Drugs, Dyes & Food Chemistry – NDA Chemistry PYQs

Solve NDA Chemistry previous-year questions on Drugs, Dyes & Food Chemistry with detailed solutions and explanations.

Chapter-wise PYQs • Concept-based explanations • Exam insights

NDA 2026-I

Q. 1. Consider the following pairs :

I. Pigment : TiO₂

II. Drier : Turpentine

III. Thinner : Naphthenates

IV. Antiskinning agent : Polyhydroxy phenol

How many of the pairs given above are correctly matched?

(a) One

(b) Two

(d) All the four

Answer: (b) Two

Explanation: Pair I: TiO₂ (titanium dioxide) is the most important white pigment – CORRECT. Pair II: Turpentine is a THINNER (solvent), not a drier – INCORRECT. Driers are cobalt/manganese naphthenates or linoleates. Pair III: Naphthenates are DRIERS (metal soaps that accelerate drying), not thinners – INCORRECT. Pair IV: Polyhydroxy phenol is an anti-skinning agent – CORRECT. Two pairs (I and IV) are correctly matched. Concept Tested: Paint components – pigment (TiO₂), thinner, drier, anti-skinning agent

★ JOVIK Exam Insight: TiO₂ = pigment. Turpentine = thinner. Naphthenates = driers. Polyhydroxy phenol = anti-skinning agent. NDA tests these distinctions.

Q. 2. Consider the following statements :

I. Pine oil is used as antifoaming agent in the emulsion paints.

II. Protective colloids are used in the emulsion paints to decrease their stability.

III. Driers having oxidizable oils are added to the emulsion paints to accelerate drying process.

Which of the statements given above is/are correct?

(a) I and II

(b) I only

(d) I and III

Answer: (b) I only

Explanation: Statement I: Correct – pine oil is used as an antifoaming (defoaming) agent to prevent excessive foam formation in emulsion paints. Statement II: Incorrect – protective colloids are used to INCREASE stability of emulsion paints, not decrease it. Statement III: Incorrect – driers added to emulsion paints typically contain oxidisable metal compounds (cobalt, manganese soaps), but the claim that these are “oxidizable oils” is imprecise. The primary drying mechanism is different in emulsion vs oil-based paints. Only Statement I is correct. Concept Tested: Emulsion paint additives – pine oil as antifoaming agent

NDA 2024-II

Q. 3. Which one among the following is commonly used as an ‘anti-skinning agent’ in paints?

(a) Gelatin

(b) N-methyl pyrrolidone

(d) Polyhydroxy phenol

Answer: (d) Polyhydroxy phenol

Explanation: Anti-skinning agents prevent the formation of a skin (dried surface film) on oil-based paints during storage. Polyhydroxy phenols act as antioxidants, inhibiting the oxidative polymerisation that causes skinning. Concept Tested: Anti-skinning agent in paints – polyhydroxy phenol

NDA 2024-I

Q. 4. Litmus, a well-known acid-base indicator, is derived from:

(a) Fungi

(b) Lichens

(d) Termite

Answer: (b) Lichens

Explanation: Litmus is a natural dye extracted from lichens (composite organisms made of fungi and algae). The commonly used lichen is Rocella tinctoria. Litmus turns red in acid and blue in base. Concept Tested: Litmus indicator source – derived from lichens

NDA 2023-I

Q. 5. Browning of the chopped apple can be minimized by :

(a) using table sugar.

(b) preserving in a container.

(d) using milk of magnesia.

Answer: (c) using lemon juice.

Explanation: Apple browning is caused by enzymatic oxidation of phenolic compounds (catalysed by polyphenol oxidase) when the apple is cut and exposed to air. Lemon juice contains citric acid and vitamin C (ascorbic acid), both of which act as antioxidants and lower the pH, inhibiting the enzyme and reducing browning. Concept Tested: Apple browning prevention – lemon juice (ascorbic acid, acidic pH)

Q. 6. Among the following, which is not the correct method for keeping the curd ?

(a) Keeping in stainless steel vessel

(b) Keeping in copper vessel

(d) Keeping in glass vessel

Answer: (b) Keeping in copper vessel

Explanation: Curd (yoghurt) is acidic. Copper reacts with acid, and copper ions leach into the curd, making it unfit for consumption. Stainless steel, plastic and glass are all inert and safe for storing acidic foods. Concept Tested: Storing acidic curd – copper vessel is unsafe

Q. 7. Toothpaste prevents tooth decay by :

(a) neutralizing the excess acidity.

(b) means of emulsification.

(d) making a coat of calcium over the teeth surface.

Answer: (c) the action of fluoride.

Explanation: Fluoride in toothpaste strengthens tooth enamel by converting hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) into the harder fluorapatite (Ca₁₀(PO₄)₆F₂), which is more resistant to acid attack by bacteria. This is the primary mechanism of tooth decay prevention by toothpaste. Concept Tested: Toothpaste prevents decay – fluoride hardens enamel

NDA 2022-I

Q. 8. Which one of the following is not a pigment?

(a) Zinc oxide

(b) Chalk

(d) Silica

Answer: (d) Silica

Explanation: Pigments give colour or opacity to paints. Zinc oxide (ZnO), chalk (CaCO₃) and white lead (2PbCO₃·Pb(OH)₂) are all white pigments. Silica (SiO₂) is not a pigment – it is a filler/extender that adds bulk and texture without providing significant colour or opacity. Concept Tested: Pigments in paints – silica is not a pigment

NDA 2021-II

Q. 9. Which one of the following compounds is used in ‘black and white’ photography?

(a) AgF

(b) AgBr

(d) Ag₂SO₄

Answer: (b) AgBr

Explanation: Silver bromide (AgBr) is the light-sensitive compound used in traditional black-and-white photographic film. It undergoes photodecomposition when exposed to light: 2AgBr → 2Ag + Br₂. The silver deposits form the dark image. Concept Tested: Black-and-white photography – silver bromide AgBr

NDA 2021-I

Q. 10. Which one of the following is the correct sequence of change in colours when a turmeric stain on white clothes is scrubbed by soap and then washed with water?

(a) Yellow – pink – blue

(b) Yellow – reddish brown – yellow

(d) Yellow – blue – pink

Answer: (b) Yellow – reddish brown – yellow

Explanation: Same question as NDA 2020 Q145. Turmeric turns reddish-brown with soap (alkali) and reverts to yellow when washed with water. This repeated question confirms it is a high-frequency concept. Concept Tested: Turmeric colour sequence with soap and water (repeated)

Q. 11. Which one of the following statements regarding Bleaching powder and D.D.T. is correct?

(a) Both are inorganic compounds

(b) Both are organic compounds

(d) Both contain calcium

Answer: (c) Both contain chlorine

Explanation: Same as NDA 2020 Q146. Both bleaching powder (CaOCl₂) and DDT contain chlorine. Concept Tested: Bleaching powder and DDT – both contain chlorine (repeated)

NDA 2020

Q. 12. Which one of the following is the correct sequence of change in colours when a turmeric stain on white clothes is scrubbed by soap and then washed with water?

(a) Yellow – pink – blue

(b) Yellow – reddish brown – yellow

(d) Yellow – blue – pink

Answer: (b) Yellow – reddish brown – yellow

Explanation: Turmeric (curcumin) is yellow in neutral/acidic conditions and turns reddish-brown in alkaline conditions. Soap is alkaline – scrubbing the stain with soap turns it reddish-brown. When washed with water (neutral), the alkaline soap washes off and the colour may revert towards yellow. Concept Tested: Turmeric colour change with alkali (soap) – yellow to reddish-brown and back

Q. 13. Which one of the following statements regarding Bleaching powder and D.D.T. is correct?

(a) Both are inorganic compounds

(b) Both are organic compounds

(d) Both contain calcium

Answer: (c) Both contain chlorine

Explanation: Bleaching powder (CaOCl₂) contains chlorine. DDT (dichloro-diphenyl-trichloroethane) is an organic compound that also contains chlorine. Both contain chlorine, making (c) the correct answer. DDT is organic; bleaching powder is inorganic – so (a) and (b) are both wrong. Concept Tested: Common element in bleaching powder and DDT – chlorine

NDA 2019-I

Q. 14. Which one of the following is used as a binder in paints?

(a) Titanium dioxide

(b) Novolac

(d) Silicones

Answer: (d) Silicones

Explanation: A binder (or vehicle) in paint holds the pigment particles together and helps the paint adhere to the surface. Silicones are used as binders in specialised paints. Titanium dioxide (TiO₂) is a white pigment. Novolac is a phenol-formaldehyde resin. Phthalocyanine is a blue/green pigment. Concept Tested: Paint components – binder (silicones)

NDA 2018-I

Q. 15. Which of the following properties is true for a tooth paste?

(a) It is acidic

(b) It is neutral

(d) It is made up of Calcium phosphate, the material of tooth enamel

Answer: (c) It is basic

Explanation: Toothpaste is mildly alkaline (basic), with pH typically 7–10. This is because it needs to neutralise the acid produced by bacteria in the mouth (which attack tooth enamel). A basic toothpaste helps counteract acid erosion. Concept Tested: Toothpaste – mildly alkaline (basic) pH

NDA 2015-I

Q. 16. The chemical used as a fixer/developer in photography is

(a) Sodium sulphate

(b) Sodium sulphide

(d) Sodium sulphite

Answer: (c) Sodium thiosulphate

Explanation: Sodium thiosulphate (hypo, Na₂S₂O₃) is used as a fixing agent in photography. It dissolves unexposed silver halide from the photographic film, leaving only the developed silver image. Without fixing, unexposed AgBr would continue to darken on exposure to light. Concept Tested: Photography fixer – sodium thiosulphate

★ JOVIK Exam Insight: Sodium thiosulphate (hypo) = photographic fixer. AgBr = used in black-and-white photographic film (NDA 2021-II Q102).

NDA 2014-II

Q. 17. Turpentine oil in paints is used as a

(a) pigment

(b) film-forming material

(d) drier

Answer: (c) thinner

Explanation: Turpentine oil is used as a thinner (solvent) in oil-based paints. It reduces the viscosity (thickness) of paint for easier application. A pigment provides colour; a drier accelerates drying; a film-forming material (vehicle) creates the protective layer. Concept Tested: Turpentine oil in paint – thinner/solvent

NDA 2013-II

Q. 18. Chromium oxide is used as an ingredient in paints to obtain

(a) green colour

(b) blue colour

(d) violet colour

Answer: (a) green colour

Explanation: Chromium(III) oxide (Cr₂O₃) is a bright green pigment. It is used in paints, ceramics and glass to give a green colour. “Chromium green” or “chrome green” is well known in the paint industry. Concept Tested: Chromium oxide as green pigment in paints

NDA 2009-II

Q. 19. Consider the following statements regarding aluminium paint:

1. It is fairly good heat-resistant.

2. It possesses good electrical resistance.

3. It cannot be used for hot water pipe.

Which of the statements given above is/are correct?

(a) 1 and 3

(b) 1 only

(d) 2 and 3

Answer: (c) 1 and 2

Explanation: Aluminium paint is heat-resistant (reflects heat radiation) – statement 1 correct. It has good electrical resistance – statement 2 correct. Statement 3 is wrong – aluminium paint IS used for hot water pipes precisely because of its heat resistance. Concept Tested: Properties of aluminium paint – heat resistance and electrical resistance

NDA 2009-I

Q. 20. Which one of the following chemicals is used in Beauty Parlours for hair-setting?

(a) Sulphur based

(b) Phosphorus based

(d) Iron based

Answer: (a) Sulphur based

Explanation: Hair-setting chemicals (perming agents) work by breaking and re-forming the disulphide bonds (S-S bonds) in keratin protein. The chemicals are sulphur-based. This is why hair perms use thioglycolate (a sulphur compound) to break S-S bonds and reform them in a new shape. Concept Tested: Hair-setting chemicals – sulphur-based (disulphide bond chemistry)

Q. 21. The natural dye present in turmeric is known as

(a) Cinnamin

(b) Phenolphthalein

(d) Curcumin

Answer: (d) Curcumin

Explanation: Curcumin is the active yellow pigment (dye) in turmeric (Haldi). It is responsible for turmeric’s colour and is also an antioxidant. Phenolphthalein and methyl orange are synthetic laboratory indicators. Concept Tested: Natural dye in turmeric – curcumin