Phase change graphs

A phase change graph can show a lot of information about a substance. It’s first important to note that a phase change graph is going to be unique to every pure substance and mixture. In the case of mixtures, concentration of its solutes can have drastic effects on its phase change graph. For example: take water for example. Water freezes at 1 atm and 0ºC. Now if you added salt to that water, it would need to be colder than 0ºC in order for it to freeze. The more salt you add to the water, the colder it will need to be before it freezes. This is why we put ice on our icy walkways in the winter, it is to decrease the melting point of the ice! (also to make more friction when walking or driving on it)

Here is an example of a Pressure-Temperature Phase graph (PT graph):

Looking at the diagram, and considering that this is true only for pure water, then you can see what phase water is in no matter the temperature or the pressure. For example, at 20ºC and 1 atm, you can see that the intersection of temperature and pressure lands in the liquid section of the PT graph. When you look at water at 1 atm and 100ºC, you can see that you land directly on a line. This means that the substance exists in both states, liquid and gas. The triple point is the conditions needed to be met in order for water to exist in all three states.