explain how the celsius scale was devised and why it is not appropriate to use it when describing the behavior of gases

Answer 1

freezing water = 0
boiling water = 100
Ideal gasses follow the rule
P V/nR = T
where the T has to be from absolute zero, where the molecules have no kinetic energy. Absolute zero is about -273 in the centigrade scale. Therefore degrees K (or Kelvin or "absolute") are used where K = degrees C +273

Answer 2

You can read about the Celsius scale here. Originally, zero degrees C was the freezing point of water and 100 was the boiling point of water. The interval between these two points was divided into units of 1/100. The definition is slightly different now.
http://en.wikipedia.org/wiki/Celsius

The behavior of gases is associated with the Kelvin scale where absolute zero is where the molecule has no kinetic energy; therefore, the Celsius scale is not appropriate for the behavior of gases.