40. Which statement(s) about changes of state of matter is/are correct?
A. A change of state is a physical change.
. A change of state is irreversible.
[RRB Group D, 10 Feb 2026, Shift 2]
(A) Only B
(B) Neither A nor B
(C) Both A and B
(D) Only A
Answer: (D) Only A
Explanation: Melting, freezing, condensation and vaporisation change how molecules are arranged and move, but generally not their chemical identities. Such state changes are physical and often reversible by controlling temperature or pressure. Therefore statement A is correct and statement B is incorrect.
39. Adding dilute hydrochloric acid to zinc granules produces hydrogen gas. This is an example of which change?
[RRB Group D, 04 Feb 2026, Shift 1]
(A) Dissolution
(B) Mixture formation
(C) Physical change
(D) Chemical change
Answer: (D) Chemical change
Explanation: Zinc reacts with hydrochloric acid to produce zinc chloride and hydrogen gas: Zn + 2HCl → ZnCl2 + H2. The appearance of new substances and evolution of gas establish a chemical change, rather than simple physical mixing or dissolving.
38. Which statement correctly defines a chemical change at the molecular level?
[Delhi Police Head Constable (AWO/TPO), 16 Jan 2026, Shift 1]
(A) A change affecting only size or shape
(B) A change producing one or more new substances
(C) A change that can always be easily reversed
(D) A change involving no energy exchange
Answer: (B) A change producing one or more new substances
Explanation: Chemical change involves the rearrangement of atoms and changes to chemical bonds, yielding substances with identities different from those of the reactants. Changes merely in shape, size or state need not create new substances and are usually classified as physical changes.
37. Which of the following processes is a physical change?
[RRB Group D, 09 Jan 2026, Shift 2]
(A) Crystallisation
(B) Rusting
(C) Combustion
(D) Fermentation
Answer: (A) Crystallisation
Explanation: Crystallisation usually separates an existing dissolved substance as solid crystals without changing its chemical formula. By contrast, rusting, combustion and fermentation involve reactions that produce new substances. Growing sodium chloride crystals from salt water is a standard example of physical crystallisation.
36. Which is an example of a physical change during the formation of a solution?
[RRB Group D, 08 Jan 2026, Shift 2]
(A) Curdling milk with lemon juice
(B) Dissolving salt in water
(C) Digestion of food in the stomach
(D) Rusting iron in salt solution
Answer: (B) Dissolving salt in water
Explanation: Sodium chloride dissolves into hydrated sodium and chloride ions distributed through water. No new chemical compound is produced, and salt can be recovered by evaporating the water. The other choices involve chemical processes such as protein coagulation, digestion or oxidation.
35. Which of the following is a chemical change?
[RRB Group D, 27 Dec 2025, Shift 2]
(A) Combustion of petrol
(B) Mixing salt and water
(C) Evaporation of water
(D) Melting of ice
Answer: (A) Combustion of petrol
Explanation: Petrol contains hydrocarbons that react with oxygen during combustion, forming principally carbon dioxide and water while releasing heat. Because new chemical products are formed, the process is chemical. Mixing salt with water, evaporation and melting usually change physical properties only.
34. Which one of the following is NOT a chemical change?
[UPSC NDA-II, General Ability Test, 14 Sep 2025]
(A) Burning of coal
(B) Rusting of metal
(C) Melting of ice
(D) Reaction of an acid with a base
Answer: (C) Melting of ice
Explanation: Melting converts solid water to liquid water without changing the molecular formula H2O. Coal combustion forms oxides and ash, iron rusting produces iron oxides, and acid-base neutralisation yields salt and water. These three processes involve chemical reactions, unlike melting.
33. Which of the following is an example of a chemical change?
[RRB NTPC Graduate Level CBT-I, 05 Jun 2025, Shift 1]
(A) Cutting paper
(B) Burning wood
(C) Melting ice
(D) Boiling water
Answer: (B) Burning wood
Explanation: Burning wood involves combustion and generates gases, mineral ash and heat. Because the original material is converted into chemically different products, it is a chemical change. Cutting paper and changing water between its physical states do not themselves create new compounds.
32. Which feature distinguishes colloids from true solutions?
[RRB JE (ECE) CBT-II, 22 Apr 2025, Shift 1]
(A) True solutions have visible particles, but colloids do not
(B) True solutions are single-phase systems, whereas colloids contain two phases
(C) Only true solutions exhibit Brownian motion
(D) True solutions show the Tyndall effect, but colloids do not
Answer: (B) True solutions are single-phase systems, whereas colloids contain two phases
Explanation: A true solution appears as one uniform phase at the molecular level. A colloid comprises a dispersed phase within a dispersion medium, although it can look uniform to the naked eye. Colloids scatter light, whereas ordinary true solutions generally do not.
31. Which statements about humus are correct?
1. It is a dark-coloured amorphous substance.
2. It is colloidal and serves as a nutrient reservoir.
[UPSC CDS-I, General Knowledge, 13 Apr 2025]
(A) 1 only
(B) 2 only
(C) Both 1 and 2
(D) Neither 1 nor 2
Answer: (C) Both 1 and 2
Explanation: Humus forms through decomposition of organic remains in soil. It is dark and largely amorphous, while its colloidal properties give it considerable surface area and nutrient-holding capacity. Thus it retains important plant nutrients and contributes to soil fertility and water retention.
