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Melting point thermometer?

So, earlier this summer I bought a jar of coconut oil from Trade Joe's. It was nice and solid.

A while after bringing it home, I noticed it had gone completely liquid. "Huh."

Turns out the melting point is a shade under 25 C. And yeah, in recent months my kitchen was typically warmer than that. Not anymore though, and it's gone solid again.

But that got me wondering about a crude thermometer, or calibration set for normal ones, made out of various substances with different melting points. Especially if household substances. "That's solid, that's liquid, must be between 25 and 30."

Doesn't seem likely though, at least not without bugging a chemist for obscure compounds. I found a table of oil and fat melting points. Butter is around 35, ghee a bit higher, tallow high 30s or 40s. Vegetable oils are mostly negative, with peanut oil sneaking in at 3 C. Chocolate is said to be about 30, though that seems low for outright melting IME. The highly non edible gallium is also 30.

But stuff between 3 and 30? Well, there's coconut oil... otherwise, something of a desert, which makes sense: the coconut oil was so surprising precisely because one doesn't encounter things melting at room temperature.

Honey is said to liquefy more rapidly under 10 C, but I'm not sure how long that takes, and it's not spontaneously reversible.

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come_to_think
Oct. 2nd, 2015 10:33 pm (UTC)
Only pure substances (and some special mixtures called eutectics) have a definite melting point. Mixed materials like chocolate, butter, & glass just get less & less viscous as you heat them, so it's a matter of judgment when to call them liquid.

Without bugging a chemist, to find a compound with a desired melting point, you can look in a melting-point index. (You can see how such a thing might be useful to a chemist: What is this stuff? Well, it melts at 10 deg C, so maybe it's bromopicrin, or....) I'm sure there are plenty on the Web, but in any case there are several in my Rubber Handbook (30th ed., 1948, given me for Christmas 1949).

I myself have fantasized a use for such a table: A big coffee spoon, hollow, containing a substance whose melting point is the right temperature for a cup of coffee. When the coffee is poured, you stir it, whereupon enough of the stuff melts to bring it down to that temperature. Thereafter, it slowly solidifies, keeping the coffee at that temperature. The shops that sell these spoons give you a free cup of coffee, which you stir with a thermometer until it is just right. Then they sell you a spoon for that temperature. I have not patented this idea, so if you will do so & manufacture it, you will have an instant sale.

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