Kinetic Theory Summary for Y9

Easy stuff for year 9 Chemists and Physicists

• Atoms = tiny particles 1 x 10-10m diameter (100 billion across a small fingernail)
• Molecules = groups of atoms bonded (chemically stuck) together
• Elements = unique number of nucleons[1], electrons, 118 in all (smallest H)
• Compounds = 2 or more elements bound together chemically (Fe2O3, NaCl, H2SO4)
• Mixtures = substances not chemically bound occupying the same space (Fe filings, sand)

Kinetic Theory

Three states of matter – solids, liquids and gases. (there are 5 actually, but we only need 3 of them)

Matter made up of very small particles in constant random motion. Accounts for diffusion and Brownian Motion. [Clicking on either of these links takes you to another post on this website. Scroll down to see them.]

This little applet shows what happens when you take the lid off a perfume bottle. There’s no way of stopping it, so just watch for a bit, then go back.

In solids the particles are packed very closely together. They vibrate (as if connected by tiny springs) about fixed positions and have strong forces of attraction between them.

 Solids : have a high density can not be compressed do not flow have a fixed shape have a fixed size

In liquids the particles are close together but not as close as they are in solids. They can move around in any direction and are not fixed in position. The forces of attraction between them are still quite strong but, again, not as strong as in solids.

 Liquids : have a medium density can not be compressed can flow have the shape of their container have a fixed size

In gases the particles are very far apart with large distances between them. They move around very quickly in all directions and the forces of attraction between them are very, very weak.

 Gases : have a very low density can be compressed can flow have the shape of their container have the size of their container

You can change the state of a substance by heating or cooling it.

When the change is from a liquid to a solid it is called freezing.

When the change is from a gas to a liquid it is called condensing.

When the change is from a solid to a liquid it is called melting.

When the change is from a liquid to a gas it is called boiling or forced evaporation.

Here’s a summary…

[1] Nucleons – the number of protons plus neutrons in the nucleus – the very tiny bit in the middle