The deviation from the ideal behavior is due to the difference between the interactions between the solute and solvent particles.
Positive deviation:- The solution of n-hexane and ethanol shows the positive deviation because the Hydrogen bonding in case of ethanol is decreased by the n-hexane molecules. Due to decreased attraction the vapour pressure of the mixture increases leading to positive deviation.
Negative deviation:- Solution of acetone and chloroform shows negative deviation because it forms Hydrogen bonding between the molecules. Due to this the vapour pressure of the mixture decreases leading to the negative deviation.