Chemistry: States of Matter: Vander Waals'equation and Liquefaction of gases

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Vander Waals’ Equation

  • The van der Waals equation of state approaches the ideal gas law PV=nRT as the values of these constants approach zero.

  • Van der Waals attributed the deviation of real gases from gas equation to the following faulty assumptions of the kinetic theory.

  • (

  • The actual volume of the gas molecules is negligible as compared with the total volume of the gas.

  • The gas molecules do not exert any appreciable attraction on each other.

Liquefaction of Gases

Liquification of Gases

Liquification of Gases

Liquification of Gases

  • The substances in the gaseous state are converted to the liquid state.

  • It generates a liquid from a solid or a gas.

  • Liquefaction occurs when vibrations or water pressure within a mass of soil cause the soil particles to lose contact with one another.

Critical Constants

  • The critical temperature, Tc, is characteristic of every gas and may be defined as: “The temperature below which the continuous increase of pressure on a gas ultimately brings about liquefaction and above which no liquefaction can take place no matter what so ever pressure be applied”.

  • The pressure required to liquefy the gas at critical temperature is called critical pressure and the volume occupied by 1 mole of gas under these conditions is called the critical volume.

Properties of Liquids

  • Liquids have fixed volume but no fixed shape.

  • Liquids flow from higher to lower level.

  • Under normal conditions liquid have their boiling points above room temperature.

  • Liquids are almost incompressible.

Vapour Pressure

  • The pressure caused by the evaporation of liquids.

  • The factors influencing vapor pressure are surface area, intermolecular forces and temperature.

  • The vapor pressure of a molecule differs at different temperatures.

Surface Tension

Image of Surface Tension

Image of Surface Tension

Image of Surface Tension

  • It is defined as the physical property equal to the amount of force per unit area necessary to expand the surface of a liquid. For eg.the molecules of a water droplet are held together by cohesive forces.

  • It is measured as the energy required increasing the surface area of a liquid by a unit of area.

  • When the attractive forces are between unlike molecules, they are said to be adhesive forces.

  • A carefully placed small needle can be made to float on the surface of water even though it is several times as dense as water is due to the surface tension phenomenon.


Image of Viscosity

Image of Viscosity

Image of Viscosity

  • Viscosity is defined as the fluid’s resistance to flow.

  • The internal friction of a moving fluid is described by the Viscosity.

  • It is the measure of a substance’s resistance to motion under an applied force.

  • Water is an example of a low viscosity liquid.

Gay Loussac’S Law of Combining Volumes

Image of Gay Loussac's Law of Combining Volumes

Image of Gay Loussac's Law of Combining Volumes

Image of Gay Loussac's Law of Combining Volumes

  • This law states that when gases react, they do so in volumes which bear a simple ratio to one another, and to the volume of the product(s) formed if gaseous, provided the temperature and pressure remain constant.

For the reaction

1 volume of nitrogen combines with 3 volumes of hydrogen to form 2 volumes of ammonia.

Avogadro’S Law

  • The Italian physicist Amadeo Avogadro was the first to propose, in 1811, a relationship between the volume of a gas and the number of molecules present in it.

  • Equal volumes of all gases at the same temperature and pressure contain equal number of molecules. Mathematically, Avogadro’s’ law is expressed as: V α N (at constant temperature and pressure) Where V and N are volume and number of molecules respectively. At a given temperature and pressure, the number of molecules present in the gas is directly proportional to the number of moles. Therefore, where n is the number of moles ∴



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