Explanation: Pure substances have characteristics different from mixtures. Constituent particles of pure substances have fixed composition.

• Copper, silver, gold, water and glucose are some examples of pure substances.
• Glucose contains carbon, hydrogen and oxygen in a fixed ratio and its particles are of same composition.
• Hence, like all other pure substances, glucose has a fixed composition.
• Also, its constituents—carbon, hydrogen and oxygen—cannot be separated by simple physical methods.

Source: Science NCERT