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We were asked as a lab experiment to measure the volume occupied by water and ethyleneglicol as a function of its temperature by having a flask inside a body of water at the desired temperature. Then, when the maximum temperature was reached, we had to to the same backwards. The same goes for the length of two metal rods

At first, the changes in temperature were slower and easier to set because they were done with a thermostat. But in the cooling process it was much faster because refrigerated water of an unknown temperature was used, without the thermostat being able to make the process slower. Hence the temperature fell continuosly, without being able to reach a stable temperature for any measurement.

Experimental setup

It was found that the process was asymmetric, as can be seen in the following charts:

Difference when heating (upper table) and cooling (lower table)

This happened in solids as well, with the lengths in the cooling process being lower.

I tried to look for information about this "hysteresis" online, but failed to find anything. I thought that it may have to do with this the process not being quasistatic, rather than phenomena similar to hysteresis in magnets, but we have only studied equilibrium thermodynamics yet and I don't know how would non-equilibrium affect the experiments.

Any possible hint or explanation would be much appreciated.

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  • $\begingroup$ Does the temperature refer to the "body of water" or the flask? If you change the temperature of water, the temperature of the flask lags behind. $\endgroup$
    – Quillo
    Commented Feb 6 at 9:00
  • $\begingroup$ It was the temperature of the body of water. During the heating process we let enough time pass, so I guess both would be at equilibrium and they wouldn't lag behind. But during the cooling, if the flask lagged behind the water, I would get a temperature that is actually higher, thus having a higher volume. But I got the opposite, lower than usual. $\endgroup$
    – Jaime Yepes de Paz
    Commented Feb 6 at 11:15

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The most probable cause of the effect was the thermal lag time between the bath temperature and the sample temperature. This hypothesis is easily testable.

Also lease note that with your equipment it is not possible to measure temperatures in the lab to 5-place accuracy as listed in your charts.

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