Glucose behaves as a typical aldehyde toward several reagents while also showing ring-specific behaviour, and reconciling both requires recognising the open-chain and cyclic forms coexist. Oxime formation with hydroxylamine and the addition of one HCN molecule are classic carbonyl reactions, proving the presence of one free aldehyde (-CHO) group in the open-chain aldohexose. Mutarotation, the gradual change in optical rotation of a fresh solution, arises because the open-chain aldehyde recyclises through its C5 hydroxyl to form a hemiacetal, generating alpha and beta anomers at the new C1 stereocentre that interconvert via the open form. Thus the equilibrium between the open-chain aldehyde and the cyclic hemiacetal explains all three facts at once. A ketone-only description fails because glucose forms an oxime characteristic of aldehydes; a carboxylic acid end is absent in glucose itself; and two aldehyde groups are not present. This is precisely the NCERT account of glucose structure. As a check, the failure of glucose to give the Schiff aldehyde test and its existence mainly as the pyranose ring are both consistent with a small equilibrium concentration of the reactive open-chain aldehyde.