# How do I make a phase diagram for water?

##### 1 Answer

By knowing where the *normal* boiling and freezing points are (at

*Note that the phase diagram is simply a pressure vs. temperature graph.*

We know that *normal* **freezing** and **boiling points**, respectively.

The **critical point** is when the liquid and vapor exist at the same time, at some

The **triple point** is when the solid, liquid, and vapor exist at the same time, at some

The **Clapeyron equation** describes the slope of a coexistence curve,

#DeltabarH_"trs"# is the molar enthalpy of the phase transition.#DeltabarV_"trs"# is the change in molar volume due to the phase transition.#T# is the temperature in#"K"# .

Using the Clapeyron equation:

- For the liquid-solid coexistence curve in the phase diagram of water,
#(dP)/(dT) < 0# , since#DeltabarV_((l)->(s)) > 0# , and#DeltabarH_"frz" < 0# , which indicates that**water expands when it freezes**(this is unusual). - For the liquid-vapor coexistence curve,
#(dP)/(dT) > 0# , since#DeltabarV_((l)->(g)) > 0# , and#DeltabarH_"vap" > 0# , which indicates that**water expands when it vaporizes**(as usual). - For the solid-vapor coexistence curve,
#(dP)/(dT) > 0# , since#DeltabarV_((s)->(g)) > 0# , and#DeltabarH_"sub" > 0# , which indicates that**water expands when it sublimates**(clearly).

Put all that together into a phase diagram:

Simply note where each point is, and you should be able to at least sketch its general shape. Note that the phase diagram is simply a pressure vs. temperature graph.