Which one among the following chemical tests is used to distinguish monosaccharide from disaccharide ?
Show Hint
To differentiate carbohydrates, remember these key tests: Barfoed's (mono vs. di), Seliwanoff's (keto vs. aldo), and Iodine (starch). Tollen's and Fehling's tests are for reducing vs. non-reducing sugars in general.
Let's evaluate the purpose of each test:
(A) Seliwanoff's test is used to distinguish between aldose and ketose sugars. Ketoses give a rapid positive test (a cherry-red color).
(B) Barfoed's test is specifically used to distinguish between reducing monosaccharides and reducing disaccharides. The reagent consists of copper(II) acetate in a weakly acidic solution. Monosaccharides, being stronger reducing agents, reduce the Cu$^{2+}$ ions to red cuprous oxide (Cu$_2$O) precipitate much faster (within 2-3 minutes) than disaccharides (which take 10 minutes or more).
(C) Tollen's test (silver mirror test) is a general test for reducing sugars (and aldehydes). Most monosaccharides and many disaccharides (like maltose and lactose) are reducing sugars, so they both give a positive test. It cannot distinguish between them.
(D) The Iodine test is used to test for the presence of starch, which is a polysaccharide. It does not give a positive test for monosaccharides or disaccharides.
Therefore, Barfoed's test is the correct choice to distinguish monosaccharides from disaccharides.
