Explain the following:
(a) ROH's with three or fewer C's are `H_(2)O` soluble, those with five or more C's are insoluble, and those with four C's are marginally soluble.
(b) When equal volumes of ethanol and water are mixed the total volume is less than the sum of the two individual volumes.
(c ) Propanol (molecular weight = 60) has a higher boiling point than butane (molecular weight = 58).

Explanation

(a) The water solubility of alcohols is attributed to intermolecular H-bonding with `H_(2)O`. As the number of carbons increases, molecular weights of the alcohols also increase and their solubility in water decreases because greater carbon content makes the alcohol less hydrophilic. Conversely, their solubility in hydrocarbon solvents increases.
(b) Due to H-bonding ethanol and water molecules.
(c ) Alcohol molecules attract each other by relatively stronger H-bonds and somewhat weaker dipole-dipole interactions, resulting in a higher boiling point, Only weaker van der Waals forces must be overcome to vapourize the hydrocarbon.