Chemistry: Solutions: Ideal and Non-Ideal Solutions: Positive and Negative Deviation (For CBSE, ICSE, IAS, NET, NRA 2022)

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Ideal and Non-Ideal Solutions

  • The ideal solutions obey Raoult՚s Law and during their formation there is no change in heat and volume.
  • The non-ideal solutions are those solutions which do not obey Raoult՚s law and whose formation is accompanied by changes of heat and volume.
  • Most of the real solutions are non-ideal and they show deviation from the ideal behavior.

Generally, deviations are of two types they are:

Positive Deviation

  • Positive deviations are shown by liquid pairs for which the A-B molecular interactions are weaker than the A-A and or B-B molecular interactions.
  • The total vapour pressure for such solutions is greater than predicted by Raoult՚s law.
  • The total vapour pressure for such a solution will be high for a particular intermediate composition.
  • Examples of non-ideal solutions showing positive deviation from the ideal behavior are mixtures of liquids such as water-propanol, ethanol-chloroform, acetone- carbon disulfide, ethanol-cyclohexane etc.
Positive Deviation

Negative Deviation

  • Negative deviations are shown by liquid pairs for which the A-B molecular interactions-are stronger than A-A or B-B molecular interactions.
  • The total vapour pressure for such solutions is less than that predicted by Raoult՚s law.
  • For a particular intermediate composition, the total vapour pressure of such a solution will be less.
  • Examples of such liquid pairs are chloroform- acetone, water - sulphuric acid, phenol-aniline, water- HCI etc.
Negative Deviation

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