Gases neither have definite size (volume) nor definite shape. Moreover, the changes in pressure
are equally important (in case of solids & liquids these changes are negligible) as the changes in
volume, when gases are heated. Therefore to determine the unique relation between changes in volume
and temperature, the pressure must be kept constant and to determine unique relation between
changes in pressure and temperature the volume must be kept constant. Hence the following two
coefficients are defined for enclosed masses of gases.
Coefficient of Volume expansion at constant pressure :
The change in volume, above the volume at
00 C per unit original volume at 0 0 C per 10 C change
in volume, at constant pressure, is called gp .
Note: Experimentally it is found that
for all known gases.
Coefficient of Pressure expansion at constant Volume:g v
The changes in pressure, above the pressure at
00C per unit original pressure at 00C per 10C change
in temperature, at constant volume, is called gv .
Note: Experimentally it is found that for
all known gases.
Kelvin or Absolute or Ideal Gas Scale of Temperature
Therefore at t = -2730C the volume of gas becomes V = 0, which must be treated as the limit.
Thus, if a gas were to always remain a gas (therefore called ideal gas) without getting liquefied then
the lowest possible temperature to which it can be cooled down must be -
2730C and not less than that (for the volume then will become negative; clearly absurd proposal). Although, the Celsius Scale
of temperature is an objective measure, it is still dependent on properties of one particular
substance viz. water; whereas all ideals gases tend to attain V = 0 at t =
-2730C. Therefore, it is better to redefine the temperature scale in terms of behavior of all the ideal gases, irrespective of their
chemical nature, rather than in terms of one particular substance viz. water.
The new temperature scale was introduced by Lord Kelvin, hence it is called the Kelvin Scale
of temperature and it is defined as
T0 K = t = 273, where t is the temperature on the Celsius Scale.
Thus, T = -273 + 273 =
00K at (t)lowest = - 2730C.
It is obvious that just as - 2730C is the lowest possible temperature that can occur in nature,
00K is also the lowest possible temperature; hence
00K is the absolute zero of temperature. Since
Kelvin Scale of temperature is based on the behaviour of Ideal gases, it is also known as the Ideal gas
scale of temperatures. In Chapter 15 it will be shown that a scale of temperature can be defined
independent of any substance; this scale of temperature can be called Absolute Scale and it will be
shown that it is uniquely related to Kelvin Scale, hence Kelvin Scale is called also as Absolute Scale of