have observed that sometimes sealed bottles with metallic lids are so tightly screwed that one has to put the lid in hot water for some time to open it. This would allow the metallic lid to expand, thereby loosening it to unscrew easily. In case of liquids, you may have observed that mercury in a thermometer rises, when the thermometer is put in slightly warm water. If we take out the thermometer from the warm Fig.
. Pressure versus temperature of a low density gas kept at constant volume. Fig. .
A plot of pressure versus temperature and extrapolation of lines for low density gases indicates the same absolute zero temperature. Fig. . Comparision of the Kelvin, Celsius and Fahrenheit temperature scales.
relationship. Notice that since PV = constant and V/T = constant for a given quantity of gas, then PV / T should also be a constant. This relationship is known as ideal gas law. It can be written in a more general form that applies not just to a given quantity of a single gas but to any quantity of any low-density gas and is known as ideal-gas equation : or PV = µ RT ( .
) where, µ is the number of moles in the sample of gas and R is called universal gas constant: R = . J mol – K – In Eq. . , we have learnt that the pressure and volume are directly proportional to temperature : PV ∝ T .
This relationship allows a gas to be used to measure temperature in a constant volume gas thermometer. Holding the volume of a gas constant, it gives P ∝ T . Thus, with a constant-volume gas thermometer, temperature is read in terms of pressure. A plot of pressure versus temperature gives a straight line in this case, as shown in Fig.
. . However, measurements on real gases deviate from the values predicted by the ideal gas law at low temperature. But the relationship is linear over a large temperature range, and it looks as though the pressure might reach zero with decreasing temperature if the