What is meant by negative deviation from Raoults law and how is the sign of $∆$ H_mix related to negative deviation from Raoult's law?

1. When the vapour pressure of a solution is lower than the predicted value by Raoult's law, it is called negative deviation.
2. In such cases intermolecular interaction between solute and solvent particles (A and B) are stronger than those between solutesolute(A-A) and solvent-solvent(B-B).
3. It leads to decrease in vapour pressure resulting in negative deviation.
4. Characteristics of a solutions showing negative deviation
$\text{ 1) }{\mathrm{P}}_{\mathrm{A}}<{\mathrm{P}}_{\mathrm{A}}^0{\mathrm{X}}_{\mathrm{A}}:{\mathrm{P}}_{\mathrm{B}}<{\mathrm{P}}_{\mathrm{B}}^0{\mathrm{X}}_{\mathrm{B}}$
$\text{ 2) }{\mathrm{\Delta H}}_{\text{mix }}<0$ : i.e -ve, because weak A - A and B - B bonds are broken and strong A - B
bonds is formed. Heat is consequently released. 
$\text{ 3) }{\mathrm{\Delta V}}_{\text{mix }}<0;\text{ i.e, }-\mathrm{ve}$
5. Examples of solution showing negative deviation
i) HNO₃ and water
ii) Chloroform and acetone
iii) Acetic acid and pyridine
iv) Hydrochloric acid and water