# Basic question regarding triple point of water and physical properties of water

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?).

Njorl
Water is more dense as a liquid than a solid. At atmospheric pressure, it reaches it's maximum density at about 4 degrees C I think. Cool it or heat it from that point, and it will expand.

When you ice skate, you actually compress ice into water that is below the atmospheric freezing point of zero C.

Njorl

Njorl
What is the "V" in a PVT digram?

Njorl

It's volume. Kind of a 3-d diagram.

Thanks for your help by the way!

Any ideas? I'm thinking it's still a point but I'm really not sure at all. Or perhaps a straight line on the 3-D diagram such that the pressure and temperature are always correct for it to be the triple point of water.

Bystander
Homework Helper
Gold Member
You've been through the phase rule? f=c-p+2?

Right, water is more dense as a liquid than as a solid, so the solid-liquid boundary line in the phase diagram for water has a slightly negative slope, as opposed to the usual positive slope for other substances. Therefore, for water, the increase in pressure at constant temperature would favor the liquid phase, not the solid phase (which is usual for other substances). But I guess that's already been said...
As for the triple point question...since the triple point is only dependent on pressure and temperature, volume can vary. I think the point in a 2D coordinate system is equivalent to a line in a 3D system. I would take the triple point and hold it constant in the PT plane, and draw a straight line with slope of zero along the V axis.
I'm not completely sure, but that's what I would answer. Good luck!

Bystander
Homework Helper
Gold Member
"As for the triple point question...since the triple point is only dependent on pressure and temperature, volume can vary."

Veerrryyyy baaddd guess. Phase rule: "f" (degrees of freedom of a system) = the sum of "c" (number of components) - "p" (number of phases) + 2. How many phases? How many components? How many degrees of freedom in PVT space?

So are we just going to have a point then on the PVT diagram?

Bystander
Homework Helper
Gold Member
One component, three phases, zero degrees of freedom. All properties of the system, T,P, &rho; of all three phases are fixed. Next question is, "What is the volume of a system m1&rho;1 + m2&rho;2 + m3&rho;3 = VTotal ?"

You get a line running from T,P of the triple point and molal volume of the vapor through the molal volume of the solid to T,P of the triple point and molal volume of the liquid.