* Mixture: A combination of two or more substances where each substance retains its own properties.

* Pure substance: A substance made up of only one type of atom or molecule.

When you dissolve sugar in water, you are not chemically changing the sugar. The sugar molecules are simply dispersed throughout the water molecules. You can still separate the sugar from the water through evaporation, leaving you with the original sugar and water.

Since the sugar and water retain their individual properties, the solution is a mixture.