Study Notes On Chemistry: PROPERTIES OF GASES

Study Notes On Chemistry: PROPERTIES OF GASES

  1. Properties of Gases

  • Gas has no definite volume or shape.
  • The other outstanding characteristic of gases is their low densities, compared with those of liquids and solids.
  • All gases expand equally due to equal temperature difference.
  • Diffusion of gases: The phenomenon in which a substance mixes with another because of molecular motion, even against gravity- is called diffusion.
  • The pressure of a gas: The molecules of a gas, being in continuous motion, frequently strike the inner walls of their container
  • Temperature and Temperature Scales: Temperature is defined as the measure of average heat. Temperature is independent of the number of particles or size and shape of the object.
  • Compressibility: Particles of a gas have large intermolecular spaces among them. By the application of pressure much of this space can be reduced and the particles be brought closer. Hence the volume of a gas can be greatly reduced. This is called compressing the gas.

Gas Laws

  • All gases, irrespective of their chemical composition, obey certain laws that govern the relationship between the volume, temperature and pressure of the gases. A given mass of a gas, under definite conditions of temperature and pressure, occupies a definite volume.

When any of the three variables is altered, then the other variables get altered. Thus these Gas laws establish relationships between the three variables of volume, pressure and temperature of a gas.

  • Boyle’s Law: “The product of the volume and pressure of a given mass of dry gas is constant, at constant temperature”.
  • Charles’ Law: “At constant pressure, the volume of a given mass of gas increases or decreases by 1/273 of its original volume at 32˚F, for each degree centigrade rise or lowering in temperature.”
  • Pressure Law: Volume remaining constant, the pressure of a given mass of gas increases or decreases by a constant fraction (=1/273) of its pressure at 0˚C for each degree Celsius rise or fall of temperature.
  • Avogadro’s Law: This is quite intuitive: the volume of a gas confined by a fixed pressure varies directly with the quantity of gas. Equal volumes of gases, measured at the same temperature and pressure, contain equal numbers of molecules. Avogadro’s law thus predicts a directly proportional relation between the number of moles of a gas and its volume.
  • Gay-Lussac’s Law: When different gases react with each other chemically to produce gaseous substances, then under the same condition of temperature and pressure, the volume of the reacting gases and product gases bear a simple ration among one another.


  • Avogadro Number: From Avogadro’s hypothesis, we know equal volume of all gases contain equal number of molecules at normal temperature and pressure.

The number is known as Avogadro Number and is equal to 6.06X1023.

  • The ideal gas equation of state: If the variables P, V, T and n (the number of moles) have known values, then a gas is said to be in a definite state, meaning that all other physical properties of the gas are also defined. The relation between these state variables is known as an equation of state.
  • An ideal gas is an imaginary gas that follows the gas laws and has 0 volume at 0 K i.e., the gas does not exist.

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