KS3 · Chemistry

Particle model of gases

Seal a syringe's tip with your finger and push the plunger in. Will it move? That depends on what is inside, and why tells you what a gas really is.

Chemistry · Particle model

Same stuff, two pictures

Everything is made of tiny particles, too small to see. Switch between Liquid and Gas and notice what changes: how close the particles are, and how they move.

Selected state

Liquid

Arrangement

Random

Spacing

Touching

Motion

Around and over each other

Picture every particle touching its neighbours. They are arranged randomly and are free to move, but they can only shuffle around and over each other.

Watch out: The picture is a snapshot, so do not read anything into the exact positions. The ideas that matter are touching in a liquid, and far apart with gaps in a gas.

Chemistry · What is in the gaps?

Look at those gaps again

Picture a balloon full of helium. Inside it are helium particles with gaps between them.

Which of these is closest to what you think right now?
How sure are you?

Gas behaviour

?

Reason it through

Why does a gas fill its whole container, and escape if the lid is taken off?

Link 1 of 4

First link · your turn

How strong are the forces of attraction between gas particles?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Predict, then check

Two syringes, one full of air and one full of water. You seal the tip of each with a finger and push the plunger.

Which plunger can you push in?

Chemistry · Liquid or gas?

What do they share?

Sort each description. Tick Liquid, Gas, or both if it fits both. Then check.

  • A Liquid
  • B Gas
  1. Particles are arranged randomly
  2. Particles have enough energy to move past each other
  3. Particles are touching
  4. Spaces between the particles
  5. Particles move very quickly in random directions
  6. Can be compressed

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why a gas flows, fills its container and squashes, and why a liquid does not.

What you need to know

  • All substances are made of tiny particles, too small to see, and how the particles move depends on whether the substance is a solid, a liquid or a gas.
  • In a gas the particles are arranged randomly, are not touching, have gaps (spaces) between them and move very quickly in random directions.
  • Between gas particles there is nothing at all, just empty space. It is not air. The forces of attraction are very, very weak, and the particles have enough energy to be free of them.
  • Because the particles are free to move in all directions, a gas can be poured, can flow, fills the entire container and escapes if there is no lid.
  • A gas can be compressed because its particles are far apart with spaces between them. A liquid cannot, because its particles are already touching.

The big picture

In a gas, the particles are far apart with nothing but empty space between them, held by only very weak forces of attraction, and moving very quickly in random directions. That one picture explains why a gas can flow, fills its container, escapes from an open jar and can be compressed, while a liquid, whose particles are already touching, cannot.

Key points

1Gas particles are far apart, not touching, arranged randomly and moving very quickly in random directions.
2The gaps between gas particles are empty space, not air, and the forces of attraction between them are very, very weak.
3Weak forces plus enough energy mean the particles are free, so a gas flows, fills its container and escapes an open jar.
4Compress means to take up less space than before. Gases can be compressed; liquids cannot.
5Liquid and gas share a random arrangement and freedom to move. Particles move faster in a gas.

Worked example

Problem

Describe how the arrangement and movement of particles in a gas are different from a liquid.

⚠ Watch out

Saying there is air between the particles of a gas, or that gas particles do not attract each other at all. Between the particles there is only empty space, and the forces are very weak but not zero.

🧠

Memory hook

Gas = gaps. The gaps are empty space (not air), and they are the room a gas has to be squashed into.

✓

Check yourself

In one breath: how are the particles of a gas arranged and moving compared with a liquid, and what is the one thing between them?

Flashcards

(12)
What are all substances made of?
Tiny particles, too small to see with your eyes. They move differently depending on whether the substance is a solid, a liquid or a gas.
How are the particles arranged in a gas?
Randomly, with gaps (spaces) between them. They are not touching.
How do gas particles move?
Very quickly, in random directions.
What is between the particles of a gas?
Nothing at all, just empty space. It is not air.
What is the gas inside a helium balloon made of?
Particles of helium with spaces in between them.
How strong are the forces of attraction between gas particles?
Very, very weak, compared with the much stronger forces in a solid. They are not zero.
Why are gas particles free of the forces between them?
They have enough energy to be free of the attractive forces, so they are not pulled together.
Why can a gas flow and fill its container?
Its particles can move in all directions and are not fixed to other particles. With no lid, the gas escapes.
What does compress mean?
To make something take up less space than before.
Why can a gas be compressed?
Its particles are far apart with spaces between them. A force moves them closer together, so the gas takes up less volume.
Why can a liquid not be compressed?
Its particles are already touching, so they cannot be made to get any closer.
Which state has faster-moving particles, liquid or gas?
Gas. Particles move faster in the gas state than in the liquid state.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

More KS3 Chemistry topics

See the full KS3 Chemistry curriculum →

How this lesson was checked. This KS3 Chemistrylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 2 October 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.