Which of the following has the maximum melting point but minimum solubility in water ?

Options

A. KCI
B. NaCI
C. CsCI
D. RbCI

Answer

B

Explanation

Lattice enthalpy is the amount of energy reuired to separate one mole of ionic solid into its gaseous ions. Thus, the greater the lattice enthalpy, the higher the melting point, and the lower the solubility. The decreasing order of lattice enthalpy is
`NaCI(-770 "kJ mol"^(-1)) gt KCI(-703) gt RbCI(-674) gt CsCI(-644)`
Thus,NaCI with the maximum lattice enthalpy has the highest melting point but the lowest solubility in water. For the same halide ion, the melting points of Li halides are lower than those of the corresponding Na halides and, thereafter, they decrease as we move down the group from Na to Cs :
The low melting point of LiCI(887 K) relative to that of NaCI(1084 K) is due to the fact that LiCI has substantial covalent character while NaCI is almost ionic. Thereafter, melting points decrease due to decrease of lattice enthalpy. It is interesting to note that LiCI `(828 "kJ mol"^(-1))`.