Question

What element is used in thermometers?

Answer 1

The cheapest source for a thermometer is being a liquid in glass medium. The liquid to be used in the thermometer must sustain the higher temperatures and must not evaporate. Water (though coloured) cannot be used in the thermometer despite being the easiest source as it will evaporate at temperatures above $100{}^\circ C$ and will wet the glass.

Complete answer:
Let us know about the thermometers we use;
Mercury is the element used in the thermometers due to various reasons such as;
Liquid at room temperature.
High coefficient of expansion.
Shiny and reflective liquid.
Low freezing point i.e. $-38.83{}^\circ C$ and high boiling temperature $356.7{}^\circ C$.
Does not wet the glass surface.
Expands evenly with temperature.

Note:
The mercury thermometer must be handled with proper care and must be avoided from breaking as Hg is highly toxic in nature. Especially, the thermometers are instructed to keep away from children for the same.
Also, alcohol thermometers are also used for various applications but as we are asked for elements used in the thermometer then its, Hg.