Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: thedoc on 09/02/2016 18:50:02

Title: How would a water-based thermometer work?
Post by: thedoc on 09/02/2016 18:50:02

Kerry Lozito asked the Naked Scientists:
   I'm developing weather curriculum for 3rd grade students and was experimenting with making a water-based thermometer.  I filled a soft "jumbo" plastic pipette with tap water (several mL at room temperature) placed the bulb of the pipette in a cup of water at the same temperature, and marked the water level in the neck of the pipette.  Then I transferred the pipette to a cup of hot tap water.  To my surprise, the water level in the neck of my so-called thermometer went down, not up.  The effect was almost immediate.  When placed in ice water, the water in the neck of the  pipette went up, not down.  Is this related to the relative densities of the water in and outside of the pipette? Thermal expansion of the plastic?  Something else?
What do you think?

Title: Re: How would a water-based thermometer work?
Post by: alancalverd on 09/02/2016 22:37:48

The key is the word "plastic". If the bulk of the water was in a cylindrical part of the pipette, this would tend to change shape when warmed. The surface/volume ratio of a sphere is minimal so the pressure of the water column above the bulb would force the bulb from a right cylinder towards a sphere, which has a larger volume and hence the level in the tube would decrease.

Use a glass bottle with a glass tube and a cork. This thdermometer can be very sensitive and is particularly interesting in the region 0 - 4 deg C where it really does "read backwards", to the consternation of those who preach disaster as the polar ice melts.

Title: Re: How would a water-based thermometer work?
Post by: puppypower on 10/02/2016 12:23:16

You also may need to calibrate your thermometer using a number of standards. Below is a curve of the density of water as a function of temperature. The line is not exactly straight from 0C to 100C, so with a fixed diameter glass tube, the volume change per degree of temperature will drift. The extra standards is a good exercise for lab practice. If they ask why you need to do so much work, this allows you to show and explain the density curve.

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww1.lsbu.ac.uk%2Fwater%2Fimages%2Fmaximum_density.gif&hash=59952bb71de54580cf120b16f97c0efd)

Title: Re: How would a water-based thermometer work?
Post by: Bored chemist on 11/02/2016 22:14:54

Alan, you explained half the problem. Why would the water level rise in cold water - why would the cylindrical bulb suddenly become "more cylindrical"?
(incidentally, a lot of thermometers have practically cylindrical bulbs,and they work just fine)
I have an alternative explanation.
The soft plastic bulb also expands on heating.
If it expends more than the water then the water level will drop.

According to this
https://en.wikipedia.org/wiki/Thermal_expansion
polypropylene has a higher expansion coefficient than water.