Besides water, what matter can exist in three states? Thankz much

Almost every chemical compound you can think of can exist in all three states. There are some substances (like carbon dioxide, for example) that tend to make the transition from solid to gas directly (this is called "sublimation") and vice-versa (this is called "deposition"), without ever going through a liquid stage. Even these substances, however, can usually be coerced into a liquid phase under the right temperature and pressure conditions in a laboratory.

The list shrinks considerably if you're talking about matter that can be found in any of the three states in nature. Mercury is another good example. It's melting point (going from solid to liquid) is around -39C. It's boiling point is much higher at something over 350C. However, even at temperatures well below their boiling point, most substances still exist in the vapor phase (Google "vapor pressure" if you want to understand more). Mercury's vapor pressure is quite high, so even at room temperature there will be a considerable amount of it in the air. (This, by the way, is why it's dangerous to keep open containers of liquid mercury lying around. Elemental mercury in the vapor phase is very toxic).

I agree that all elements and many simple covalent compounds can exist in all three states. Almost any compound that is a liquid or gas at room temperature can be liquified or solidified at lower temperatures. But many solid compounds will decompose before they can melt or vaporize, look at CaCO3 for example. So, while there are many compounds that can exist in all three phases, there are also many examples of compoumds that do not.

Good point.

Thankz much

Thankz much