Scientists believe that all matter is made of tiny pieces that they call particles.

Matter can exist in three states: solid, liquid and gas.

The particles in each state are arranged differently.

• In a solid the particles are arranged in an orderly way, all touching, very close together.

• Solids are dense because of this close-packing

• In a liquid all the particles are still touching but are a little further apart. 

• There are spaces between some particles because the arrangement is not orderly but liquids are still dense because the particles are still close.

• In a gas, none of the particles are touching. They are all very far apart.

• So gases are not dense.

• And, unlike liquids and solids, they can be squashed in to smaller spaces by pushing their particles closer - they are compressible.

We call the particle model Kinetic Theory because all particles can move, depending on how much energy they have.

• Particles in a solid have the least energy.

• They can't change place but they can vibrate.

• So they have a fixed shape and volume - in other words, if you pick up a single piece of a solid and move it, it doesn't get any larger or smaller or change in any way (unless you break it).

• In a solid the particles are arranged in an orderly way, all touching, very close together. Solids are dense because of this close-packing

• In a liquid all the particles are still touching but are a little further apart. 

• There are spaces between some particles because the arrangement is not orderly but liquids are still dense because the particles are still close.

• In a gas, none of the particles are touching. They are all very far apart.

• So gases are not dense.

• And, unlike liquids and solids, they can be squashed in to smaller spaces by pushing their particles closer - they are compressible.

• Gases have no fixed shape and no fixed volume. In other words, if you burst a Helium balloon the Helium particles drift apart until they are evenly spread around the room however large it is and whatever its shape. This is diffusion. 

Liquids diffuse too but, because the particles can only slip slowly over each other in a liquid, diffusion is much slower than in gases. Solids cannot diffuse because their particles do not change place.

Forces of attraction between neighbouring particles in solids keep them from splitting apart.

The closer the particles the stronger these forces.

• When you heat a solid its particles vibrate more - this pushes its neighbours a little further apart, weaken weaken the forces and the solid expands.

• When particles are far enough apart the forces are weak enough for particles to slip past each other - the solid has melted.

• Particles in a liquid don't all have exactly the same amount of energy.

• The fastest moving ones sometimes have enough energy to break the surface and escape as a gas - this is evaporation.

• When all the particles have enough energy to overcome the forces the liquid is boiling - all turning to gas at the same time.

• If you cool down a gas the particles lose energy and slowly come together to condense back into a liquid.

• Further cooling takes away enough energy so that the particles can't move about any more and so they become a solid again - it freezes.

• A few substances can turn straight from a solid to a gas without melting into a liquid. We call this sublimation.

• The substances then turn straight back to a gas when cooled - we call this deposition.

Different substances melt at different temperatures because forces of attraction between particles are different for each substance.

Substances with very strong forces will have high melting and boiling points - eg diamonds, titanium, or salt.

Substances with very weak forces will have very low melting and boiling points - eg Hydrogen, Nitrogen, Helium etc