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

In summary: Triple point is still a point on a PT diagram.The triple point is when water reaches its maximum density as a liquid. When you ice skate, you compress the water below the atmospheric freezing point into a solid. And finally, when you answer a question, don't just say what the question is, but also what you did to try to answer it. In summary, you have been through the phase rule.
  • #1
Claire84
219
0
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!
 
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  • #2
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
 
  • #3
What is the "V" in a PVT digram?

Njorl
 
  • #4
It's volume. Kind of a 3-d diagram.

Thanks for your help by the way! :smile:
 
  • #5
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.
 
  • #6
You've been through the phase rule? f=c-p+2?
 
  • #7
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!
 
  • #8
"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?
 
  • #9
So are we just going to have a point then on the PVT diagram?
 
  • #10
One component, three phases, zero degrees of freedom. All properties of the system, T,P, ρ of all three phases are fixed. Next question is, "What is the volume of a system m1ρ1 + m2ρ2 + m3ρ3 = 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.
 

1. What is the triple point of water?

The triple point of water is the temperature and pressure at which all three phases of water - solid (ice), liquid, and gas (water vapor) - can coexist in equilibrium.

2. What is the significance of the triple point of water?

The triple point of water is a fundamental physical constant that is used as a reference point for defining the Kelvin temperature scale. It is also used in various scientific experiments and as a standard for calibrating thermometers.

3. How does the triple point of water relate to the melting and boiling points of water?

The triple point of water is the only temperature and pressure at which all three phases of water can coexist. At this point, the melting and boiling points of water are equal, making it a unique and important reference point.

4. What are the physical properties of water at the triple point?

At the triple point, water has a temperature of 0.01 degrees Celsius and a pressure of 611.657 pascals. Its density is 999.97 kg/m^3 and its specific heat capacity is 4.186 joules per gram per degree Celsius.

5. Can the triple point of water be altered or changed?

The triple point of water is a fundamental physical constant and cannot be altered or changed. However, it can be affected by external factors such as impurities or atmospheric pressure, which may slightly shift the temperature and pressure at which the triple point occurs.

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