For example, the system of chloroform (CHCl 3) and acetone (CH 3 COCH 3) has a negative deviation from Raoult's law, indicating an attractive interaction between the two components that have been described as a hydrogen bond. The system HCl–water has a large enough negative deviation to form a minimum in the vapor pressure curve known as a (negative) azeotrope, corresponding to a mixture that evaporates without change of composition. When these two components are mixed, the reaction is exothermic as ion-dipole intermolecular forces of attraction are formed between the resulting ions (H 3 O and Cl) and the polar water molecules so that Δ H mix is negative.