[Goharshady]

This is just a simple question:
Why is Avogadro's law only for gases?
It seems really silly to ask such a question but the point is that it's really hard to answer it.

[rosco_pk]

It is because Avogadro's law deals only with ideal gases. So at equal volumes, pressures and temps, ideal gases will all contain the same ammount of molecules. I hope this helped. Someone else may also want to confirm or disporve this! This is my understanding of it anyway!

[Twitchy5]

What you said, Rosco, is correct, but I think he was looking for why it can't be used for solids, or liquids.
To Goharshady:
This is Avogadro's law:
V = k
n

V - Volume of container
k - Temperature in Kelvins
n - number of moles

It can't be used in solids because solids cannot fill their container.
Nor can it be used in liquids, because it has different properties than a gas; the molecules are more packed together than in a gas.

[Borek]

This is Avogadro's law:
V = k
n

V - Volume of container
k - Temperature in Kelvins
n - number of moles

So 1 mole of gas put in 1 L container has temperature of 1K?

You better reread the definition, as you are completely off.

[renge ishyo]

Avogadro's law only holds perfectly for ideal gases (and holds well for non-ideal gases at high temperatures and low pressure). The reason being that the law fails in the presence of intermolecular attractions as these can change the volume of a given gas so that it deviates from that of a gas of weaker intermolecular attractions at a given temperature and pressure. For example, suppose you have a mole of helium gas at say, 101⁰C and at 1 atm of pressure. The intermolecular attractions are very weak in Helium and it would occupy a certain volume. Now compare this to a mole of water as a gas at this same temperature and pressure. Avogadro's law would predict that since both gases are at the same temperature and pressure and both contain one mole of substance that they should have the same volume (as in V=nRT/P).

In reality, the water would occupy a smaller volume than the helium. Why? Because the intermolecular attractions between water molecules are stronger than those of helium. Each water molecule would pull those other water molecules around it towards itself to some extent to make the distance between molecules less than it would be in the ideal case. These interactions would tend to compress the gaseous water somewhat so that it's volume is smaller than that of helium when we measure it.  If we changed the conditions we could make this difference less and less (by raising the temperature and lowering the pressure so that, in both the water and the helium, the distance between species is so great that intermolecular attractions play a minor role), but Avogadro's Law would still only hold perfectly in the ideal case regardless.

[Goharshady]

Thanks everyone I think renge's reply was the best.
Since actually i wasn't looking for where the law came from. as someone said about PV=nRT only.
Thanks