Answer: Sodium chloride is an ionic solid, and when it dissolves in water, its individual sodium and chloride ions are dissociated from one another. Sugar molecules, on the other hand, maintain the bonds between their constituent atoms when dissolved, and thus remain as whole molecules in the solution.
Explanation: Here's why: Salt is about 25% more dense than sugar. Therefore a teaspoon of salt weighs more than a teaspoon of sugar by almost 25%. Your dissolving test would not be accurate because you would be starting out with a larger mass of salt than sugar.