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this post was submitted on 16 Jul 2023
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O F is the freezing temperature of a saturated brine solution, while 100 F was the body temperature of a human. Yes, body temperature has been revised a bit, but the two points were chosen as stable points that anyone could access that would generally be unchanged by pressure changes, etc. Human homeostasis is quite good at keeping a temperature in a narrow range. Also, boiling is massively affected by air pressure. At 5000' elevation, boiling is approximately 202 F and continues to get lower as altitude increases. Lots of people live at higher altitudes. (Hi! I am one of them !)
Edit: I was a little off on the temperature selected for body temp, but still pretty close: https://en.m.wikipedia.org/wiki/Fahrenheit
This is really interesting and I think there is a lot of support for the body temperature point. I was curious about whether the method of deriving 0F is insensitive to pressure changes and I can't find any evidence of that. But I don't know enough about chemistry or physics myself. Do you know, or have any details on where you learned this?
So I was a little off on the temperature chosen for the body, but the Wikipedia page has some good details: https://en.m.wikipedia.org/wiki/Fahrenheit
Re: freezing temperature of brine and pressure sensitivity, of course it is sensitive but we are talking about MPa-GPa of pressure, way beyond small pressure changes due to changes in altitude. You can get started by looking at physical chemistry of solutions if you are interested! A good place to start is "freezing point depression" and "boiling point elevation" of solutions. Also, single component phase diagrams: here it is for water.