【正确答案】(a) The boiling point is the temperature at which the vapor pressure of the liquid equals the surrounding pressure. The normal boiling point is the temperature at which the vapor pressure is 1 atm.
   (b) The freezing point is the temperature at which solid first crystallizes from solution;the freezing point depression is the difference between that temperature and the freezing point of the pure solvent. For example, a certain benzene solution starts to freeze at4. 5℃ ; its freezing point depression is (5. 5 4.5)℃ =1.0℃