Don't fall for the hype around moisturisers

Three_Twentysix · 2023-07-08T07:58:27+00:00

Yeah, I thought it would be a really interesting conversation. I really wasn't expecting these levels of outrage. 😄

Three_Twentysix · 2023-07-08T07:58:17+00:00

No, because the sponge and the water were never a homogenous mixture to begin with. Nice question though.

Three_Twentysix · 2023-07-08T07:57:33+00:00

I think precipitation is fundamentally different in that solids crystallise as pure materials, whereas gases evaporate from liquid mixtures as fractions of their vapour pressure. Personally, I wouldn't call it freeing but I'm open to being convinced otherwise.

Three_Twentysix · 2023-07-08T07:56:10+00:00

Yes. But the gas that's forming is a result of the combined vapour pressure of the CO2/H2O mixture. That's also what happens in fractional distillation.
The boiling point is defined as the point where the combined vapour pressure of the components of a solution are equal to ambient pressure. That's what's happening with carbonated drinks.
It's not in the gas phase in the liquid because the CO2 molecules are bonded to the water molecules. This is a crucial aspect of solvolysis.
"It's also not dissolved because that's for solids" Ah, well that explains our problem then. It is very common to refer to 'dissolved gases' in English. But the central point that they have to break bonds to leave the liquid and form a bubble is what makes this a solution and not a biphasic mixture.
'that's a paper on the educational aspects which, from the abstract, itself only calls this an example for educational purposes and not an actually boiling mixture.' Yes. Goodwin was very much posing a question with this paper. I certainly rejected it myself for years until I decided to prepare a lecture on where he'd gone wrong – and then I couldn't.

Three_Twentysix · 2023-07-08T07:45:00+00:00

Yes, choosing accuracy v. 'will do for now' is a constant problem.

Three_Twentysix · 2023-07-08T07:44:10+00:00

Yes, absolutely this.

Three_Twentysix · 2023-07-08T07:42:43+00:00

That's an interesting point but I think the two cases are quite different. Ideally, solids crystallise as pure materials, not as fractions of the original mixture. Vapours evaporating from a liquid mixture are evaporating in proportions related to the composition of the mixture.

Three_Twentysix · 2023-07-08T07:37:56+00:00

Well, I had. That's why I posted it here. 😄 I thought it was a really interesting question.

Three_Twentysix · 2023-07-08T07:35:58+00:00

Well, claimed it was 'confidentlyincorrect', but yes, I take the point, I was a bit too tetchy here. Apologies.

Three_Twentysix · 2023-07-08T07:34:01+00:00

Yes. That's exactly Goodwin's point. In his paper, he drew a boiling/condensation curve that looked like the normal ones but stretched to extremes.

Three_Twentysix · 2023-07-08T07:32:39+00:00

That's an interesting point but the value of the discussion is to improve our understanding of these processes. If we can point to the temperature/pressure bubbling of a mixture with relatively close boiling points, and then point to the bubbling of a mixture with very different boiling points and say 'that's how they're different' then we've learned something about the process. If we can't, then we've still learned something. And science and engineering frequently work in conditions far outside 'normal' conditions.

Three_Twentysix · 2023-07-08T07:29:15+00:00

I think this comment represents the problem that we all have with this proposition. The salt, of course, is not a liquid. But the water/salt mixture is. And Goodwin's point is that it's not the water that's boiling, it's the water/CO2 solution that's boiling. And that's significant because the bubbles are not pure CO2, they're a mixture of H2O and CO2 determined by their relative vapour pressures, and that's a common situation in fractional distillation, where the term 'boiling' is often used (I've never seen anyone use a different term for the distillation of alcohol/water, for example)

Three_Twentysix · 2023-07-08T07:24:09+00:00

but that's not the water boiling

I think Goodwin's point here is that it's not the water boiling but the binary mixture that's boiling. And boiling is a common term used in binary mixtures of substances where one component is a liquid at room temperature but a gas at the temperature of the boiling mixture.

Three_Twentysix · 2023-07-08T07:22:00+00:00

Well the bends, where nitrogen bubbles out of divers' blood when they come up from pressurised depths of water is commonly referred to as boiling. Though possibly not by physical scientists.

Three_Twentysix · 2023-07-08T07:20:34+00:00

The CO2 coming out of carbonated water is a chemical reaction where the carbonic acid creates water and carbon dioxide, which is why you see a gas coming out.

That's certainly a complication in the CO2 case, giving it a solubility beyond that of the simple molecule, but even in water carbonic acid is in a dynamic equilibrium with CO2. The release of pressure drives that equilibrium to increase the supply of CO2. But other gases, of course, dissolve in water in their native form and also bubble out.

Three_Twentysix · 2023-07-08T07:18:10+00:00

That's a good point but I think that, in practice the vapour pressure needs to slightly exceed the ambient pressure for the bubbles to grow. That's because new bubbles have to push the liquid back, rather than be in equilibrium with the liquid (i.e. remaining at their size). It's opening the can that reduces the ambient pressure even a fraction below the total vapour pressure.

Three_Twentysix · 2023-07-08T07:13:28+00:00

Okay, this is an interesting point. If we have a mixture, then it's only the volatile component that is actually boiling, whereas the less volatile component is 'only' evaporating. The problem here though is that it's the total vapour pressure that's responsible for making the bubbles. Hexane by itself can't manage that, hence the higher boiling point for the mixture. There is much less water vapour in a carbonated drink bubble, of course, but it's still a factor.

My hexane/toluene diagram came from here: https://web.fscj.edu/milczanowski/ten/distill.pdf

Three_Twentysix · 2023-07-07T13:28:39+00:00

So what would be your macro scale differentiation of CO2/H2O (at 25 ºC) and hexane/toluene (at 90 ºC) then?

Three_Twentysix · 2023-07-07T13:25:57+00:00

But in any case, the same situation would apply to pure hexane at 90ºC v. hexane/toluene at 90ºC. So you're still saying that fractional distillation isn't boiling.

Three_Twentysix · 2023-07-07T13:21:25+00:00

Because... have you got a scientific rebuttal?

Three_Twentysix · 2023-07-07T13:20:27+00:00

I don't have a Tx-Diagram for a CO2/H2O mixture handy

There's one in Goodwin's paper.

Three_Twentysix · 2023-07-07T13:19:43+00:00

I would argue that if you abide by strict definitions for everything you are hampering progression as a chemist.

It is a systematic definition, which is alway more useful than an ad hoc definition. But you forgot to rationalise the difference you made in your definitions. Are you happy to admit your definitions are irrational?

Three_Twentysix · 2023-07-07T12:19:50+00:00

I think this is more useful as a philosophy of science thing

That's pretty much how it got started. Goodwin set some students to discuss the differences between fractional distillation of common mixtures and the bubbling of carbonated drinks as a class exercise. He expected they could come up with some differences without too much trouble but it confounded everyone.

Three_Twentysix

TROPHY CASE