ds1 has been teaching me science (basic chemistry) and it blows my mind

[roisin]

I have a very sketchy background in even basic science with very many gaps.

Did you know that if a solid dissolves into a liquid the volume of the water doesn't increase? So you can stir 4 big spoons of salt into, say, 300 ml of water and it will still be 300 ml. This is because the particles of salt fit in the gaps between the water particles. I think this is fascinating.

But if you do it with sand the volume of water does increase because the solid doesn't dissolve.

Now is this because the sand particles are too big? or too heavy? or what?

And by 'particles' does he mean molecules? ie water molecules would be H2O ie 3 atoms?

I've been looking on google to try and find some diagrams of these things, or electron-microscope pictures, but with no real success. Does anyone have any?

Molecules are any 2 atoms joined together.

so O2 is a molecule, because it is 2 atoms joined together, but it is still an element because it only consists of one type of atom

H2O is also a molecule, because it cosists of more than one atom, but in this case it is a compound, because it consists of more than one element

Re pictures. the things that you have been talking about are really tiny, but here are some lines to bigger molecules...DNA!

Here

The sand particles don't dissolve because they are insoluble. Their atoms are held together by covalent bond and not ionic bonds.

With ionic bonds, as in salt, the NaCl actually breaks up into sodium and chloride ions, so are small enough to fit in the gaps in the water. The sand molecules don't break up so are always too big, and therefore the volume increases when you add it to water.

Particles mostly means molecules.

BBC Bitesize should have good graphics.

Thanks MB.
So what he's talking about - the salt particles fitting between the gaps in the water particles: is that actually talking about molecules and the gaps between the molecules?

It seems to go against what I thought I knew about physics, archimedes and all that.

We got the jug and salt out in the kitchen!

I just find learning new stuff so exciting

and the absolute effect of the thing that you disolve (the solute) on the solven will depend on the characteristics of the solute

water is wonderfully wierd stuff.

When a salt disloves it breaks into two charged particles (as SP explained) the water molecules will actually be drawn towards those charged particules and 'stick' to them by weak hydrogen bonds.

Hydrogen bonds are ace things, and life on earth as we know it can only exist because of them!!!