Q.Two liquids X and Y on mixing give a warm solution. The solution is

A non-ideal solution fails to obey Raoult's law across the whole range, and shows $\Delta H_{mixing}\ne0$, $\Delta V_{mixing}\ne0$.

Positive deviation: if A-B attraction is weaker than A-A and B-B attraction, molecules escape more readily than ideal, so $p_A>p^\circ_Ax_A$, $p_B>p^\circ_Bx_B$, and $p_{total}$ exceeds the Raoult's-law prediction. Example: ethanol-water -- weaker ethanol-water hydrogen bonding than ethanol-ethanol or water-water bonding; mixing is endothermic ($\Delta H_{mixing}>0$) with a slight volume increase. Other pairs: ethyl alcohol & cyclohexane, benzene & acetone, CCl$_4$ & chloroform, acetone & ethyl alcohol.

Negative deviation: if A-B attraction is stronger than A-A and B-B attraction, escaping tendency is lower than ideal, so $p_{total}$ is lower than predicted. Example: phenol-aniline -- new phenol-aniline hydrogen bonds are stronger than either component's self-bonding; mixing is exothermic ($\Delta H_{mixing}<0$) with a slight volume decrease. Other pairs: acetone + chloroform, chloroform + diethyl ether, acetone + aniline, chloroform + benzene. …