In solution showing positive deviation, the partial pressure of each component of solution is greater than the vapour pressure as expected according to Raoul's law.
In solution of methanol and benzene, methanol molecules are held together due to hydrogen bonding as shown below.
On, adding benzene, the benzene molecules get in between the molecules of methanol, thus breaking the hydrogen bonds. As the resulting solution has weak intermolecular attraction, the escaping tendency of alcohol and benzene molecule from the solution increases. Consequently the vapour pressure as expected from Raoult's law increases.
