Why is only mercury used in a thermometer?

Reasons that make mercury a thermometric liquid are:

1) Visibility - Mercury is more visible than most of the liquids.

2) Melting and Boiling points - Mercury has high melting and boiling points making it a liquid over a broad range of temperatures.

3) Metallic nature - Since it is metallic in nature, it has a stable coefficient of expansion. Therefore it expands at a uniform rate throughout a broad range of temperatures. 
 4) Stickness - Mercury is a non-sticky metal. Therefore it makes a clear meniscus and helpful to take readings.

5) Expansion - It will not expand when it gets solidified. Therefore it doesn't break the tube of the thermometer.


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