Read the passage carefully and answer the questions.
Carbohydrates are naturally formed organic compounds from various plants. The common chemical formula, \( (C_x(H_2O)_y) \) was considered as hydrates of carbon to derive the various carbohydrates. For instance, the general formula for glucose \( (C_6H_{12}O_6) \) easily fits into the above formula. Carbohydrates also called saccharides are classified on the basis of their behavior towards hydrolysis. Monosaccharides cannot be hydrolyzed further to simpler units of polyhydroxy aldehyde or ketones. About 20 monosaccharides are known to occur in nature. Carbohydrates that yield large number of monosaccharide units are called polysacccharides. Amino acids contain \( (-NH_2) \) and \( (-COOH) \), which can be classified in various forms. Proteins are polymers of alpha amino acids which are connected to each other by a peptide bond or peptide linkage. When a native protein is subjected to physical change, its globules unfold and helix get uncoiled and the protein loses its biological activity. The protein is said to be denaturated.
Which of the following statements is incorrect for Lactose?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → It is an invert sugar
Invert sugar is the equimolar mixture of glucose and fructose obtained by the hydrolysis of sucrose, in which the sign of rotation changes from (+) to (−). Lactose is a different disaccharide: its hydrolysis gives galactose and glucose, and no such inversion is involved.
\( \displaystyle \text{Lactose} + \ce{H2O} \xrightarrow{\text{dil. acid or lactase}} \beta\text{-D-galactose} + \beta\text{-D-glucose} \)
Option (1) Lactose is a reducing sugar — Correct for lactose. The units are joined between C1 of galactose and C4 of glucose, so C1 of the glucose unit is free; a free aldehyde group can form there, and lactose reduces Tollens’ reagent and Fehling’s solution.
Option (2) It is known as milk sugar — Correct for lactose. Lactose is the disaccharide found in milk.
Option (3) On hydrolysis it yields galactose and glucose — Correct for lactose. It is made of a β-D-galactose unit and a β-D-glucose unit, set free on hydrolysis.
Option (4) It is an invert sugar — Incorrect for lactose (invert sugar comes from sucrose). Sucrose (+) gives dextrorotatory glucose and laevorotatory fructose; the larger laevorotation of fructose makes the mixture laevorotatory, so the sign is inverted. Lactose shows no such change.
NCERT (2026-27 edition):
Disaccharides – …
Disaccharides – “… Sucrose is dextrorotatory but after hydrolysis gives dextrorotatory glucose and laevorotatory fructose. Since the laevorotation of fructose (–92.4°) is more than dextrorotation of glucose (+ 52.5°), the mixture is laevorotatory. Thus, hydrolysis of sucrose brings about a change in the sign of rotation, from dextro (+) to laevo (–) and the product is named as invert sugar.”