What are wetting liquids in thermometer?

Mercurial thermometers

In general, thermometers use a liquid that expands or contracts when heated or cooled. The liquid is confined to a narrow glass tube, and the height of the liquid column indicates the temperature.

In mercurial thermometers, the liquid is mercury. Mercury is a good conductor of heat, so it responds quickly to changes in temperature. It also has a high coefficient of expansion, so it expands or contracts significantly when heated or cooled. However, mercury is also a toxic metal, so it is important to handle mercurial thermometers with care.

Alcohol thermometers

Alcohol thermometers use alcohol (usually ethyl alcohol) as the liquid. Alcohol is less toxic than mercury, so alcohol thermometers are safer to use. However, alcohol also has a lower coefficient of expansion than mercury, so alcohol thermometers are less sensitive to changes in temperature.

Other liquids

Other liquids that are sometimes used in thermometers include pentane, hexane, and toluene. These liquids are all less toxic than mercury, but they also have lower coefficients of expansion.

Wetting liquids

The liquid in a thermometer must wet the glass tube in order to form a continuous column. If the liquid does not wet the glass, it will break up into droplets and will not be able to indicate the temperature accurately.

The surface tension of a liquid determines whether or not it wets a glass tube. Surface tension is the force that causes a liquid to contract and form a droplet. If the surface tension of a liquid is greater than the adhesive force between the liquid and the glass, the liquid will not wet the glass.

Mercury has a high surface tension, so it wets glass well. Alcohol has a lower surface tension, so it does not wet glass as well. In order for alcohol thermometers to work properly, they need to be specially treated to make the glass more hydrophilic (water-loving).

Conclusion

The liquid in a thermometer must wet the glass tube in order to form a continuous column and indicate the temperature accurately. Mercury is a good wetting liquid, while alcohol is not as good. Alcohol thermometers need to be specially treated to make the glass more hydrophilic.