When a dilute salt solution if freezing why does the solvent come out first??

and also why in saturated solution solute comes out first????

THE MAXIMUM VALUE OF INDEX OF REFRACTION

OF A MATERIAL OF A PRISM WHICH ALLOWS THE

PASSAGE OF LIGHT THROUGH IT WHEN THE

REFRACTING ANGLE OF THE PRISM IS 'A' IS

 

a)(1-sinA/2)^1/2           b)(1-cotA/2)^1/2

 

c)(1-(tanA/2)^2)^1/2     d)(1+(cotA/2)^2)1/2