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Properties, states and phases of a pure substance

Zhigang Suo's picture

These slides are part of a course on thermodynamics.

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Office presentation icon Pure Substance 2015 10 25.ppt4.31 MB

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Zhigang Suo's picture

After class a student asked about a charge of phase as discontinuous change of state.  I added two slides (11 and 16) to make this point visual.    I also modified the page on the rule of mixture.

Ruobing Bai's picture

Dear Zhigang,

The discussion about "how to define phase" and "change of phase" in class was really interesting. It is surprising to see the seemingly "simple" idea still deserves deep thinking.

Dear Zhigang and Ruobing:

0. Excellent slides!

Since I just recently (during the last a few weeks) covered the same topic, let me share my experience (as usual, at length!):

1. Is it really a phase above the critical point?

If we define the phase as a spatially and temporally contiguous portion of a thermodynamically pure substance having the same state, then above the critical point, the very idea of the phase would not seem to apply any longer. The density fluctuations become too large not only across space but also over time, even very small durations of time.

In my class I skipped this part, but on the second thoughts, here is a good resource I found just now [^].

(For the introductory 2nd year course for the mechanical engineering branch, in Indian universities, we are supposed to get to the numerical examples involving the use of steam tables and the Mollier chart ASAP, and not worry too much about the exact nature of the configurations of matter at the micro-scale. That's why, I skipped this part during my lectures.)

2. The quaint idea of the pure substance in thermodynamics

Thermodynamics theory got developed at a time when the atomic hypothesis was several decades old, but still had not yet been accepted. [The conversion from the hypothesis to the theory took about a century!] So, thermodynamicists must have grappled with the problem of defining what constitutes a ``pure'' substance as without any benefit of the atomic theory.

To understand their definition of a pure substance, today's student basically has to temporarily unlearn the atomic theory. Today, the idea of treating a mixture of steam and water as a ``pure'' substance looks puzzling to students on at least two separate counts: (i) H_2O is a compound, not an element, (ii) steam + water is a mixture of multiple states, and hence of phases, but they are too new to the distinctions between states, elements, compounds, components and phases.

3. The difference between the water vapour and steam!

This happened to be the ``simple'' point where my students got stumped for what to me was a suprisingly long time!

I told them about the existence of micro-droplets of the liquid-phase water in the wet steam, as in contrast to their absence in the dry stream (and of course, also their absence in the superheated steam). I told them that the dry steam had the spatially separated and individually existing H_2O molecules alone (which was again to be distinguished from a mixture of 1 part hydrogen gas molecules + 1/2 part oxygen gas molecules). But the students didn't immediately get all the subtlties.

The reason: at the beginnig of their second year of engineering, they still don't have the idea of multi-scaling well-fixed in their mind. (Their first course on engineering materials science is only concurrent.)

So, the students were basically confusing the macro-scale droplets with the micro-scale ones. Indeed, the terms: nano-meter, micro-meter and 1/10th of a mm were all almost the same to them.

In fact, from their faces, I could make out that they were finding all of the following distinctions confusing to make: (i) the wet steam as it comes out of a pressure cooker's regulator, (ii) the mixture of the wet steam inside a pressure cooker plus any condensation of water droplets which happens on its inner walls some time after the heat is turned off (their imagination was that the wet steam produced by a boiler had similarly big droplets at all times), (iii) the dry steam as it exists at precisely the saturated vapor line, (iv) the superheated steam, and (v) a fine spray of water (as used in, say, hair-cutting salons). They could have easily confused (v) with any and all of the first four.

[My best stab at explaining the difference turned out to be: making a reference to the vaporization of camphor. It has no confusing liquid droplets coming in (and I didn't tell them about coagulation of camphor vapour!), and so, that did it!]

Even then, I can't be sure if all of them have gained sufficient clarity about it even by now (a few weeks later). At the same time, I am also sure that on the university examination, questions will be asked not on this distinction, but only on the use of the steam tables or the Mollier chart---for pretty good reasons---and the emphasis will be only on getting the right numerical value in the final step---for not so pretty reasons. ... So, there... Let me stop right here.

Best,

--Ajit
[E&OE]

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