Matter: Elements, Compounds and Mixtures – NDA Chemistry PYQs

Q1. Which of the following is a mixture?
(a) Distilled water (b) Iodised salt (c) Baking soda (d) Common salt

Answer: (b) Iodised salt
Iodised salt is common salt (NaCl) mixed with a small amount of potassium iodide (KI). The two are physically mixed — not chemically joined. So it is a mixture, not a compound. Distilled water, baking soda, and common salt are all pure substances.
[NDA 2010-II]

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Q2. Which of the following is NOT a compound?
(a) Water (b) Glass (c) Salt (d) Sugar

Answer: (b) Glass
Glass is an amorphous solid with no fixed composition. It is made by melting silica with other substances, and its composition can vary. It is not a pure compound. Water (H₂O), salt (NaCl), and sugar (C₁₂H₂₂O₁₁) all have fixed compositions and are compounds.
[NDA 2013-II]

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Q3. Which one of the following represents a physical change?
(a) Burning of magnesium ribbon (b) Souring of milk (c) Rusting of iron (d) Melting of ice

Answer: (d) Melting of ice
Melting of ice is a physical change. Ice changes to water — the chemical composition (H₂O) remains the same. No new substance is formed. Burning, souring, and rusting all produce new substances — they are chemical changes.
[NDA tested via CDS]

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Q4. After cutting an apple or banana, the colour of the cut surface becomes brown. Which of the following correctly explains this?
Statement I: The colour of the cut surface becomes brown after cutting an apple or banana.
Statement II: Polyphenolic compounds present in fruits get oxidised in air and show colour.
(a) Both true, II correctly explains I (b) Both true, II does not explain I (c) I true, II false (d) I false, II true

Answer: (a) Both true, Statement II correctly explains Statement I
When a fruit is cut, its inner surface is exposed to oxygen in the air. Polyphenol oxidase enzyme in the fruit converts polyphenolic compounds into dark-coloured quinones. This is an oxidation reaction. It is a chemical change — a new coloured substance is formed.
[NDA 2012-I]

Q5. Which one among the following elements is polyatomic?
(a) Phosphorus (b) Sulphur (c) Chlorine (d) Aluminium

Answer: (b) Sulphur
Sulphur exists as S₈ — eight atoms per molecule. This makes it polyatomic. Phosphorus is P₄ — tetraatomic (4 atoms). Chlorine is Cl₂ — diatomic (2 atoms). Aluminium is a metal — monoatomic.
[NDA 2024-II]

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Q6. A mixture of sand, common salt, and ammonium chloride can be separated by which sequence of methods?

Answer: First sublimation (ammonium chloride sublimes and is separated), then add water and filter (sand is filtered out as it is insoluble), then evaporate the filtrate (water evaporates and salt is left behind).
The correct sequence is: Sublimation → Filtration → Evaporation.
[NDA 2010-II]

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Q7. Which one of the following is used to separate cream from milk?
(a) Filtration (b) Sedimentation (c) Centrifugation (d) Distillation

Answer: (c) Centrifugation
Cream and milk have different densities. When milk is spun at high speed in a centrifuge, the denser components move outward and the lighter cream rises to the centre and can be separated. This process is called centrifugation.
[NDA tested via CDS]

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Q8. Iron filings can be separated from sulphur powder by which method?
(a) Sieving (b) Magnetic separation (c) Filtration (d) Sublimation

Answer: (b) Magnetic separation
Iron is magnetic. Sulphur is not. A magnet is brought near the mixture. Iron filings are attracted to the magnet and separate from the sulphur powder. This is the fastest and simplest method for this particular mixture.
[NDA tested via CDS]

Q9. Which one of the following is a chemical change?
(a) Dissolving sugar in water (b) Cutting of paper (c) Melting of wax (d) Souring of milk

Answer: (d) Souring of milk
Souring of milk is a chemical change. Bacteria convert lactose in milk into lactic acid. A new substance (lactic acid) is formed. The change cannot be reversed. Dissolving sugar, cutting paper, and melting wax are all physical changes — no new substance is formed.
[NDA tested via CDS]

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Q10. Which one of the following is considered as a compound?
(a) Silicon (b) Tin (c) Sugar solution (d) Calcium carbonate

Answer: (d) Calcium carbonate
Calcium carbonate (CaCO₃) is a pure compound with fixed composition. Silicon and tin are elements — made of only one type of atom. Sugar solution is a homogeneous mixture of sugar and water.
[NDA tested via CDS]

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Q11. Which one of the following methods is used to separate anthracene from a mixture of salt and anthracene?
(a) Distillation (b) Sublimation (c) Evaporation (d) Chromatography

Answer: (b) Sublimation
Anthracene sublimes — it converts directly from solid to vapour on heating — while salt remains as residue. The vapour is collected and cooled to recover pure anthracene.
[NDA 2021-II]

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Q12. Alloys are:
(a) Compounds (b) Elements (c) Mixtures (d) Pure substances

Answer: (c) Mixtures
Alloys are homogeneous mixtures of two or more metals (or a metal and a non-metal). Their composition can vary. They are not chemically bonded — they are physically mixed in the molten state. Because composition is variable, they are mixtures — not compounds.
[NDA tested via CDS]