Raoult's law states that a solvent's partial vapour pressure in a solution (or mixture) is equal or identical to the vapour pressure of the pure solvent multiplied by its mole fraction in the solution.

The compounds which show negative deviation from Raoult's law are a mixture of acetone and chloroform. This results in the formation of the hydrogen bonding between atoms of molecules. There is a decrease in the vapour pressure of the mixture that results in a negative deviation from Raoult's law.

Solution with negative deviation has
$$\begin{gathered} {\mathrm{P}}_{\mathrm{T}}< {{\mathrm{P}}_{\mathrm{A}}}^0{\mathrm{X}}_{\mathrm{A}} + {{\mathrm{P}}_{\mathrm{B}}}^0{\mathrm{X}}_{\mathrm{B}} \\ {\mathrm{P}}_{\mathrm{A}}< {{\mathrm{P}}_{\mathrm{A}}}^0{\mathrm{X}}_{\mathrm{A}} \\ {\mathrm{P}}_{\mathrm{B}}< {{\mathrm{P}}_{\mathrm{B}}}^0{\mathrm{X}}_{\mathrm{B}} \end{gathered}$$
 If vapour pressure decreases, so the boiling point increases.