Temperature is the measure of kinetic energy of molecules.
According to zeroth law of thermodynamics, if A&B are in thermal
equilibrium and B&C are in thermal equilibrium then A&C are in thermal
equilibrium .This idea led to the development of thermometer.If B is
thermometer then A&C are at the same temperature without actually being in
contact. Water being abundant and easily available was used to calibrate the
thermometer. Celsius scale and Fahrenheit scale were developed using water as
reference substance. In the Celsius scale, the freezing point of water is
marked as 0 deg C and the boiling point of water as 100 deg C. The problem with
using water as a reference substance ;1)BP and MP of water are a function of
pressure and temperature .2) what if we want to measure the temperature of a
distant planet which is devoid of water.
This lead to the development of absoulte temperature scale
/kelvin scale. Kelvin scale is based on activity of atoms. At 0K, all
substances have minimum molecular activity irrespective of the nature of
atoms(element) . 0K is - 273 deg C