11th Chemistry NCERT notes

Physical Properties of Gaseous State

  • Gases have no definite volume and they do not have specific shape.
  • Gases mix evenly and completely in all proportions without any mechanical aid.
  • Their density is much lower than solids and liquids.
  • They are highly compressible and exert pressure equally in all directions.

Boyle’s law (Pressure-Volume Relationship)

At constant temperature, the volume of a given mass of gas is inversely proportional to its pressure.

Boyle’s law (Pressure-Volume Relationship)

Here, V is the volume, P is the pressure.

K = Constant of proportionality.

The value of constant k, depends upon the amount of gas, the temperature of the gas and the units in which P and V are expressed.

Let V1 be the volume of the gas at a pressure P1. By keeping the temperature constant, if the pressure is increased to P2 then the volume will decrease to V2. According to the law,

Boyle’s law (Pressure-Volume Relationship)

Isotherms: Graphs of V vs p or pV vs p at constant temperature known as Isotherms.

Boyle’s law (Pressure-Volume Relationship)

Charles’ Law (Temperature-Volume Relationship)

The volume of the given mass of a gas increases or decrease by 1/273 of its volume for each degree rise or fall of temperature respectively at constant pressure.

Charles’ Law (Temperature-Volume Relationship)

Charles’ Law (Temperature-Volume Relationship)

The volume of a given mass of a gas is directly proportional to the absolute temperature at constant pressure.

Charles’ Law (Temperature-Volume Relationship)

Charles’ Law (Temperature-Volume Relationship)

Absolute zero is the theoretically possible temperature at which the volume of the gas becomes zero. It is equal to 00C or 273.15K.

Isobars: A graph of V vs T at constant pressure is known as isobar.

Charles’ Law (Temperature-Volume Relationship)

Charles’ law explains that gases expand on heating, so hot air is less dense than cold air.

Gay Lussacs’ Law (Pressure-Temperature Relationship)

At constant volume, pressure of a given mass of a gas is directly proportional to the temperature.


Gay Lussacs’ Law (Pressure-Temperature Relationship)

Pressure Vs. temperature (Kelvin) graph at constant molar volume is shown in figure.

Gay Lussacs’ Law (Pressure-Temperature Relationship)

Isochore: Each line of graph is called isochore.

Avogadro Law (Volume-Amount Relationship)

Avogadro’s law states that equal volumes of all gases under the same conditions of temperature and pressure contain equal number of molecules.

V α n

Where n is the number of moles of the gas.

Avogadro constant: The number of molecules in one mole of a gas = 6.022 x 1023.


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