Chemistry > States of Matter

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  • Intermolecular forces are the forces of attraction and repulsion between interacting particles.
  • Attractive intermolecular forces are known as van der Waals forces. it is important to note that attractive forces between an ion and a dipole are known as ion-dipole forces and these are not Van der Waals forces.
  • London forces

    The London dispersion force is the weakest intermolecular force. It is a temporary attractive force that results when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles. This force is sometimes called an induced dipole-induced dipole attraction. London forces are the attractive forces that cause non-polar substances to condense to liquids and to freeze into solids when the temperature is lowered sufficiently.

  • These forces are always attractive and interaction energy is inversely proportional to the sixth power of the distance between two interacting particles (i.e., 1/r 6 where r is the distance between two particles).

Dipole-dipole forces

  • Dipole-dipole forces are attractive forces between the positive end of one polar molecule and the negative end of another polar molecule. Dipole-dipole forces have strengths that range from 5 kJ to 20 kJ per mole. They are much weaker than ionic or covalent bonds and have a significant effect only when the molecules involved are close together (touching or almost touching).

  • Polar molecules have a partial negative end and a partial positive end.
  • The partially positive end of a polar molecule is attracted to the partially negative end of another

Dipole Induced Dipole Forces

  • A dipole-induced dipole attraction is a weak attraction that results when a polar molecule induces a dipole in an atom or in a non-polar molecule by disturbing the arrangement of electrons in the non-polar species.

Hydrogen bonding

  • The hydrogen bond is really a special case of dipole forces. A hydrogen bond is the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. Usually the electronegative atom is oxygen, nitrogen, or fluorine, which has a partial negative charge. The hydrogen then has the partial positive charge. Hydrogen bonding is usually stronger than normal dipole forces between molecules.

Boyle's Law

  • At constant temperature, the pressure of a fixed amount (i.e., number of moles n) of gas varies inversely with its volume. This isknown as Boyle's law.

    pV = K

    p- Pressure, V-volume, K-constant.

  • At a constant temperature, pressure is directly proportional to the density of a fixed mass of the gas.

Charles Law

  • Charles' Law describes the direct relationship of temperature and volume of a gas. Assuming that pressure does not change, a doubling in absolute temperature of a gas causes a doubling of the volume of that gas. A drop of absolute temperature sees a proportional drop in volume. The volume of a gas increases by 1/273 of its volume at 0°C for every degree Celsius that the temperature rises

    Temperature = Constant x Volume


    Volume = Constant x Temperature


    Volume/Temperature = Constant

  • Mathematically,

Gay Lussac's Law

  • At constant volume, pressure of a fixed amount of a gas varies directly with the temperature.
  • Mathematically,

     = constant

Avogadro's Law

  • It states that equal volumes of all gases under the same conditions of temperature and pressure contain equal number of molecules.
  • Mathematically,

    V = k*n

    k = Avogadro number =

Ideal Gas Equation

  • A gas that follows Boyle's law, Charles' law and Avogadro law strictly is called an ideal gas
  • Mathematically,

    pV = n RT.

  • R is called gas constant. It is same for all gases. Therefore it is also called Universal Gas Constant and its value is = 8.314 J K-1mol-1.

Combined Gas Law

Density and Molar Mass of a Gaseous Substance

  • M =  (d=density)

Dalton's Law of Partial Pressures

  • The total pressure exerted by the mixture of non-reactive gases is equal to the sum of the partial pressures of individual gases.
  • p1+p2+p3+......(at constant T, V) Pressure exerted by saturated water vapour is called aqueous tension. Aqueous tension of water at different temperatures.

Partial pressure in terms of mole fraction

  • where  is mole fraction.


  • Due to the failure of the following two assumptions of the Kinetic gas theory the deviation is observed.
    • There is no force of attraction between the molecules of a gas.
    • Volume of the molecules of a gas is negligibly small in comparison to the space occupied by the gas.
  • The deviation from ideal behaviour can be measured in terms of compressibility factor Z, which is the ratio of product pV and nRT.
  • At high pressure all the gases have Z > 1. These are more difficult to compress. At intermediate pressures, most gases have Z < 1. Thus gases show ideal behaviour when the volume occupied is large so that the volume of the molecules can be neglected in comparison to it.
  • The temperature at which a real gas obeys ideal gas law over an appreciable range of pressure is called Boyle temperature or Boyle point. Boyle point of a gas depends upon its nature. Above their Boyle point, real gases show positive deviations from ideality and Z values are greater than one. The forces of attraction between the molecules are very feeble. Below Boyle temperature real gases first show decrease in Z value with increasing pressure, which reaches a minimum value.


Sample Examples



A balloon is filled with hydrogen at room temperature. It will burst if pressure exceeds 0.2 bar. If at 1 bar pressure the gas occupies 2.27 L volume, upto what volume can the balloon be expanded ?


According to Boyle's Law p1V1 = p2V2

If p1 is 1 bar, V1 will be 2.27 L

If p2 = 0.2 bar, then

V2 = p1V1/ p2 = 1*2.27/0.2

V = =11.35 L

Since balloon bursts at 0.2 bar pressure, the volume of balloon should be less than11.35 L.



At 25°C and 760 mm of Hg pressure a gas occupies 600 mL volume. What will be its pressure at a height where temperature is 10°C and volume of the gas is 640 mL.


P1 = 760 mm Hg, V1= 600 mL

T1 = 25 + 273 = 298 K

V2 = 640 mL and T2 = 10 + 273 = 283 K

  • According to Combined gas law,

Substituting the values of  in the above equation,

P2 = 676.6 mm Hg



On a ship sailing in Pacific Ocean where temperature is 23.4 °C, a balloon is filled with 2 L air. What will be the volume of the balloon when the ship reaches Indian ocean, where temperature is 26.1°C ?


V1 = 2 L

T2 = (26.1 + 273) K = 299.1 K

T1 = (23.4 + 273K) = 296.4 K

From Charles law,

Substituting the values of  in the above equation, we get


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