Mixture of chloroform and acetone forms a solution with negative deviation from Raoult’s law due to:

When two liquids are mixed, the total vapor pressure is often predicted by Raoult’s law, which assumes that each component behaves as if it were pure, with its own escaping tendency.

In a mixture of chloroform (CHCl₃) and acetone ((CH₃)₂CO), the observed vapor pressure is lower than the value calculated from Raoult’s law. This phenomenon is called a negative deviation.

Reason for negative deviation: The two molecules can form specific attractive interactions that are stronger than the average intermolecular forces present in the pure components. Acetone possesses a carbonyl group (C=O) that is a good hydrogen‑bond acceptor, while chloroform contains a slightly acidic hydrogen attached to a carbon bearing three chlorines. This hydrogen can engage in hydrogen bonding with the carbonyl oxygen of acetone.

The formation of these hydrogen bonds creates a **new, stronger intermolecular force** between unlike molecules. As a result:

• The molecules are held more tightly together, reducing their tendency to escape into the vapor phase.
• The effective escaping tendency (or vapour pressure) of each component is lowered compared with the prediction from Raoult’s law.

Thus, the mixture exhibits a negative deviation because the attractive hydrogen‑bonding interaction between acetone and chloroform is stronger than the average forces in the pure liquids.

In summary, the negative deviation from Raoult’s law for the chloroform‑acetone system is caused by the formation of hydrogen bonding between acetone and chloroform.