30. Which of the following is NOT a property of a colloid?
[RRB Technician Grade III, 29 Dec 2024, Shift 1]
(A) Colloids cannot normally be separated by ordinary filtration
(B) A colloid is a homogeneous mixture
(C) A colloid is a heterogeneous mixture
(D) Colloidal particles do not settle readily when undisturbed
Answer: (B) A colloid is a homogeneous mixture
Explanation: Colloids can appear uniform macroscopically but contain distinct dispersed and continuous phases, making them heterogeneous systems. Their particles remain dispersed for substantial periods and usually pass through ordinary filter paper. A true molecular solution, unlike a colloid, is homogeneous.
29. What is the direct change of a solid into a gas without entering the liquid state called?
[SSC MTS, 14 Nov 2024, Shift 1]
(A) Sublimation
(B) Fusion
(C) Evaporation
(D) Condensation
Answer: (A) Sublimation
Explanation: Sublimation is the direct transition of a solid into vapour. Dry ice and naphthalene provide familiar examples under suitable conditions. Fusion means melting, evaporation changes liquid to vapour, and condensation changes vapour to liquid. Sublimation itself is a physical change.
28. Which solution is used to test for starch in food?
[SSC MTS, 21 Oct 2024, Shift 2]
(A) Ethylene glycol–water solution
(B) Magnesium solution
(C) Talc solution
(D) Iodine solution
Answer: (D) Iodine solution
Explanation: Dilute iodine solution turns blue-black in the presence of starch, particularly amylose, because iodine interacts with its helical structure. The observation is used as a quick qualitative food test. A negative test normally retains the brownish colour of iodine solution.
27. Which of the following is NOT a physical change?
[SSC CGL Tier-I, 24 Sep 2024, Shift 1]
(A) Heating an iron rod until red hot
(B) Curdling of milk
(C) Evaporation of diesel
(D) Sublimation of ammonium chloride
Answer: (B) Curdling of milk
Explanation: Milk curdling involves the coagulation of proteins and can be associated with acidification, changing its properties. Heating a metal, evaporating diesel and subliming a substance primarily alter temperature or state. The examination treats milk curdling as a chemical change.
26. In which of the following colloids is the dispersion medium a liquid?
[UPSC CDS-II, General Knowledge, 01 Sep 2024]
(A) Mist
(B) Foam
(C) Jelly
(D) Shaving cream
Answer: (D) Shaving cream
Explanation: Shaving cream is a foam containing gas bubbles dispersed through a liquid medium. Mist consists of liquid droplets dispersed in air, while jelly has a solid continuous network. The general term foam can cover more than one medium; shaving cream identifies the liquid-medium case explicitly.
25. Which of the following is an example of a solid-in-liquid type of solution?
[SSC CHSL Tier-I, 02 Jul 2024, Shift 2]
(A) Brass
(B) Vinegar
(C) Tincture of iodine
(D) Bronze
Answer: (C) Tincture of iodine
Explanation: Tincture of iodine contains iodine, ordinarily a solid, dissolved in a liquid alcohol solvent. It is therefore classified as a solid-in-liquid solution. Brass and bronze are solid alloys, while vinegar is an aqueous solution of liquid acetic acid.
24. What happens when potassium iodide solution is mixed with lead nitrate solution?
[WBCS (Executive) Preliminary, 16 Dec 2023]
(A) A white precipitate of lead iodide forms
(B) A yellow precipitate of lead iodide forms
(C) A red precipitate of lead iodide forms
(D) A black precipitate of lead iodide forms
Answer: (B) A yellow precipitate of lead iodide forms
Explanation: Lead(II) nitrate and potassium iodide undergo a double-displacement reaction. Insoluble yellow lead(II) iodide separates as a precipitate, while potassium nitrate remains dissolved: Pb(NO3)2 + 2KI → PbI2 + 2KNO3. A new insoluble compound demonstrates chemical change.
23. Which of the following substances can dissolve in water?
[WBCS (Executive) Preliminary, 16 Dec 2023]
(A) Glucose
(B) Urea
(C) Acetone
(D) All of the above
Answer: (D) All of the above
Explanation: Glucose and urea are polar molecules that interact favourably with water through hydrogen bonding. Acetone is a polar liquid and is miscible with water. Consequently, all three substances can form solutions with water, though their molecular structures and uses differ.
22. Mg(s) + O2(g) → MgO(s) + energy: How may this reaction be classified?
[WBCS (Executive) Preliminary, 16 Dec 2023]
(A) Combination reaction
(B) Oxidation reaction
(C) Exothermic reaction
(D) All of the above
Answer: (D) All of the above
Explanation: Magnesium and oxygen combine to form one principal product, magnesium oxide. Magnesium gains oxygen and is oxidised, and the reaction releases heat and bright light, making it exothermic. The correctly balanced reaction is 2Mg + O2 → 2MgO.
21. In which case do both physical and chemical changes take place?
[UPSC CDS, General Knowledge, 03 Sep 2023]
(A) Burning a candle
(B) Freezing water
(C) Cooking food
(D) Rusting iron
Answer: (A) Burning a candle
Explanation: When a candle burns, some wax melts and may later solidify, representing physical change. Simultaneously, vaporised wax reacts with oxygen, producing carbon dioxide, water and heat: a chemical change. Therefore burning a candle illustrates both types of transformation together.
