Mercury is used in thermometers becuase it expands easily and does not stick to the sides of the thermometer.
Difference between clinical and lab thermometer-
The range of clinical thermo.is between 35 degree celsius to 42degree celsius whereas the range of lab. thermo. is between -10degree celsius to 110degree celsius.
CT are used to measure temp. of human body whereas LT is used to measure the temp. of objects other than the human body.
Mercury is used in thermometers with a higher range, as its freezing and boiling points are -39c and 360c while alcohol is used in those needing to measure a lower range with -114c and 78c respectively.