MPSC 2024 Prelims Paper-I · Question 70 of 100

Correct — option (2), 'Iodine value'. The iodine value of an oil or fat is defined as the mass of iodine, in grams, taken up by a hundred grams of the sample, and it measures unsaturation directly. The chemistry behind the definition is the addition reaction of a halogen across a carbon-carbon double bond: each double bond in the fatty acid chains absorbs one molecule of halogen, so the quantity of iodine consumed is a straightforward count of the double bonds present. A highly unsaturated oil therefore has a high iodine value and a saturated fat a low one, which is why the number tracks the familiar distinction between liquid oils and solid fats — the more double bonds a chain carries, the more its kinks prevent close packing, and the lower the melting point. The measurement has practical uses beyond the laboratory. Drying oils used in paints and varnishes are identified by a high iodine value, because it is the unsaturation that lets them polymerise on exposure to air. The hydrogenation of vegetable oils to make vanaspati works by saturating those same double bonds with hydrogen over a nickel catalyst, so the iodine value of the product is markedly lower than that of the oil it was made from, and the fall in iodine value is the standard way of following the process. The other three quantities offered all say something about an oil, but none of them counts double bonds.

• (1)Saponification value — The saponification value is the number of milligrams of potassium hydroxide needed to saponify one gram of the fat — that is, to hydrolyse its ester linkages into glycerol and the salts of the fatty acids, which are soap. What it measures is the average chain length, and therefore the average molecular mass, of the fatty acids present: a fat built from short chains has more ester groups per gram and so consumes more alkali, giving a high saponification value, while one built from long chains gives a low one. It is entirely indifferent to whether those chains contain double bonds, which is precisely the information the question asks for.
• (3)Acid value — The acid value is the number of milligrams of potassium hydroxide required to neutralise the free fatty acids present in one gram of the sample. It measures how much of the fat has already been broken down out of its ester form, and so it is used as an index of deterioration — a rising acid value in a stored oil signals hydrolytic rancidity and poor keeping quality. The quantity being counted is free acid groups, not double bonds; an oil can be thoroughly unsaturated and perfectly fresh, with a high iodine value and a very low acid value.
• (4)Viscosity value — Viscosity is a physical property, a measure of a liquid's resistance to flow, and it is not one of the standard analytical constants by which oils and fats are characterised in the way that the iodine, saponification and acid values are. It does vary with composition and strongly with temperature, so it cannot serve as a specific measure of unsaturation: two oils of quite different double-bond content can be brought to the same viscosity simply by warming one of them. The option is included as the one choice that does not belong to the family of chemically defined values at all.

Oils and fats are triglycerides — esters of glycerol with three fatty acid chains — and they are characterised in the laboratory by a small set of defined numbers, each of which isolates one property of those chains. The iodine value counts double bonds, since halogen adds across each one, and so measures unsaturation; it is high for liquid oils such as linseed, sunflower and safflower and low for solid fats. The saponification value measures the alkali needed to hydrolyse one gram of the fat and therefore reflects the average molecular mass of the fatty acids, being high for short-chain fats such as coconut oil and butter. The acid value measures free fatty acid and so indicates rancidity and storage quality. Behind all three lies the structural point that governs the whole subject: saturated chains are straight and pack closely, giving higher melting points and solids at room temperature, while unsaturated chains carry cis double bonds that put a permanent bend in the molecule, prevent close packing and give liquids. Hydrogenation removes those bends by adding hydrogen across the double bonds over a nickel catalyst, converting an oil into a semi-solid fat; the by-product of partial hydrogenation is the formation of trans fatty acids, which is why the process has become a public health as well as an industrial subject.

MPSC's chemistry section includes a strand of applied and food chemistry, and the analytical constants of oils and fats sit in it alongside questions on hydrogenation, rancidity, soaps and detergents. The questions are almost always definitional — which value measures what — so the reliable preparation is a four-line table pairing each named value with the property it reports. This particular item is printed as a fill-in-the-blank stem, one of the several stem shapes this paper uses, and it carries no statement list, so the whole question rests on a single definition. It is worth noting that the trio of iodine, saponification and acid values also underpins food-standards work: specifications for edible oils are written partly in terms of these numbers, which makes them a plausible source of questions in the current-affairs register as well as the scientific one.

• The iodine value is the mass of iodine in grams absorbed by 100 grams of an oil or fat, and it measures the degree of unsaturation, since halogen adds across each carbon-carbon double bond.
• A high iodine value indicates many double bonds, characteristic of liquid oils and of the drying oils used in paints and varnishes.
• The saponification value is the milligrams of potassium hydroxide needed to saponify one gram of fat, and it reflects the average chain length of the fatty acids.
• The acid value is the milligrams of potassium hydroxide needed to neutralise the free fatty acids in one gram of fat, and it is used as an index of rancidity.
• Hydrogenation of vegetable oils over a nickel catalyst saturates the double bonds and lowers the iodine value, converting a liquid oil into a semi-solid fat.

• Confusing the saponification value with the iodine value; the first reports chain length, the second reports double bonds
• Reading the acid value as a measure of composition when it is a measure of deterioration
• Assuming a physical property such as viscosity can substitute for a chemically defined analytical constant
• Forgetting that hydrogenation lowers the iodine value, since it removes the very double bonds the value counts

Applied and food chemistry appears in MPSC papers as definitional recall of exactly this kind — which quantity measures which property, which process produces which change, which additive serves which purpose. The questions are short, carry no statement list as often as not, and are answered instantly by anyone holding the definitions; they are among the cheapest marks in the science section for a prepared candidate. Expect companion items on hydrogenation and vanaspati, on rancidity and antioxidants, and on the difference between soaps and detergents.

No directly related past PYQ was found.

• practice — not a real PYQ

  The hydrogenation of a vegetable oil to make vanaspati has which of the following effects on its iodine value ?

  • (a)The iodine value rises, because hydrogen is added
  • (b)The iodine value falls, because double bonds are saturated
  • (c)The iodine value is unchanged, since only the melting point alters
  • (d)The iodine value becomes zero in every case

  Answer(b) The iodine value falls, because double bonds are saturated — hydrogenation adds hydrogen across the carbon-carbon double bonds of the fatty acid chains, and since the iodine value counts those double bonds, removing them lowers it. Partial hydrogenation leaves some double bonds intact, so the value falls without reaching zero.

• practice — not a real PYQ

  The saponification value of a fat is a measure of :

  • (a)The degree of unsaturation of its fatty acid chains
  • (b)The average molecular mass, and hence chain length, of its fatty acids
  • (c)The quantity of free fatty acid present as a result of rancidity
  • (d)Its resistance to flow at a stated temperature

  Answer(b) The average molecular mass, and hence chain length, of its fatty acids — a fat of short chains has more ester linkages per gram and so consumes more alkali on saponification. Unsaturation is measured by the iodine value, free fatty acid by the acid value, and resistance to flow is viscosity, a physical property.