When sugar is added to water, the sugar dissolves and the resulting liquid is clear. Do the sugar and water form a pure substance, or do they form a mixture?
It forms a solution.
A pure substance is when only one type of atom or molecule is present. In this case the substance contains two different molecules; sugar and water, assuming sugar is all one type (e.g. glucose) and there are no contaminants in the water or sugar before combining the two.