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Question from my textbook: Jason says 'The mercury in the thermometer can be replaced by coloured water. The thermometer will function well after recalibrating using a similar method to calibrating a mercury-in-glass thermometer." Comment on his idea.

The answer is NO, but why?

Please use simple english to answer my question, as I am only grade 9, I haven't learnt much about physics, therefore I may not understand some technical words. Thanks!

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    $\begingroup$ I musty confess that I can't see why water wouldn't work in the thermometer. The thermal expansion coefficients are similar. Obviously water has a more limited range as it's only liquid from 0°C to 100°C while mercury is liquid from -39°C to 357°C. $\endgroup$ – John Rennie Nov 7 '15 at 5:56
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    $\begingroup$ How do you know that the answer is "no"? Water was used in early thermometers (although alcohol works better because of its lower freezing point). My guess is that there's some non-fundamental reason that the author is focusing on, but that it really isn't that important. $\endgroup$ – Daniel Griscom Nov 7 '15 at 12:55
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    $\begingroup$ Just from the wording of the problem I suspect the author is focusing on the calibration step. But since we don't know what the author has told you about that (and I don't recall the usual calibration scheme for Hg-in-glass) we can't help you there. $\endgroup$ – dmckee --- ex-moderator kitten Nov 7 '15 at 14:54
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A mercury-in-glass thermometer is traditionally calibrated by measuring two temperatures: the freezing point of water (0°C), and the boiling point of water (100°C).

There's an obvious reason why this won't work for a water-in-glass thermometer: the water in the thermometer will freeze when when you stick it in the ice bath, and boil when you stick it in the boiling water, and in neither case will you get a good reading.

There's also a less-obvious problem with measuring the freezing point: even if you try to measure the height of the water column just before the water freezes, you won't get good information: water has a maximum density around 4°C, and expands as it cools from there to the freezing point. The height of the water column just before it freezes will be about the same as it was at 8°C.

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The colored water will rise in the thermometer to the top by capillary action and show the max temperature all the time. So a glass thermometer that uses mercury will not work if mercury is replaced by water even if you intend to use that thermometer just between 0 and 100 C.

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    $\begingroup$ That isn't true. The water has a fixed volume and cannot just rise. You could equally argue that because mercury has a contact angle greater than 90° it would shrink to the bottom of the tube. Anyway I remember making water thermometers in science classes at school and they do work. $\endgroup$ – John Rennie Nov 7 '15 at 6:59

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