Q.Which of the following acids is a vitamin?
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Water Soluble Vitamins: The First Meeting
Imagine you drop a spoonful of sugar into a glass of water. It dissolves completely, disappears into the liquid, and you can't see it anymore — but it's still there, molecule by molecule, spread evenly throughout the water. That's exactly what "water soluble" means: the substance mixes with water at the molecular level.
Now, a vitamin is a tiny organic compound your body needs in small amounts to function properly — it helps enzymes do their jobs, keeps your skin healthy, helps you see, and so on. Your body cannot make most vitamins on its own, so you must get them from food.
Put the two together: a water soluble vitamin is a vitamin that dissolves in water. This single property — solubility — determines almost everything about how these vitamins behave in your body.
The Precise Statement
Water soluble vitamins are vitamins that dissolve in water, are not stored in significant amounts in the body, and are excreted in urine when consumed in excess. They must be consumed regularly (daily or almost daily) to prevent deficiency.
The group includes the B-complex vitamins (B1, B2, B3, B5, B6, B7, B9, B12) and Vitamin C. That's 9 vitamins total.
Why Solubility Matters So Much
Think about fat. Fat and water don't mix — oil floats on top of water. Fat soluble vitamins (A, D, E, K) behave like oil: they dissolve in fat, get stored in your liver and fatty tissues, and can stay in your body for weeks or months. You don't need them every single day.
Water soluble vitamins are the opposite. Because they dissolve in water, they travel freely in your blood (which is mostly water). But your kidneys are constantly filtering your blood and removing excess water-soluble substances into urine. So if you take more Vitamin C than your body needs right now, the extra gets flushed out within hours.
This does not mean you can take unlimited amounts safely. Very high doses of some water soluble vitamins (especially B6 and niacin/B3) can cause toxicity — just not as easily as fat soluble vitamins.
The Practical Consequences
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You need them frequently — ideally every day. Missing a day won't cause immediate harm, but weeks of low intake will lead to deficiency diseases (scurvy for Vitamin C, beriberi for B1, pellagra for B3, etc.).
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Cooking destroys them — because they dissolve in water, boiling vegetables leaches vitamins into the cooking water. If you throw that water away, you lose the vitamins. Steaming or microwaving with minimal water preserves them better.
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They are fragile — heat, light, and air can break them down. That's why fresh fruits and vegetables lose vitamin content over time.
The Two Families
| Family | Members | Key Role |
|--------|---------|----------| …
Why this formula?
Water-Soluble Vitamins: Understanding Why They Behave the Way They Do
Let’s start with the core idea: Water-soluble vitamins are a group of organic compounds that dissolve in water and are not stored in large amounts in the body. The key “formulae” here are not mathematical equations but chemical structures and physiological principles that explain their solubility, absorption, and excretion.
1. Why Are They Water-Soluble? The Chemical Reason
The fundamental “formula” is the presence of polar functional groups (like –OH, –COOH, –NH₂, –C=O) in their molecular structure.
- Polar groups form hydrogen bonds with water molecules.
- Example: Vitamin C (ascorbic acid) Its structure has multiple –OH groups and a lactone ring.
C6H8O6(with 4 –OH groups)
Each –OH can hydrogen-bond with water, making it highly soluble.
Why this matters:
Unlike fat-soluble vitamins (A, D, E, K) which are nonpolar and stored in fatty tissues, water-soluble vitamins dissolve in blood and interstitial fluid. This means they are rapidly absorbed but also quickly excreted by the kidneys.
Key takeaway: Solubility is not magic — it’s a direct consequence of molecular polarity.
2. The “No Storage” Principle: Why Excretion Happens
The body has no specific binding proteins to store most water-soluble vitamins (except B₁₂).
- Once absorbed, they circulate freely in blood.
- Kidneys filter blood continuously.
- If plasma concentration exceeds the renal threshold, excess is excreted in urine.
Derivation of the excretion logic:
Let Cp = plasma concentration of vitamin, T = renal threshold.
- If Cp≤T → all filtered vitamin is reabsorbed.
- If Cp>T → excess (Cp−T) is lost in urine.
Why this holds:
The transport proteins in kidney tubules have a maximum capacity (saturation). Once saturated, the extra vitamin cannot be reabsorbed.
Exam tip: This is why water-soluble vitamins need daily intake — no storage means deficiency can develop in weeks.
3. The B-Complex “Formula”: Coenzyme Activation
Most B vitamins are precursors to coenzymes. The “formula” here is:
Vitamin (inactive)enzymeCoenzyme (active)
Example: Thiamine (B₁) → Thiamine pyrophosphate (TPP)
- Thiamine has a thiazole ring and a pyrimidine ring.
- In the body, it gets phosphorylated (adds two phosphate groups).
- The active form TPP is a coenzyme for decarboxylation reactions (e.g., in Krebs cycle).
Why this matters for exams:
You don’t need to memorize every step — just understand that the vitamin itself is not the active molecule. The body modifies it to create the functional coenzyme.
4. Vitamin C: The Redox “Formula”
Vitamin C (ascorbic acid) acts as a reducing agent. Its key reaction:
Ascorbic acid⇌Dehydroascorbic acid+2H++2e−
Why this holds:
The molecule has an enediol group (–C(OH)=C(OH)–) which can donate electrons.
- In the reduced form (ascorbic acid), it has two –OH groups on adjacent carbons.
- Upon oxidation, these become a diketone (C=O groups).
- This reversible redox pair allows vitamin C to scavenge free radicals and regenerate other antioxidants (like vitamin E).
Key exam point: The ability to donate electrons is why vitamin C is essential for collagen synthesis (proline hydroxylation requires reducing power).
5. Absorption: The “Active Transport” Formula …
The key idea is that ascorbic acid is the chemical name for Vitamin C, a water-soluble vitamin. The other options are not vitamins: aspartic acid is an amino acid, adipic acid is a dicarboxylic acid used in nylon production, and saccharic acid is an oxidation product of glucose.
- Identify which of the given acids is an essential micronutrient required in the diet. …
The question asks which acid is a vitamin. Ascorbic acid is vitamin C, a water-soluble vitamin essential for collagen synthesis and antioxidant function. The correct answer is (ii) Ascorbic acid.
This is a straightforward classification question from biochemistry. The key is to recall which of these compounds is an essential micronutrient that the human body cannot synthesise and must obtain from the diet.
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Aspartic acid is a non-essential amino acid. It is a building block of proteins and can be synthesised by the body. It is not a vitamin.
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Ascorbic acid is the chemical name for vitamin C. It is a water-soluble vitamin, meaning it dissolves in water and is not stored in large amounts. It acts as a cofactor for several enzymes (e.g., in collagen synthesis) and as a powerful antioxidant. Humans lack the enzyme L-gulonolactone oxidase needed to synthesise it, so it must come from food.
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Adipic acid is a dicarboxylic acid used industrially in nylon production. It has no vitamin activity and is not required in the human diet.
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Saccharic acid (also called glucaric acid) is a sugar acid derived from glucose. It is not a vitamin; it is a metabolic byproduct. …
Concept: Vitamins as Essential Organic Compounds
Vitamins are organic compounds required in small amounts for normal metabolic functions. They are not synthesized in sufficient quantities by the body and must be obtained from diet. The key is to recognize which acid among the options is a vitamin.
Method: Recall of Common Vitamin Names
Steps:
- Identify the vitamin form of each acid
- Aspartic acid → an amino acid, not a vitamin
- Ascorbic acid → Vitamin C (water-soluble vitamin)
- Adipic acid → a dicarboxylic acid used in nylon production, not a vitamin
- Saccharic acid → an oxidation product of glucose, not a vitamin …
Here are the common mistakes students make on this question, along with how to avoid each.
1. Confusing "Vitamin" with "Amino Acid"
- The Mistake: Students see the word "acid" and the suffix "-ic" and immediately think of amino acids (the building blocks of proteins). Aspartic acid (A) is a well-known amino acid, so many pick it without reading the question carefully.
- Why it happens: In biology and chemistry chapters, "amino acids" are drilled heavily. The brain defaults to the most familiar option.
- How to Avoid: Read the question stem twice. The question asks for a vitamin, not an amino acid. Before looking at the options, ask yourself: "Which of these is a vitamin I have studied?" If you don't recognize the name as a vitamin, eliminate it.
2. Assuming "Ascorbic" is a Complex Name for a Lab Chemical
- The Mistake: Students think "Ascorbic acid" sounds like a synthetic lab compound (like adipic or saccharic acid) and dismiss it.
- Why it happens: The common name for Vitamin C is rarely used in everyday conversation. Students memorize "Vitamin C" but forget its chemical name.
- How to Avoid: Memorize the chemical names of common vitamins.
- Vitamin C = Ascorbic acid (think: anti-scorbutic, meaning prevents scurvy).
- Vitamin B1 = Thiamine
- Vitamin B2 = Riboflavin
- Vitamin B3 = Niacin
- Vitamin B12 = Cyanocobalamin
- Vitamin A = Retinol
- Vitamin D = Calciferol
- Key trick: The word "ascorbic" is directly linked to scurvy (the disease caused by its deficiency). This is a classic exam linkage.
3. Falling for the "Acid" Trap (Adipic & Saccharic)
- The Mistake: Students pick Adipic acid (C) or Saccharic acid (D) because they sound like "vitamin-like" names or because they confuse "Saccharic" with "Saccharin" (an artificial sweetener).
- Why it happens: The names sound scientific and important. Students guess based on sound rather than knowledge.
- How to Avoid: Know the industrial uses of these acids.
- Adipic acid: Used to make nylon (a polymer). Not a vitamin.
- Saccharic acid: An oxidation product of glucose (sugar). Not a vitamin.
- Aspartic acid: An amino acid (found in proteins). Not a vitamin.
- Ascorbic acid: Vitamin C. This is the only one that is an essential micronutrient. …
- CBSE 2026Set ANNUAL1 markMCQQ.Vitamin C is(a) Water soluble(b) Fat soluble(c) Soluble in both(d) None of these
›Reveal solutionSolution
Vitamins are classified as either fat-soluble (A, D, E, K) or water-soluble (B-complex and C); Vitamin C belongs to the water-soluble group.
Vitamin C (ascorbic acid) dissolves in water due to its polar hydroxyl-rich structure, and like the B-complex vitamins, it is not stored in significant amounts in the body - excess is excreted in urine, and it must be replenished regularly th …
- CBSE 2025Set D1 markMCQQ.Water soluble vitamin is(a) Vitamin E(b) Vitamin D(c) Vitamin K(d) Vitamin B
›Reveal solutionSolution
Vitamins B and C are water soluble; A, D, E, K are fat soluble — so the answer is Vitamin B.
Vitamins are classified by their solubility:
- Water soluble: vitamin B-complex (B1, B2, B6, B12 etc.) and vitamin C. These are not stored in the body and must be supplied regularly in diet; excess is excreted in urine. …
- CBSE 2025Set ANNUAL1 markQ.Fill in the blank: Water soluble Vitamins are Vitamin B and ________.
›Reveal solutionSolution
The water-soluble vitamins are vitamin B-complex and vitamin C; the rest (A, D, E, K) are fat-soluble.
Vitamins are classified by solubility:
- Fat-soluble: A, D, E, K (stored in the body's fatty tissue and liver). …
- CBSE 2025Set ANNUAL1 markQ.Write the chemical name of Vitamin C.
›Reveal solutionSolution
Vitamin C is L-ascorbic acid, a water-soluble vitamin.
Vitamin C is chemically known as ascorbic acid. It is a water-soluble vitamin (unlike A, D, E, K which are fat-soluble) and is not stored in the body, so it must be supplied regul …
- CBSE 2023Set 56/3/11 markMCQQ.Which of the following vitamins is water soluble? (A) Vitamin A (B) Vitamin D (C) Vitamin E (D) Vitamin C
›Reveal solutionSolution
Vitamins divide into fat-soluble (A, D, E, K) and water-soluble (B-complex and C) based on their chemical structure and solubility. Vitamin C is the only water-soluble option here.
Understanding vitamin solubility
Vitamins fall into two fundamental categories determined by their molecular architecture. Fat-soluble vitamins possess long hydrocarbon chains or ring structures that make them nonpolar—they dissolve in lipids and organic solvents. Water-soluble vitamins, by contrast, carry multiple hydroxyl groups (−OH) or other polar functional groups that form hydrogen bonds with water molecules.
This distinction isn't academic trivia; it governs how your body handles each vitamin. Fat-soluble vitamins (A, D, E, K) get stored in liver and adipose tissue, accumulating over time. Water-soluble vitamins (the B-complex family and vitamin C) cannot be stored in significant amounts—excess quantities are excreted in urine, which is why you need regular dietary intake.
Analyzing each option
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Vitamin A (retinol): A long-chain alcohol with a β-ionone ring and an isoprenoid tail. Completely hydrophobic. Stored in the liver as retinyl esters. Fat-soluble.
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Vitamin D (calciferol): A steroid derivative with four fused rings and a side chain. Synthesized in skin from cholesterol upon UV exposure. Stored in fat tissue. Fat-soluble.
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Vitamin E (tocopherol): Features a chromanol ring attached to a long phytyl tail (16 carbons). Acts as a lipid-phase antioxidant in cell membranes. Fat-soluble. …
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- CBSE 2023Set F1 markMCQQ.Which of the following vitamins are soluble in water?(a) A and B(b) C and D(c) B and C(d) A and D
›Reveal solutionSolution
The B-complex vitamins and vitamin C are water-soluble; A, D, E and K are fat-soluble.
Vitamins are grouped by solubility:
- Water-soluble: the B-group (B1, B2, B6, B12, etc.) and vitamin C. Because they are not stored appreciably in the body, they must be supplied regularly in the diet and are excreted in urine. …
- CBSE 2020Set ANNUAL1 markQ.Give an example for water soluble vitamin.
›Reveal solutionSolution
Vitamin C (ascorbic acid) — and the B-complex vitamins — are water-soluble; vitamin C is a standard example.
Concept. Vitamins are classified by solubility into fat-soluble (A, D, E, K) and water-soluble (B-group and C). Water-soluble vitamins dissolve in water, are not stored in the body, and are regularly excreted in urine, so they must be supplied continuously in the diet.
…
- CBSE 2019Set ANNUAL1 markQ.Beri-beri occurs due to the deficiency of which vitamin?
›Reveal solutionSolution
Beri-beri is a nerve and cardiovascular disorder caused specifically by a deficiency of vitamin B1 (thiamine).
Thiamine (vitamin B1) is a water-soluble B-vitamin required as a coenzyme in carbohydrate metabolism. Its prolonged deficiency impairs nerve conduction and heart function, producing the disease beri-beri, characterised by weakness, nerve damage (polyneuritis) and in severe cases heart failure. …
- CBSE 2018Set ANNUAL1 markMCQQ.Which among the given vitamins is water soluble ?(a) A(b) B(c) D(d) E
›Reveal solutionSolution
Vitamins are classified as fat-soluble (A, D, E, K) or water-soluble (B-complex and C); among the given options, vitamin B is water soluble.
Vitamins A, D, E and K are fat-soluble — they dissolve in fats/oils and are stored in the body's fatty tissue and liver.
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- CBSE 2017Set ANNUAL1 markQ.Fresh tomatoes are a better source of vitamin C than those which have been stored for some time. Why ?
›Reveal solutionSolution
Vitamin C degrades on storage because it is a water-soluble vitamin that is easily oxidised.
Vitamin C (ascorbic acid) belongs to the water-soluble vitamins, which — unlike the fat-soluble ones — are not stored in the body and, importantly here, are also chemically unstable outside the body: they are readily decomposed by exposure to heat, light, and atmospheric oxygen (oxidation). As tomatoes are stored over time, their vitamin C content is progressively oxidised/degraded, so freshly picke …
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