Question

Why entropy of mixing is not zero for ideal solution?

Answer 1

while mixing ideal solutions, the mixing process is not coupled with heat tinting (delta H = 0) as well as with changes in the total volume of the mixture (delta V = 0). Although delta H = 0, delta S does not need to be zero, since it is defined as follows:

delta S = - n R ( x1 lnx1 + x2 lnx2 )

Since 0 < x1, x2 < 1, lnx1 and lnx2 will both be negative and lead to a positive delta S.