20. When milk is converted into curd, what type of change is it?
[SSC CHSL Tier-I, 09 Aug 2023, Shift 4]
(A) Reversible change
(B) Physical change
(C) Isothermal change
(D) Chemical change
Answer: (D) Chemical change
Explanation: During curd formation, bacteria ferment lactose and generate lactic acid. Increased acidity causes milk proteins to coagulate, creating a product with properties different from fresh milk. New chemical substances are formed, so curd formation is classed as chemical change.
19. Which of the following statements about mixtures is NOT correct?
[SSC CHSL 2022 Tier-I, held 09 Mar 2023, Shift 1]
(A) Colloids are heterogeneous mixtures in which particle size is large
(B) The major component of a solution is called the solvent
(C) The minor component of a solution is called the solute
(D) A solution is a homogeneous mixture of two or more substances
Answer: (A) Colloids are heterogeneous mixtures in which particle size is large
Explanation: Compared with coarse suspension particles, colloidal particles are relatively small and remain dispersed. Their size lies between molecular-scale solutes and large suspended particles. The other statements correctly distinguish solute, solvent and homogeneous solutions. The term large in the first statement is misleading.
18. Formation of slaked lime when calcium oxide reacts with water is an example of which reaction?
[RRB Group D, 11 Oct 2022, Shift 3]
(A) Displacement reaction
(B) Combination reaction
(C) Endothermic reaction
(D) Decomposition reaction
Answer: (B) Combination reaction
Explanation: Quicklime, CaO, combines with water to form calcium hydroxide, Ca(OH)2, commonly called slaked lime. As two reactants produce a single compound, the process is a combination reaction. It releases considerable heat and is therefore also exothermic.
17. What is the process called when electricity decomposes water into hydrogen and oxygen?
[RRB Group D, 11 Oct 2022, Shift 1]
(A) Hydrolysis of water
(B) Atomisation of water
(C) Electrolysis of water
(D) Ionisation of water
Answer: (C) Electrolysis of water
Explanation: Electrolysis uses electrical energy to split water into hydrogen and oxygen at different electrodes. The balanced overall reaction is 2H2O → 2H2 + O2. Unlike boiling, this changes the chemical identity of the substance and is a chemical decomposition.
16. 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) is an example of which type of reaction?
[RRB Group D, 22 Sep 2022, Shift 2]
(A) Precipitation reaction
(B) Decomposition reaction
(C) Neutralisation reaction
(D) Redox reaction
Answer: (C) Neutralisation reaction
Explanation: Sodium hydroxide is a base and sulfuric acid is an acid. Their reaction produces sodium sulfate and water. Hydrogen ions from the acid combine with hydroxide ions from the base, which is the defining feature of acid-base neutralisation.
15. Milk of magnesia, used to relieve indigestion, is a what?
[UPSC CDS-II, General Knowledge, 04 Sep 2022]
(A) Strong base
(B) Mild base
(C) Strong acid
(D) Mild acid
Answer: (B) Mild base
Explanation: Milk of magnesia is a suspension containing magnesium hydroxide, Mg(OH)2. It is a sparingly soluble, relatively mild base that neutralises excess stomach acid. Its milky appearance comes from fine undissolved solid particles rather than dissolved milk components.
14. Dialysis is the process used for separation of which materials?
[WBCS Mains 2021, Paper IV, held May 2022]
(A) Suspended particles from colloids
(B) Suspended particles from crystalloids
(C) Colloidal particles from crystalloids
(D) None of the above
Answer: (C) Colloidal particles from crystalloids
Explanation: Dialysis separates small dissolved ions or molecules, called crystalloids, from larger colloidal particles using a selectively permeable membrane. The smaller species diffuse through membrane pores, while colloidal particles are retained, allowing purification of a colloidal dispersion.
13. Why does water come out of sliced cucumber after salt is sprinkled on it?
[UPPSC Combined State/Upper Subordinate Services Prelims, 24 Oct 2021]
(A) Active transport
(B) Passive transport
(C) Osmosis
(D) Diffusion
Answer: (C) Osmosis
Explanation: Salt makes the fluid outside cucumber cells more concentrated than the cell sap. Water then moves outward through selectively permeable cell membranes by osmosis. This passive movement of solvent explains why the sliced cucumber releases liquid without requiring cellular energy.
12. What is meant by dry ice?
[WBCS Preliminary, 2021]
(A) Ice at −23°C
(B) Ice at 4°C
(C) Solid sulfur dioxide
(D) Solid carbon dioxide
Answer: (D) Solid carbon dioxide
Explanation: Dry ice is solid carbon dioxide, CO2, rather than frozen water. At normal atmospheric pressure it sublimes directly to carbon dioxide gas, bypassing a liquid phase. The transformation is physical because the molecules remain CO2 before and after sublimation.
11. LPG is principally a mixture of which two gases?
[WBCS Preliminary, 2020]
(A) Methane and butane
(B) Propane and butane
(C) Ethane and propane
(D) Ethane and butane
Answer: (B) Propane and butane
Explanation: Liquefied petroleum gas for domestic use commonly contains propane and butane in proportions that can vary by supplier and climate. These hydrocarbons are liquefied under pressure and vaporise when released. Mixing them does not itself form a new chemical compound.
