# Positive deviation from ideal behaviour [closed]

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.

• 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 $V \gt V_{ideal}$. – porphyrin Mar 26 '17 at 13:18
• @porphyrin I am talking with respect to vapour pressure. – TrY iS CheM Mar 26 '17 at 13:36

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.