Gas is one of the four basic states of matter. A pure gas consists of individual molecules(like helium, xenon, neon, etc), diatomic molecules (like oxygen, sulphur, hydrogen, etc) or a compound molecule made of different atoms(as in CO2). A mixture of gases contains variety of gaseous molecules like that of air.

Physical Properties

Gases don't have definite shape and volume. They tend to fill all the space available to them and take on the shape and volume of the container. Molecules are relatively far apart and thus influence each other to a lesser extent.

Fundamental measurement of Gases


The volume of any sample of a gas is considered to be the space of the container that it occupies. The space occupied by the molecules of gas is negligible as compared to the volume of the container. The volume is expressed in litres(or millilitres) or m3 or cm3.


The molecules of gases are in continuous random motion. They frequently collide with each other and with the walls of the container. The collisions of the molecules with the walls of the container give rise to what is called as the pressure. It is measured as force per unit area and is uniform in all the directions. It is measured by instruments such as Manometer and Barometer(for atmospheric pressure). It is expressed in N/m2 or mm of Hg or atmospheres(atm) or torr.
1 atm = 1.013 × 105 ≡ 1.013 × 105
1 bar = 105 N/m2
1 atm = 760 mm of Hg column = 76 cm of Hg column = 760 torr = 1.013 bar


The temperature is discussed in terms of hotness or coldness. The measurement is based on the expansion of certain material(most often it is mercury) with increasing temperature.
One of the scale to measure the temperature is taken as Celsius(oC) scale. The freezing point of water is taken as 0oC and its boiling point as 100oC.
It was observed by Gay Lussac that the rise in volume of a given mass of gas for each degree rise in temperature is nearly equal to 1/273 times of the gas at 0oC. If Vo is the volume of the gas at 0oC and VT is the volume of gas at To, then:
 V_T = V_o \bigg(1 + \frac{T}{273} \bigg)
Thus, VT = 0 if T = -273 oC i.e., the volume of a given mass of a gas is zero at constant pressure or we can say that the gas would completely disappear if T = -273 oC. So T = -273°C is the lowest possible temperature that can be achieved. This temperature is called absolute zero. Now a new scale called as absolute scale or Kelvin scale is defined where -273oC = 0 K.


 T(^oF) = 32 + \frac{9}{5} \times T(^oC)

Ideal Gas>>

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