Is there an Ideal Liquid Law? Or Solid Law? There is an Ideal Gas Law, but why isn't there one for liquids or solids? Is it because they are much too hard to predict or that solids and liquids vary drastically in their reaction to temperature and pressure then do gases, thereby nullifying the need for a Law that could apply to all?
 A: In gases, under normal conditions, the average distance between molecules is large compared to size of the molecules so the molecules spend most of their time far apart. Interactions between molecules, or at least strong interactions between molecules, tend to be short range. This means that interactions between molecules don't have much effect on the overall properties of the gas because the molecules spend most of the time too far apart to interact strongly with each other. This means that to a good approximation we can ignore interactions between molecules completely, and this is what we mean by an ideal gas. An ideal gas is one where the gas particle do not interact.
So we don't have to worry whether the gas is made up from helium, or oxygen, or chlorine or whatever. As long as the molecules spend most of their time far apart the gas will behave as an ideal gas so all gases behave pretty much the same way. All we need to do is correct for the mass of the gas molecules, which is a straightforward numerical factor.
The trouble is that in liquids and solids the molecules are very close together. In fact in solids they are touching and in liquids they aren't far off that. Because the molecules are so close the interactions between them are strong, and therefore can't be ignored. That's why, for example, solid iron is very different to solid paraffin wax. Since we can't ignore interactions in liquids and solids there can be no universal ideal liquid or ideal solid equations that all liquids and solids obey.
