Matter is made of tiny moving particles — Physical education, 11–13 years
A particle model helps us explain what happens inside solids, liquids and gases, even though we cannot see their particles directly.
A hidden crowd of particles
Every piece of matter is made from extremely small particles. In a solid they stay close and vibrate, in a liquid they move past one another, and in a gas they spread out and move freely. The material looks still, but its particles are not.
Why invent particles we cannot see?
People needed one picture that could explain why perfume spreads, why gases can be squeezed, and why liquids flow while solids keep their shape. The particle model grew from this problem: many visible effects had to be explained by an invisible structure. It is useful because one model links many observations.
Diffusion in a glass of water
Put one drop of food colouring into a glass of still water. First, the colour is concentrated in one small region. Next, coloured particles move between water particles and spread in all directions. After some time the colour is more even, although nobody has stirred it; random particle motion caused the change.
The particles do not disappear
A reasonable mistake is to think that particles vanish when a puddle dries or that new space appears when air is squeezed. What changes is their spacing and movement, not their existence. Water particles spread through the air as vapour, and air particles are pushed closer together in a smaller volume.
Explaining smells and sprays
The particle model helps explain why a smell can reach you across a room and why a spray spreads into the air. It also guides engineers who design filters, air cleaners and breathing equipment. The model is useful here because gases carry moving particles from one place to another.
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