C6H12O6, which is glucose, is a compound and therefore a pure substance. Elements and compounds are pure substances. A pure substance has a fixed composition which you can see glucose has from the chemical chemical formula. All particles in a sample of a pure substance are the same. Compounds are only broken down into their elements when a chemical reaction takes place.
A mixture contains more than one type of particle so its composition can vary. For example, you can prepare a solution of water and alcohol with any proportions. You can purchase rubbing alcohol that's 70% or 90% alcohol. The components of mixtures can be separated by physical means such as filtration or evaporation. While pure substances are always uniform, mixtures can be uniformly distributed (homogeneous) or unevenly distributed (heterogeneous).