Why should an ethanol-filled thermometer NOT be used at temperatures close to #100# #""^@C#?

1 Answer
Mar 24, 2016

The alcohol-filled thermometer will be off the scale when the temperature is #230# #""^@F#.


The boiling point of ethanol is #+78# #""^@C#. This limits the top measurement of the thermometer to this temperature. Of course, the MELTING POINT of ethanol is #-100# #""^@C# or so, which allows measurement of temperatures lower than #0# #""^@C#.

A thermometer may be filled with mercury, or (for lower temperatures), it may be filled with ethyl alcohol, toluene, xylenes, or some liquid whose melting point/boiling point extends the range of measurement. Because (as far as I know) the rate of liquid expansion when heated gives a LINEAR SCALE, the thermometer may be calibrated by placing in an ice bath (or a colder bath), marking off this freezing point, then placing in a bath of boiling water, and marking off this boiling point at #100# #""^@C#. The THINNER the glass capillary, the greater the distance between the two points and the more accurate the reading on the thermometer.

Note that often thermometers are handled carelessly, and develop breaks in the column of mercury or breaks in the column of ethanol. This typically happens when new graduate students use these instruments carelessly. The columns can be restored by careful heating so that the liquid expands in the top bulb and the break is joined, and then slow cooling. Of course, though, when you really need to take a temperature measurement, you find that all the thermometers in the lab have been abused. Thermometers should be stored UPRIGHT, with a sponge in the bottom of the jar to protect the bulbs. Some graduate chemists treat these thermometers like family heirlooms and are very fussy about who uses them, and how they use them (I know this because that's what I did!).