If you have two solutions with different amounts of salt (or something else) dissolved in them, and you separate them by something that will let water through but not the salt, water will move towards the side with more salt in it. This is why and snails shrivel up if you poar salt on them.

If you put two liquids or gases next to each other, even if they are totally still they will mix with one antother slowly

Here we have two gases, air and bromine, sitting in a glass tube so they are perfectly still, and despite the fact that bromine is heavier than air, they slowly mix.

At start	30mins later	45mins later
from www.schoolscience.co.uk

What is happening?

Even when a gas or liquid is perfectly still the molecules that make it up are bouncing around. the hotter it is the more they are bouncing around. So over time the molecules bounce around and mix. This mixing is very slow and takes a long time, but if you leave the two gases next to each other for long enough they will mix perfectly. This effect is called diffusion and is why balloons go down slowly and how oxgen gets from your blood into your cells among many other processes.

Here is a model of what is going on, press start, and leave it for a little while and see what happens:

And now for something slightly more complex

Now there are some kinds of material that have an interesting effect on diffusion. Imagine putting a piece of material seperating the two types of molecule that will let one type of molecule through, but not the other. What is going to happen?

Find out with this model, the blue molecules (water) on the right can get through the dotted bit of material in the middle, but the red ones (salt) can't.

Some of the water molecules should have gone through the material, and then sort of got lost in amongst the red ones. Even though there is more stuff on the left hand side and there is actually a pressure pushing them back to the right (the water level on the left is higher) the water still moves to the left.

Even if there is some water amongst the salt more will come through the material

70% salt on left

This is called osmosis, the reason it is important is that all living cells are covered with these materials which will let water through but not salt or sugars ( they are called partially permiable membranes by the way), and for the same reason snails shrivel up when you pour salt on them.... oh and it drives lots of important biological processes.