A Whole Cylinder of Gas Squeezes Into a Carryable Tank

Because a gas is mostly empty space. Its particles are far apart with almost nothing holding them together, so pressure simply pushes them closer — and a large volume becomes a small one.

Try the same with water and nothing happens, because its particles are already almost touching. This page covers everything in the CBSE Class 8 Science chapter's second part: compressibility, particle motion and flow, melting and boiling, and predicting a change of state.

Because of how much space lies between the particles in each state.

The order of interparticle spacing is:

**gas $\gg$ liquid $>$ solid

- In a gas the particles are very far apart with negligible attraction. Pressing them together closes those gaps easily, so a gas is highly compressible.
- In a liquid the particles are much closer with moderate attraction, so a liquid is only very slightly compressible.
- In a solid the particles are closely packed with very strong attraction and almost no gaps, so a solid is practically incompressible.

Demonstrate it with a syringe. Block the nozzle with a finger and press the plunger:

- Filled with air, the plunger moves in a long way.
- Filled with water, it barely moves at all.

An LPG cylinder is the everyday proof. A very large volume of cooking gas is squeezed into a steel cylinder you can carry, and squeezed so hard that it is stored as a liquid inside. Compressed natural gas in a bus and the air in a cycle tyre are the same idea.

The order is what to state in an answer, and the reason with it: gases compress most because their particles have the largest spaces and the weakest attraction** — it is the spacing that is being reduced, never the particles themselves.

A substance flows when its particles can change places. Whether they can depends on how strongly they are held.

- Solid — particles only vibrate about fixed positions. They cannot move past each other, so a solid cannot flow and keeps its shape.
- Liquid — particles are free to slide over one another while staying in contact. So a liquid flows and takes the shape of its container, but keeps its volume.
- Gas — particles move freely and rapidly in all directions, colliding and rebounding. So a gas flows and spreads to fill the entire container.

Liquids and gases are together called fluids, because both can flow.

Raising the temperature makes the particles of any state move faster. A solid's particles vibrate more vigorously, a liquid's slide past each other more easily — which is why honey flows more readily when warmed and more stiffly on a cold morning.

The motion also explains diffusion. Gas particles moving rapidly in all directions spread through a room, which is why incense is smelt across it. Liquid particles move too but far more slowly, so a drop of ink takes minutes to colour still water, and a solid barely diffuses at all.

The point worth being exact about is that particles in a solid are not still. They are in constant motion — but that motion is vibration in place, not travel, which is precisely why a solid holds its shape while its particles never stop moving.

Heating supplies energy that makes the particles move faster and overcomes the forces of attraction holding them together.

Melting — solid to liquid. As a solid is heated, its particles vibrate more and more strongly. At the melting point the vibration becomes energetic enough to break the rigid arrangement, and the particles begin to slide over each other. Ice melts to water at $0°C$.

Boiling — liquid to gas. Further heating makes the liquid particles move faster still. At the boiling point they gain enough energy to escape the attraction of their neighbours altogether and move away freely. Water boils to steam at $100°C$.

Cooling reverses both. Removing heat slows the particles, so attraction pulls them back together — a gas condenses to a liquid, and a liquid freezes to a solid.

Watching a pan of ice on a stove shows the whole sequence: ice, then water, then steam, as energy is supplied step by step.

There is one boundary case the syllabus emphasises. During a change of state the temperature does not rise, even though heat is still being supplied. A mixture of ice and water stays at $0°C$ until the last of the ice has melted.

The reason is that the heat is being used to break the forces of attraction between the particles rather than to speed them up further. Only once every particle has been freed does the temperature begin climbing again — which is why an ice-and-water drink stays cold rather than warming gradually.

Decide which way the energy is going, then name the change with the correct term.

On heating, particles gain energy and move further apart:

- Solid to liquid — melting. Ice to water; wax to molten wax; ghee softening in summer.
- Liquid to gas — vaporisation (boiling or evaporation). Water to steam; wet clothes drying.
- Solid straight to gas — sublimation, skipping the liquid stage. Camphor and naphthalene balls disappear from a cupboard without leaving any liquid behind.

On cooling, particles lose energy and move closer:

- Gas to liquid — condensation. Steam to water; dew forming on a cold bottle taken from the fridge; mist on a window.
- Liquid to solid — freezing (solidification). Water to ice; molten wax setting.
- Gas straight to solid — deposition, the reverse of sublimation. Frost forming on a cold surface.

Worked predictions:

- Ice cream left out in the sun — melts, solid to liquid.
- A steel spoon in hot tea becoming warm — no change of state; the particles simply vibrate faster.
- A camphor tablet in a wardrobe — sublimes away, solid to gas.
- Water droplets on the outside of a cold glass — condensation of water vapour from the air, not water leaking through the glass.

That last one is worth stating plainly, because it is a genuine misconception. The glass is not porous and nothing escapes from inside it. Water vapour already present in the room touches the cold surface, loses energy and condenses on the outside — which is why the droplets appear only on a cold glass and never on a warm one.