States of matter

Matter is particles, not a continuous blob. Everything that has mass and occupies space — the pencil, the water, the air — is built from enormous numbers of tiny particles (atoms or molecules) with gaps between them. Those particles are never sitting still and isolated: two competing effects act on them at once. An attractive force pulls neighbouring particles toward each other, and kinetic energy (motion, which increases with temperature) tends to fling them apart. Which state you get depends on which effect dominates.

Three states, one sliding scale. As you move from solid to liquid to gas, particle spacing grows, inter-particle attraction weakens, and particle motion becomes freer — all in step with each other, not independently:

• Solid — particles are packed close together in fixed positions. Attraction is far stronger than the particles' motion, so each particle can only vibrate about its own spot. Result: definite shape and definite volume.
• Liquid — particles sit farther apart than in a solid, with a moderate attraction: strong enough to hold the particles together as a fixed volume, but too weak to lock them into place, so they slide past one another. Result: no definite shape (it takes the shape of its container) but definite volume.
• Gas — particles are far apart with large empty gaps, and attraction is negligible. They move fast and randomly in every direction, colliding with each other and the container walls. Result: no definite shape and no definite volume — a gas expands to fill whatever space is available.

Why this explains squeezability. A gas's particles have large gaps between them, so there is plenty of empty space to push them into and a gas compresses easily. A liquid's particles are already almost touching, so a liquid resists compression strongly, and a solid's still tighter packing makes it the hardest of all to compress. Compressibility is not a separate fact to memorise — it falls straight out of particle spacing.

where: the inequality is a spacing/freedom ranking, not a numeric formula — it is the one-line summary examiners expect you to state and then justify with the particle picture above.

There is no formula to derive here — the topic is entirely explained by particle behaviour, not calculated with numbers. The goal is to see why the definite-shape/definite-volume rules follow logically from attraction and motion, so you can predict the behaviour of any unfamiliar substance rather than memorising the table by rote.

Step 1. Start from the two competing effects: inter-particle attraction pulls particles together, while particle motion (kinetic energy) pushes them apart. Neither effect ever switches off completely — every particle, in every state, is always attracting its neighbours a little and always moving a little.

Step 2. In a solid, attraction dominates so heavily that particles cannot leave their positions at all — they can only vibrate in place. Since the particles cannot rearrange themselves, the overall outer shape cannot change either. That is exactly what "definite shape" means: shape is fixed because the internal arrangement is fixed.

Step 3. In a liquid, the balance shifts. Attraction is still strong enough to keep particles from drifting far apart — so the total space they occupy (the volume) stays fixed — but it is too weak to hold them in fixed positions, so they can slide past one another. Sliding particles mean the outer boundary can flow and take on the shape of any container, while the volume inside that boundary stays the same.

Step 4. In a gas, motion wins outright. Attraction becomes negligible compared to the particles' speed, so nothing holds the particles at a fixed spacing. They spread apart until they meet a physical boundary (a container wall), which is why a gas has neither fixed shape nor fixed volume — it simply expands to occupy whatever space is available to it. This single attraction-versus-motion argument is what lets you reason out shape and volume for any solid, liquid, or gas, even one you have never studied before.

Question. A sample of iron filings, 50 mL of cooking oil, and a small balloon of helium gas are each transferred, one at a time, from a small test tube into a much larger, differently shaped jar. For each sample, state whether its shape changes, whether its volume changes, and explain both answers using the particle picture.

Step 1. Iron filings (solid). Moving the filings to the larger jar does not change their shape or total volume — a solid piece of iron keeps its own shape wherever it is placed. Strong inter-particle attraction locks the particles into fixed positions relative to each other, so an ordinary container change cannot rearrange them: shape unchanged, volume unchanged.

Step 2. Cooking oil (liquid), 50 mL. Poured into the larger jar, the oil spreads out and takes on the base shape of the new container instead of the tall, narrow shape of the test tube — so shape changes. But however the jar is shaped, the oil still measures exactly 50 mL, because moderate attraction between its particles keeps them together as a fixed total volume even while they slide past each other into a new outer form: volume unchanged.

Step 3. Helium gas (gas). Released into the much larger jar, the gas does not stay confined to a 50 mL-sized region in one corner — it spreads out and fills the entire jar, however large. Both shape and volume change, because helium's particles have negligible attraction and move fast enough to occupy all the available space, however much space that is.

Answer: the iron filings show no change in shape or volume (solid), the oil changes shape but keeps its 50 mL volume (liquid), and the helium gas changes both shape and volume, expanding to fill the jar completely (gas). The single test that separates all three: does the sample's outer shape change with the container (no for a solid, yes for a liquid and a gas), and does its volume change with the container (no for a solid or a liquid, yes only for a gas).

• Thinking a gas is empty space with no matter in it — a gas is made of real particles with mass; they are simply far apart and moving freely, not absent.
• Confusing "takes the shape of the container" with "has no fixed volume" — a liquid does the first but not the second; only a gas does both.
• Believing solid particles are completely motionless — they vibrate continuously about a fixed position; they just cannot travel from place to place.
• Treating particle spacing as the only difference between states — inter-particle attraction and freedom of motion matter just as much, and all three change together as you move from solid to liquid to gas.

Ask the tutor: Want to see how heating or cooling actually moves a substance between these three states — ask the AI tutor to walk you through what happens to the particles during melting, boiling, and freezing.