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.
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.