Why is it sometimes difficult to distinguish between a pure liquid and a solution, and how can they be distinguished?

It is difficult to distinguish between a pure liquid and a
solution created using it when there is no change in appearance. For example it is not
possible to differentiate between pure water and a solution of common salt just by
looking at the two as the salt is transparent and does not impart any color to the
water.

To identify the solution, other changes in the
properties of the liquid can be used. For example, when a solute is dissolved in a
liquid to create a solution, the density of the solution is higher than that of the
liquid. In many cases the freezing and boiling point of the liquid changes. These other
changes can be used to easily identify a solution if there is no change in the way it
looks.