this post was submitted on 24 Jul 2024
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671
freezing point curves (mander.xyz)
submitted 5 months ago by [email protected] to c/[email protected]
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[–] arandomthought 92 points 5 months ago (5 children)

Dare someone smarter than me explain what the ever loving fuck is going on there?

[–] [email protected] 99 points 5 months ago* (last edited 5 months ago) (1 children)

This is what happens when some kind of new compound is formed between these two, here it'll be series of sulfuric acid-water complexes. Same thing happens with metals when intermetallic compounds form, see titanium-nickel phase diagram. Normal case would be eutectic, see aluminum-silicon phase diagram

[–] [email protected] 29 points 5 months ago (2 children)
[–] [email protected] 25 points 5 months ago (1 children)

I see. So these are actually many sulfuric compounds in a trenchcoat chart.

[–] [email protected] 8 points 5 months ago

Some of which are probably only a bit stable and so only exist in a mixture

[–] [email protected] 16 points 5 months ago (1 children)

Never mind between 80 and 90%, WTF is happening at 42.5%‽

[–] [email protected] 21 points 5 months ago (1 children)

There's a metastable phase somewhere out there and change of the most stable solid phase between 4-hydrate and 6.5-hydrate

[–] [email protected] 23 points 5 months ago (2 children)

My uneducated understanding is that the chart shows at which temperatures sulfuric acid freezes depending on the concentration. Also in my very basic understanding of physics and chemistry I would have thought that it's linear or exponential or something predictable and not that jumpy.

[–] [email protected] 18 points 5 months ago* (last edited 5 months ago)

In normal cases you'd see two curves going away from pure compounds downwards to a common minimum, which is eutectic point. It's generally only vaguely predictable, but always monotonic

[–] phdepressed 14 points 5 months ago

Sulfuric acid and water has various H2SO4 and H2O ratios. So like 1 H2SO4 and 6, 3, 2, or 1 H2O it also has just the H2SO4 and H2S2O7. These are present as local points within solutions and with different prominence depending on the amount of water added. These 8 different ratios each have different freezing points.

[–] TheSlad 12 points 5 months ago (1 children)

If I had to guess, I would assume that there are different molecular lattices that sulfuric acid and water can form at different concentrations and that these different lattices have different freezing points. I will now go look it up.

[–] [email protected] 8 points 5 months ago

What you're describing are different crystalline phases of pure compounds, but this does not give you new minima, you need some new compound to form for that

[–] [email protected] 4 points 5 months ago

Physics.