# Understanding what goes on along the triple line (for water)

1. Nov 3, 2013

### Urmi Roy

So I have a lot of trouble understanding what goes on along the triple line.

1. So within the dome, along the triple line, is the 'quality' defined anymore? If it is, what fraction of the pure substance is solid and what fraction is liquid?

2.At the point on the extreme right end of the triple line, meeting the dome,is there solid and liquid or is it only vapor? (point C on the picture I've attached).
Similarly what phases exist at point A and B?

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2. Nov 3, 2013

### Staff: Mentor

At point C, there is pure vapor.
At point A, there is pure solid.
At point B, at a slightly higher pressure, there is pure liquid; at a slightly lower pressure there is mixture of solid and vapor.

3. Nov 3, 2013

### Urmi Roy

I get what you said about A and C, but not B. So at B, specifically, what do we get? Further, if we increase the pressure, we might move along the saturated liquid curve OR along the solid-liquid equilibrium line.

More importantly, please look at my first question about the 'quality' on the triple line.

4. Nov 3, 2013

### Staff: Mentor

It's not possible to be "exactly" at point B. If you are even the slightest amount above the pressure at point B, you have pure liquid. For example, if you say you are on the saturated liquid curve, you are at the 100 % liquid boundary. If you say you are at the right boundary of the solid-liquid equilibrium region (what you call the solid-liquid equilibrium line), you have 100 % liquid (0% solid). Slightly below point B, you have a combination of solid and vapor. You can calculate the fraction of solid and the fraction vapor at this point using the lever rule. From the figure, it looks like about 90% solid and 10% vapor.

Chet

5. Nov 3, 2013

### Urmi Roy

But say we're at any point between B and C, how do we know how much solid and liquid we have? We're on the triple line, so we'd get some liquid, some vapor and some solid, all together right?

6. Nov 4, 2013

### Urmi Roy

Okay I'm getting the impression that at any point along the triple line, even though we 'can' in principle have any of the 3 phases, it does not mean we actually have all three states co-existing. The secret is in the 'latent heat'. So depending upon the amount of latent heat added/taken away, we can stay at the same temperature and pressure but still have a change in phase. This also applies to point B.

7. Nov 4, 2013

### Urmi Roy

By extension of what I said, does it mean that it is not possible to explicitly define the mass ratio of any of the phases along the triple line? Meaning that the concept of 'quality' isn't valid along the triple line?

8. Nov 4, 2013

### dauto

Yes and yes for your last two posts. The solution for any point in the triple line except at the extremes (A an C) is not unique so there is more than one way to combine the solid liquid a gas states for any point in the line.

9. Nov 4, 2013

### dauto

Between A and B there must be some solid component, Between B and C there must be some gas. At B no phase is required to be present

10. Nov 4, 2013

### Urmi Roy

Thanks a lot, dauto! But what do you mean that at B there is no phase required to be present? B lies on the saturated liquid line, so isn't it a must for liquid to be present?

When we move from C to B, we are basically taking out some of the latent heat. So at any point between B and C, how do we increase the amount of liquid/solid present? (the phase other than vapor can be either liquid/solid but is there any way of knowing how much solid/liquid there is?)

11. Nov 4, 2013

### Staff: Mentor

If you are even slightly below the line and you are between A and B, you have solid and vapor in well-defined proportions. If you are even slightly above the line and you are between A and B, you have solid and liquid in well-defined proportions. If you are even slightly below the line, and you are between B and C, you have solid and vapor in well-defined proportions. If you are even slightly above the line and you are between B and C, you have liquid and vapor in well-defined proportions. All this quantification can be accomplished by appropriate use of tie lines and the lever rule.

Chet

Last edited: Nov 4, 2013
12. Nov 5, 2013

### Urmi Roy

13. Nov 6, 2013