Just a quick question about a homework I have been set this week! One of the questions is to say what the triple point is on a PVT diagram when it is a point on a PT diagram. Is it still a point or is it a line? Also, we have to state what physical property of water is different from that of most other substances. I've got a few properties here but can't narrow it down to one, so any help would be appreiated (e.g the only natural substance that is found in three states?). Thanks for your help!