If CuSO4. 5H2O(s)  ⇌  CuSO4. 3H2O(s)  +  2H2O(l)  KP= 1.086  ×  10-4 atm2 at 250C. The efflorescent nature of CuSO4. 5H2O can be noticed when vapour pressure of H2O in atmosphere is

An efflorescent salt is one that loses H2O to atmosphere.For the reactionCuSO4. 5H2O(s)  ⇌  CuSO4. 3H2O(s)  +  2H2O(l)KP= pH2O'2= 1.086 × 10-4 pH2O'= 1.042 × 10-2 atm=7.92 mm∵    If   pH2O'  at  250C < 7.92 mm only, then reaction will proceed in forward direction.