A mixture of ethanol and acetone shows a positive deviation from Raoult's law. In pure ethanol, the molecules are strongly held together by intermolecular hydrogen bonding. When acetone is added, its molecules get in between the ethanol molecules and break some of these hydrogen bonds. This weakens the overall intermolecular attractive forces (i.e., ethanol-acetone interactions are weaker than pure ethanol-ethanol and acetone-acetone interactions). Consequently, the escaping tendency of the molecules increases, resulting in a higher vapour pressure than predicted by Raoult's law (a positive deviation).