[research]

temperature has a direct relationship with K (kelvin) but not with ⁰C pls tell me the reason.

[tamim83]

The volume of an ideal gas has a linear relationship with temperature regardless of the scale you use.  If you are using the Celsius scale and extrapolate the volume vs. temperature data you will see that when V = 0, T = -273.15 Celsius.  So V is proportional to T + 273 when T is in Celsius. 

Later on, Lord Kelvin decided to define a new temperature scale based on the "Charles' Law" experiment.  He defined the 0 of the temperature scale to be at -273.15 Celsius so that all of the temperatures would be positive.  This is the Kelvin scale.  As a consequence, V is proportional to T in Kelvin. 

It all has to do with how the temperature is defined.  It happens that the Kelvin scale is based on the gas expansion experiment. 

[Ulagatin]

Of course, it should also be noted that the Kelvin scale is an absolute scale; that is, it is founded upon an absolute measure of temperature. Heat is a measure of the average kinetic energy of a particle. If this kinetic energy is at zero, then the temperature is at absolute zero on the Kelvin scale (which requires an infinite amount of energy input to achieve). Degrees Celsius is a relative scale, it is based upon the boiling and freezing points of water - and therefore it is relative.

Tamim's response answers to the role in which Charles' Law comes into play. The Kelvin scale is the natural scale of science in many levels.