× in every direction and the volume each one occupies grows by 10³ = 1000× — so the density falls by about 1000×.
WE 1
In a solid or liquid the particles are packed right up against each other. In a gas the average spacing between particles is about 10 times larger. Estimate how many times less dense the gas is than the liquid it came from.
Density = mass squeezed into a volume. Same particles, so compare the space each one takes up.Each particle now sits in a “box” with edges about 10× longer.volume of box ∝ (edge)³ = 10³ = 1000Same mass spread over 1000× the volume → density is 1000× smaller.gas ≈ 1000× less denseThat’s exactly why data booklets list gas densities about 1000× below solids and liquids — e.g. liquid water ≈ 1000 kg m⁻³, steam ≈ 1 kg m⁻³.
WE 2
A sample has a fixed volume and is very hard to compress, yet it flows and takes the shape of its container. Which state is it, and what does each clue tell you about the particles?
Fixed volume + hard to compress → particles are packed close, almost touching (little empty space).Flows / takes the container’s shape → particles are NOT locked in place; they can slide past one another.It’s a liquidClose-packed like a solid, but free to move like a gas — the particles have enough energy to partly overcome the forces holding them, but not enough to break away completely.
🔧 Spotting a state from its clues
Shape? Fixed = solid. Takes the container’s shape = liquid or gas.
Volume? Fixed = solid or liquid. Fills all the available space = gas.
Compress it? Barely budges = solid or liquid (no gaps). Squashes easily = gas (big gaps).
Density? High = solid or liquid. About 1000× lower = gas.
Picture the particles. Locked lattice, sliding-but-touching, or far-apart-and-flying — that pins down the rest.
SOLID fixed lattice
add energy melt →
LIQUID random, close
add energy boil →
GAS far apart
Quick recap: Same particles, different arrangements. Solids = fixed lattice, just vibrating (fixed shape & volume, dense). Liquids = close but random, free to flow (fixed volume, no fixed shape). Gases = far apart and fast (no fixed shape or volume, ~1000× less dense). Add energy and you loosen the forces’ grip, moving solid → liquid → gas.
💡 Top tips
One model explains all three. Don’t memorise three separate lists — track two things: the particles’ energy and the forces between them.
“Close-packed” is not the same as “fixed pattern”. Both solids and liquids are close-packed; only the solid has a fixed, ordered lattice.
Shape and volume are separate questions. A liquid has a fixed volume but no fixed shape — keep them apart.
Gas density ~1000× smaller comes straight from the ~10× spacing (10³). A tidy number to quote in an answer.
Temperature is coming next. “More energy” will soon become “higher temperature” — this page is the foundation for all of it.
⚠ Common mistakes
Saying liquids have “lots of space” between particles — they’re close-packed, just disordered; it’s gases that have the big gaps
Claiming a liquid has no fixed volume — it does; only its shape changes to fit the container
Writing that solid particles “don’t move” — they vibrate constantly, just around fixed positions
Mixing up shape and volume in the comparison table — a classic dropped mark
Assuming the solid is always densest — usually true, but water/ice is the famous exception
You’ve now got the particle picture that the whole of thermal physics is built on. The obvious next question: solids and liquids are both “dense” — but exactly how dense? Time to put a number on it. Next up: density, mass packed into a volume, plus the neat trick of finding the volume of awkward shapes before you divide.
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