> > > The Celcius scale is at 0 when water freezes and 100 when water boils.
> > > This makes sense to me. I can related it to things I see every day.
> > >
> > > I find the Fahrenheit system horribly confusing.
> > > What's so special about 0 degrees Fahrenheit?
> >
> > It's the temperature at which salt water freezes. Duh.
> >
> > -eric "EVERYone knows THAT" sleator
> > Sun 9 Apr A.D. 2000
>
> Doesn't that depend on how much salt is in the water?

Yes, it does indeed, eric. It depends a *lot* on how much salt is in solution: what kind of salt; whether the solution is saturated, or whether it's super-saturated; whether the water is in an ice-bath or the ice-slurry has been put directly in the water solution, making the immediate ambient conditions STP (Standard Temperature and Pressure, i.e. 0°C and 1 atm, which is sea-level pressure). If you don't specify the conditions, there are salt solutions that can easily remain in liquid state down to -51°C (-59°F).

I spent two weeks once, measuring freezing-point depressions in salt solutions. A 10% sodium chloride [NaCl] salt solution freezes around -5 to -6°C (20°F), and a 20% NaCl solution freezes at -16°C (2°F). Add a bit of 30% calcium chloride salt [CaCl2], and the new mixture will freeze only at -27°C (-17°F), because calcium chloride is exothermic -- it *produces* heat, and it's also hygoscopic -- so it'll aggressively melt the ice in the slurry. A wetted mixture of these two types of salt are regularly used to de-ice roads in the winter.

In other words, Fahrenheit chose a totally arbitrary zero point to demarcate his temperature scale. I would have guessed he'd tare his scale at the temperature that Scandinavian sea-water brine might freeze in the winter... but he didn't choose anything as plainly observable as that, eh. Why make it easy.

Fortunately, Stephen clued me into online The Straight Dope archive recently, and TSD has a couple interesting articles on both the Fahrenheit and overall metric issues.


¤ / / On the Fahrenheit scale, do 0 and 100 have any special significance?
http://www.straightdope.com/classics/a4_188.html

¤ / / Why didn't the US ever go metric?
http://www.straightdope.com/columns/950602.html