Particles of matter — Chemistry, 11–13 years
Matter may look still and continuous, but it is made of tiny particles that are always moving. This model helps explain solids, liquids and gases.
The particle picture
Imagine matter as a huge crowd of tiny particles. In a solid they stay close and mainly vibrate, in a liquid they slide past one another, and in a gas they spread out and move freely. The particles are too small to see directly.
Why use this model?
A solid ice cube can become liquid water and then invisible water vapour, even though the substance stays the same. The particle model explains this change: heating makes particles move more, while cooling makes them move less. It turns an invisible process into a picture we can reason with.
Heating water
Put 100 mL of water in a beaker and warm it. At room temperature, the particles are close together but slide past one another. Near boiling, they move much faster; at 100 °C, some escape as water vapour. The amount of water may fall, but its particles have not vanished.
The disappearing water
It is tempting to say that boiling water disappears, because the steam is hard to see. In fact, the particles have spread through the air as a gas. They are less visible, not gone; a cold window can make some return to liquid drops.
Smells travelling
When someone peels an orange, its smell can reach you across a room. Tiny scent particles leave the orange and spread through the air, moving between air particles. Perfume, cooking smells and the scent of rain work in a similar way, although the particles themselves remain unseen.
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