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I am confused about Positive behaviour from ideal behaviour as to why it occurs.

1) Molecular attractions plays a dominant role in positive deviation

OR 2) Molecular volume plays dominant role in positive deviation

I am confused these two facts.

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  • $\begingroup$ Are you asking about deviations of compressibility $pV/RT$ from unity as gas pressure increases ? If so, at low T and p molecular attractions dominate vs average thermal energy. At higher temperatures and pressures repulsive forces dominate which tend to make $V \gt V_{ideal}$. $\endgroup$ – porphyrin Mar 26 '17 at 13:18
  • $\begingroup$ @porphyrin I am talking with respect to vapour pressure. $\endgroup$ – TrY iS CheM Mar 26 '17 at 13:36
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If in a binary solution of two components A and B, the interaction between unlike components A-B is weaker than the interaction between the like components A-A and B-B, then it means that in such solutions molecules of A (or B) will find it easier to escape in the state A-B, than when they are in their pure states (A-A or B-B). It means that molecules of A (or B) will find it easier to escape from the A-B form.

This will result in having greater vapour pressure of each component of the solution than expected on the Basis of Raoult's law, and hence the total vapor pressure will also be higher than in the case of ideal solution. Thus the deviation will be positive.

In such solution $\Delta_{mix}H$ is positive because energy is required to break A-A and B-B bonds. For such solutions, the dissolution process is endothermic, i.e, the solubility will increase with temperature (Le Chatelier's principle) .

$\Delta_{mix}V$ is also positive because there is decrease in the magnitude of intermolecular forces in the solution, the molecules are loosely held, and therefore the volume of mixing increases.

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