The SI unit of temperature is the kelvin (K), which spans the same temperature change as the degree Celsius. The Kelvin scale is a thermodynamic scale, meaning that its zero point is at absolute zero rather than the freezing point of water. The second reference point for this scale as it is currently defined is the triple point of water, which is a unique point on the phase diagram of water (a specific combination of pressure and temperature) where ice, liquid water and water vapor are all in equilibrium. The triple point is assigned the temperature of 273.16 K.
The old centigrade scale used the freezing and boiling temperatures of water as its reference points, with one degree centigrade equal to 1/100 of the temperature span between the freezing and boiling points of water. The definition of the Kelvin scale was chosen to make the kelvin the same size as the centigrade degree.
The Celsius scale is defined in terms of the Kelvin scale but is equivalent to the old centigrade scale, which it replaces. It is convenient for reporting weather and cooking temperatures and so on, but is not particularly useful for scientific purposes. For instance, the behavior of gases which approximate ideal gases is such that at zero degrees C they experience a volume change of 1/273 for a one degree change in temperature. This observation provided one of the first indications for the value of absolute zero.
When using the ideal gas law:
PV = nRT
where P is pressure
V is volume
n is the quantity of gas in moles
R is a constant
T is the temperature
it is necessary to use a thermodynamic scale, usually Kelvin.
Another thermodynamic scale, the Rankine scale, has a relationship to the Fahrenheit temperature scale analogous to that between the Kelvin and Celsius scales.
