How Does Pressure Remain Constant During a Phase Transition?

[member 392791]

In reference to the graph at the bottom, my textbook has a statement '''..A process of vaporization or condensation at a constant temperature is illustrated by the lines G-H-I or I-H-G, respectively, in figure 16.3. Water would vaporize or condense at a constant temperature as the pressure-reaches point H on the vapor-pressure curve. The change that occurs at H is the increase or decrease in the fraction of vapor or liquid, respectively, at a fixed temperature. The pressure does not change until all of the vapor, or liquid, has completed the phase transition.''

This part of the statement is not clear to me. How is it that there is no pressure change until all of the vapor, or liquid, has completed the phase transition? It seems like as vapor is condensing the pressure would decrease.

 

[Chestermiller]

In reference to the graph at the bottom, my textbook has a statement '''..A process of vaporization or condensation at a constant temperature is illustrated by the lines G-H-I or I-H-G, respectively, in figure 16.3. Water would vaporize or condense at a constant temperature as the pressure-reaches point H on the vapor-pressure curve. The change that occurs at H is the increase or decrease in the fraction of vapor or liquid, respectively, at a fixed temperature. The pressure does not change until all of the vapor, or liquid, has completed the phase transition.''

This part of the statement is not clear to me. How is it that there is no pressure change until all of the vapor, or liquid, has completed the phase transition? It seems like as vapor is condensing the pressure would decrease.

The book doesn't explain it very well. If the volume of the container were fixed, then as heat is removed, both the temperature and the pressure of the vapor and liquid in the container would decrease (as you correctly concluded). But, if the pressure were held constant and heat was removed while the volume of the container were allowed to decrease, the volume of gas would decrease and the volume of liquid would increase; and the temperature would remain constant until all the vapor was condensed (and you only had liquid left).

 

[member 392791]

I am not understanding how heat could be removed along G-H-I? That line is at a constant temperature and volume, so the only thing decreasing is pressure.

 

[Chestermiller]

I am not understanding how heat could be removed along G-H-I? That line is at a constant temperature and volume, so the only thing decreasing is pressure.

The line G-H-I is a constant temperature line, but not a constant volume line. Point I is in the superheated vapor region. If you increase the pressure at constant temperature, you will be compressing the gas, and, to maintain the temperature constant, you will have to remove some heat. Eventually, by doing this, you will reach point H at which the vapor becomes saturated. Reducing the volume any further at constant temperature will require additional removal of heat. But, the pressure will no longer change. Instead, you will start forming liquid in your vessel. You will stay at point H until all the vapor has condensed to liquid. Trying to reduce the volume any further than this at constant temperature will require you to compress the condensed liquid. This will result in a very rapidly increasing pressure. This will move you to point G.

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