Phase Transitions: Why is the pressure constant?

In summary, the conversation discusses the assumption in textbooks that dP=0 during a phase transition and the reasons behind it. The main question relates to why temperature can be assumed to be constant during a phase transition and why pressure and temperature cannot both change along the phase transition line. The answer lies in the fact that for a phase change in the open atmosphere, the pressure is always equal to the ambient pressure, but in other environments, the ∫VdP term must be taken into account.
  • #1
madshiver
9
0
I have a question regarding phase transitions:

It all textbook I've seen the assumption that dP=0 during a phase transition (so that for example dH=TdS is true during a phase transition). Could anyone explain why this should be true?

I guess I could relate this question to a question I had during intro (Halliday) Physics: Why can we assume that temperature is constant during a phase transition? (Back then I imagined that after a certain point putting in energy to get the molecules to move faster does not really lead to a substantial increase in entropy, so that the rigid structure e.g. of the solid or the liquid has to be broken down, so that subsequent temperature increase would actually lead to a decent entropy increase, at the cost of reduced internal energy.)

And then the final related question: Why can't the pressure and temperature both change by "wandering" along the phase-transition line?
 
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  • #2
madshiver said:
I have a question regarding phase transitions:

It all textbook I've seen the assumption that dP=0 during a phase transition (so that for example dH=TdS is true during a phase transition). Could anyone explain why this should be true?
For a phase change in the open atmosphere, the pressure of the system is always equal to the ambient pressure of the surroundings ie the atmosphere. If the phase change takes place in an environment in which pressure is not constant, you would have to take the ∫VdP term into account. In such a case ΔH = ∫dH = ∫TdS + ∫VdP

AM
 
  • #3
Thanks :).
 

1. What is a phase transition?

A phase transition is a physical phenomenon in which a substance changes its state from one phase to another, such as from solid to liquid or from liquid to gas.

2. How does pressure affect phase transitions?

Pressure is a crucial factor in determining the phase of a substance. Changes in pressure can cause a substance to undergo a phase transition, such as increasing pressure causing a gas to condense into a liquid.

3. Why is pressure constant during a phase transition?

During a phase transition, the pressure remains constant because the energy added or removed from the substance is used to change its state, rather than causing a change in pressure. This is known as an isobaric process.

4. What is the difference between a first-order and a second-order phase transition?

In a first-order phase transition, there is a discontinuous change in the properties of the substance, such as a sudden change in volume or energy. In a second-order phase transition, the change is continuous and there is no sudden jump in properties.

5. Can phase transitions occur without a change in temperature?

Yes, phase transitions can occur without a change in temperature if there is a change in pressure or other external factors. For example, water can undergo a phase transition from liquid to solid (freezing) at 0°C, but increasing pressure can cause it to remain in a liquid state even at temperatures below 0°C.

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