What holds particles together?

The idea

Constituent particles are held together by attractive interparticle forces whose strength depends on the substance and the interparticle distance, falling drastically as distance increases; this strength decides a substance's physical state.

A piece of chalk

A piece of chalk is made of a huge number of tiny particles. Yet it does not fall apart into powder by itself.

What keeps its particles together?

• A pull (attraction) between the particles
• Glue between the particles
• The air pressing on the chalk

Pull them apart

Move the two particles apart, one small step at a time. Watch the pull between them.

Interparticle attractions

Constituent particles are held together by attractive forces called interparticle attractions. Their strength depends on the nature of the substance and the interparticle distance. Even a slight increase in distance decreases them drastically. The strength of these forces decides the physical state of a substance.

• State | Interparticle attraction
• Solid | Strongest
• Liquid | A little weaker than in a solid
• Gas | Weakest (negligible)

Notes

Particles are held together by interparticle attractions. Their strength depends on the substance and the distance, and falls drastically as the distance increases. It decides the physical state.

Check yourself

Two particles move a little farther apart. What happens to the attraction between them?

The strength of the interparticle attraction depends on...

What decides whether a substance is a solid, a liquid or a gas?

• It stays the same. Even a slight increase in distance decreases it drastically.
• It increases. The attraction gets weaker, not stronger, as the distance increases.
• It decreases a lot — correct. Yes. Even a slight increase in distance decreases it drastically.
• only the colour of the substance. Colour does not decide it. The substance and the interparticle distance do.
• the nature of the substance and the distance between the particles — correct. Yes. Both decide how strong it is.
• only the size of the container. The container does not decide it. The substance and the interparticle distance do.
• The strength of the attractions between its particles — correct. Yes. This strength decides its physical state.
• How heavy the substance is. Heavy and light things can be in any state. The strength of the attractions decides it.
• The shape of the container it is kept in. A container only holds the substance. The strength of the attractions decides the state.