Question

Why can we separate salt water and sugar solution with easily?

Answer 1

If they are mixed in water, then you could try reducing the volume by boiling, but not too long in case one or other comes out of solution.  Addition of an excess of acetone to the solution should precipitate the salt, which can be filtered off.  Evaporation of the acetone will yield the sugar.  An alternative is to mix vigorously with toluene, which will dissolve the sugar but not the salt, and separate the layers using a separating funnel (as the toluene does not mix with water).

Answer 2

Sugar is soluble in alcohols while salt is not. Simply mix the two in alcohol and filter the solution to get the salt. Then evaporate the alcohol sugar mixture and you will be left with the sugar.