Gaseous state
Release smoke inside a closed jar, or open a bottle of perfume in a corner of a room, and within moments the smell or the haze has spread everywhere, reaching even the farthest corner nobody aimed it at. That everyday magic trick is the gaseous state of matter, and this article stays close to it, asking exactly why gases behave so differently from solids and liquids. The CBSE Class 8 chapter "Particulate Nature of Matter" characterises solids, liquids and gases using three features: how far apart the particles sit, how strongly they attract one another, and how freely they can move. We will use those same three features to explain why a gas has no fixed shape and no fixed volume, why it spreads to fill any container instantly, and why it is so easily squeezed into a smaller space — reasoning purely from the particle picture, with no formula needed at this level.
A gas has particles so far apart, with negligible attraction between them, that they move freely through all available space — which is why a gas has no fixed shape or volume, spreads to fill any container, and is highly compressible.
The core idea
The defining behaviour. A gas is a state of matter that has no fixed shape and no fixed volume — whatever container you place it in, however oddly shaped, the gas expands until it occupies the entire space available.
The particle picture behind it. Three features, always used together, explain that behaviour:
- Interparticle space — particles in a gas sit very far apart from one another, much farther apart than in a solid or a liquid.
- Interparticle attraction — because the particles are so far apart, the attractive force between them is negligible (not exactly zero, but too weak to matter).
- Particle motion — gas particles move freely through all the space available to them, unlike solid particles (fixed in place) or liquid particles (confined to a limited region near their neighbours).
Why the three features add up to "no fixed shape or volume." With almost nothing holding gas particles back from one another, and with free motion through open space, there is no force keeping the particles together in any particular arrangement. So a gas cannot hold a shape of its own, and it cannot hold a fixed volume of its own either — it simply keeps expanding until it meets the walls of whatever container confines it, however large that container is.
Why a gas is compressible. Since gas particles are so far apart, there is a great deal of empty interparticle space between them. Pushing on a gas — for instance, pressing the plunger of a syringe filled with air — squeezes some of that space out, letting the particles crowd closer together, so the gas's volume shrinks noticeably. A liquid, by contrast, already has its particles close together with very little space left to squeeze, so it strongly resists compression.
Why it works
Every observable property of a gas — no fixed shape, no fixed volume, filling a container instantly, and easy compressibility — traces back to the same starting point: gas particles are far apart and barely attract each other.
Why gases fill a container completely. Because interparticle attraction is negligible, there is essentially nothing holding gas particles back from drifting apart from one another. Combined with their free motion in every direction, this means a gas released anywhere in a container keeps spreading outward, unopposed, until every available bit of space has some particles in it. Nothing stops the particles short of the container's walls, so the gas ends up evenly filling the entire volume, corner to corner.
Why a gas has no fixed shape or volume of its own. A solid holds its shape because strong attraction locks particles into fixed positions; a liquid at least holds its own volume because moderate attraction keeps particles loosely together even as they slide past one another. A gas has neither of these anchors — with attraction too weak to matter, the particles simply occupy whatever shape and volume the container gives them, expanding to fit a larger container and being compressed into a smaller one.
Why compressibility follows from interparticle space. The large gaps between gas particles are, in effect, unused space. An external push — such as pressing a syringe plunger — squeezes particles closer together, using up that space, so the gas occupies less volume than before. A liquid has almost no such gap to squeeze, which is exactly why liquids strongly resist compression while gases readily give way.
Worked example
Question: A syringe is filled with 20 mL of air and its opening is sealed with a finger. Pushing the plunger easily reduces the air's apparent volume inside the syringe to about 10 mL. A second syringe filled with 20 mL of water and sealed the same way barely moves when pushed the same way. Explain the difference using interparticle space in gases versus liquids.
Step 1. Identify what fills each syringe. The first syringe holds air, a mixture of gases; the second holds water, a liquid.
Step 2. Compare interparticle space. In the air-filled syringe, the gas particles are very far apart, leaving large interparticle spaces between them. In the water-filled syringe, the particles are already close together, as in any liquid, leaving only small interparticle spaces.
Step 3. Apply the push. When the plunger is pushed on the air, the external force closes up much of the large interparticle space between the widely-spaced gas particles, so the air's volume can shrink substantially, from 20 mL down to about 10 mL. When the same push is applied to the water, there is very little extra space left to squeeze out between the already-close particles, so the volume barely changes.
Step 4. State the conclusion. The large interparticle spaces in a gas are exactly why 20 mL of air compresses to about 10 mL under a plunger push, while the much smaller interparticle spaces in a liquid make 20 mL of water practically incompressible under the same push.
Common traps
- Thinking gas particles have zero attraction between them — the chapter describes the attraction as negligible, not exactly zero.
- Assuming a gas has "no volume" — a gas does occupy volume, filling whatever container holds it; what it lacks is a fixed volume of its own.
- Confusing the free motion of gas particles with the limited motion of liquid particles — gas particles move throughout all available space, while liquid particles move only within a limited region near their neighbours.
- Believing gases cannot be compressed because "gas is just empty space" — gases are highly compressible precisely because of the large interparticle spaces available to be reduced under pressure.
Ask the tutor: curious why perfume spreads across an entire room within minutes, or how this same particle picture explains why gases mix completely when two different gases share a container?
Why does a gas have no fixed volume, when a liquid at least keeps its own volume?
A liquid's particles still attract each other moderately, enough to stay loosely together even while sliding past one another, so a fixed amount of liquid keeps roughly the same volume regardless of container shape. A gas's particles barely attract each other at all, so nothing holds them together — they simply spread out until they fill whatever container they are in, however large. That is why a gas takes both the shape and the volume of its container, while a liquid takes only the shape.
If gas particles barely attract each other, why doesn't all the air in a room just float away?
Gravity, not interparticle attraction, is what keeps the air near the Earth's surface rather than drifting into space — that is a separate effect from the negligible attraction between individual gas particles that this chapter focuses on. Within a sealed container, the free motion of gas particles fills the container evenly in every direction, including upward and sideways, which is exactly what the smoke-in-a-jar activity demonstrates.
Is it true that gases have absolutely no interparticle attraction?
No, and this is a common trap. The chapter describes the attraction between gas particles as negligible, meaning it is far too weak to hold the particles together or give the gas a fixed shape or volume, not that it is precisely zero. Real gases do have very weak attractive forces between particles, which is part of why gases can eventually be cooled or compressed enough to turn into liquids, but at this level you only need to know the attraction is negligible.