High quality distilled water is not a mixture because the distillation process involves the removal of impurities from water via evaporation; however, distilled water from toxic sources, or that has been made with dirty equipment, may still contain impurities. This low quality distilled water is considered a mixture.
A mixture is made when two or more substances are combined in a single volume yet still retain their unique chemical properties. For example, lemonade is a mixture because the acid, water and sugar in the drink do not chemically bond together to form a new substance. Instead, the acid and sugar are dissolved in the water and float around within it.
Distillation of water removes most minerals dissolved within it. Certain impurities can survive the distillation process, however; namely, chlorine. Contaminated equipment can also introduce impurities after distillation has been completed. Proper distillation from a clean source produces a pure substance.