Question 5
In recrystallisation, why do pure crystals form while the impurities stay in solution?
The right answer is the one marked above.
Because the impurities are too dilute to reach saturation as it cools. The wanted solute becomes saturated as the solution cools and so crystallises. The impurities, present only in small amounts, never reach saturation and stay dissolved.
The hot filtration earlier in the method removes the impure parts that would not dissolve at all. These two steps deal with two different kinds of impurity.