Glucose is a substance.
A substance, in chemistry, refers to a form of matter that has a constant composition and distinct properties. Given that every sample of glucose consistently consists of 8 parts oxygen, 6 parts carbon, and 1 part hydrogen, it demonstrates a consistent and well-defined chemical composition. This indicates that glucose is a pure substance, not a mixture.
In contrast, a mixture is a combination of two or more substances that are physically intermingled but not chemically combined. Mixtures can have varying compositions and properties depending on the proportions of the substances involved. Since glucose always has the same composition, it fits the definition of a pure substance.