Question

Mercury, not water, is used as the liquid inside common laboratory and clinical thermometers, even though water has a higher specific heat. What are the physical reasons mercury is preferred over water as a thermometric liquid?

Hint:

Compare the two liquids on visibility inside a glass tube, their liquid temperature range, and whether they wet the glass, rather than comparing their specific heats.

Answer 1

Step 1: A good thermometric liquid must be easy to see clearly inside a thin glass tube. Mercury is a shiny, opaque, silvery metal at room temperature, so its level inside the capillary tube stands out sharply and is easy to read. Water is transparent and colourless, so its level is very hard to see without adding a dye.

Step 2: A thermometric liquid should also cover a wide range of temperatures while staying liquid. Mercury stays liquid from about minus 39 degree Celsius up to about 357 degree Celsius, a very wide range that covers everyday temperatures as well as fairly hot ones. Water freezes at 0 degree Celsius and boils at 100 degree Celsius, so it cannot be used at all below freezing or above the boiling point.

Step 3: Mercury does not wet or stick to the walls of the glass tube, so it moves up and down smoothly and leaves no droplets behind, giving accurate and repeatable readings. Water wets glass and clings to the tube walls, leaving a trailing film that would give an unreliable reading.

Step 4: Mercury also expands very uniformly and predictably with temperature over its working range, and it conducts heat quickly, so it reaches the temperature of its surroundings fast. Water expands and contracts unevenly, and has an unusual behaviour near 4 degree Celsius where it briefly contracts on heating, which would make a water thermometer scale non uniform and confusing.

So even though water can absorb more heat for a given rise in temperature, mercury is chosen because of its visibility, wide liquid range, non wetting property, and uniform, quick response to temperature change.