50. Which of the following is NOT an example of a colloid?
[RRB Group D, 25 Aug 2026, Shift 2]
(A) Milk
(B) Blood
(C) Sugar solution
(D) Ice cream
Answer: (C) Sugar solution
Explanation: A sugar solution is a true homogeneous solution: individual sugar molecules are distributed throughout water and are too small to produce appreciable Tyndall scattering. Milk and ice cream are colloidal systems; blood contains dispersed components and colloidal plasma constituents.
49. Which of the following is a characteristic of a true solution?
[RRB Group D, 25 Aug 2026, Shift 1]
(A) Solute particles are easily separated by filtration
(B) Solute particles can be seen with the naked eye
(C) Solute particles settle rapidly at the bottom
(D) Solute particles are evenly distributed in the solution
Answer: (D) Solute particles are evenly distributed in the solution
Explanation: A true solution has a uniform composition because dissolved particles spread throughout its solvent. The particles are molecular or ionic in size, do not settle on standing and pass through ordinary filter paper. Salt dissolved in water is a familiar example.
48. Which of the following is a property of a suspension?
[RRB Group D, 21 Aug 2026, Shift 3]
(A) Its particles never settle down
(B) It is a homogeneous mixture
(C) Its particles are smaller than 1 nm
(D) It can be separated by filtration
Answer: (D) It can be separated by filtration
Explanation: A suspension contains comparatively large, undissolved particles distributed in a fluid. These particles can settle when undisturbed and are retained by ordinary filter paper. Muddy water illustrates the distinction between a suspension, a colloid and a true solution.
47. Which of the following is a correct property of a colloidal solution?
[RRB Group D, 18 Aug 2026, Shift 1]
(A) The solute settles at the bottom after some time
(B) The particles scatter a beam of light
(C) The dispersed particles are separated by ordinary filtration
(D) All colloidal particles are visible to the naked eye
Answer: (B) The particles scatter a beam of light
Explanation: Colloidal particles are large enough to scatter incident light, producing the Tyndall effect. Unlike the larger particles in a suspension, they generally remain dispersed instead of settling quickly. Ordinary filter paper cannot normally separate the dispersed colloidal phase from its medium.
46. What are the two main components of a colloidal solution?
[RRB Group D, 17 Aug 2026, Shift 2]
(A) Metal phase and non-metal medium
(B) Reactant phase and product medium
(C) Dispersed phase and dispersion medium
(D) Solute phase and solvent medium
Answer: (C) Dispersed phase and dispersion medium
Explanation: The dispersed phase consists of fine particles distributed throughout the continuous dispersion medium. For example, in milk, tiny fat droplets form part of the dispersed phase while the surrounding aqueous liquid acts as the medium. Their states determine the colloid type.
45. Which of the following is a chemical change and NOT a physical change?
[RRB Group D, 12 Aug 2026, Shift 3]
(A) Melting of ice
(B) Rusting of iron
(C) Dissolving sugar in water
(D) Boiling of water
Answer: (B) Rusting of iron
Explanation: Iron reacts with oxygen in the presence of moisture and forms hydrated iron oxides known as rust. Since a new material results, rusting is chemical. Melting, boiling and the ordinary dissolution of sugar alter physical state or distribution without forming new compounds.
44. Which of the following is an example of a homogeneous mixture?
[RRB Group D, 10 Aug 2026, Shift 2]
(A) Sand and water
(B) Sugar dissolved in water
(C) Oil and water
(D) Iron filings and sulfur
Answer: (B) Sugar dissolved in water
Explanation: Dissolved sugar molecules spread uniformly throughout water, giving a single-phase mixture with essentially identical composition in every small sample. Sand and water remain a suspension, while oil and water separate into layers. Mixed iron filings and sulfur are also heterogeneous.
43. Which of the following statements is NOT correct?
[SSC GD Constable, 29 May 2026, Shift 2]
(A) Evaporation causes heating
(B) The states of matter are interconvertible
(C) Boiling is a bulk phenomenon
(D) Evaporation is a surface phenomenon
Answer: (A) Evaporation causes heating
Explanation: Evaporation normally causes cooling, not heating, because escaping molecules take energy from the liquid and its surroundings. Boiling occurs throughout a liquid at its boiling temperature under a given pressure. Evaporation can occur at the surface over a range of temperatures.
42. What is the amount of solute present in a given amount of solution known as?
[SSC MTS, 18 Feb 2026, Shift 3]
(A) Dilution
(B) Saturation
(C) Concentration
(D) Diffusion
Answer: (C) Concentration
Explanation: Concentration describes how much solute is present relative to a specified mass or volume of solution. It may be expressed as a mass percentage, molarity or another suitable unit. Dilution lowers concentration by adding solvent; saturation describes a solubility limit.
41. Why do we feel cool after applying water to our skin in summer?
[RRB ALP CBT-I, 17 Feb 2026, Shift 2]
(A) Evaporation absorbs heat from the skin surface
(B) Water raises body temperature by absorbing sunlight
(C) Condensation releases energy onto the skin
(D) Water stops sweating and traps body heat
Answer: (A) Evaporation absorbs heat from the skin surface
Explanation: To evaporate, water molecules need energy to escape into the gaseous state. They absorb this energy partly from the skin, which loses heat and feels cooler. The process is a physical change; water remains chemically H2O before and after evaporation.
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.
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.
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.
10. Which cation most effectively coagulates negatively charged Sb2S3 colloid?
[RPSC Senior Teacher Grade II Science, 04 Jul 2019]
(A) Ba2+
(B) Ca2+
(C) Al3+
(D) Na+
Answer: (C) Al3+
Explanation: For a negatively charged sol, positively charged counter-ions cause coagulation. Under the Schulze–Hardy rule, coagulating power generally increases sharply with counter-ion valency. Trivalent aluminium ions are therefore more effective than the listed divalent or monovalent cations.
9. Which is an example of gas dispersed in a solid?
[RPSC Senior Teacher Grade II Science, 18 Feb 2019]
(A) Cloud
(B) Froth
(C) Fog
(D) Pumice stone
Answer: (D) Pumice stone
Explanation: Pumice develops when bubbles of gas become trapped in cooling volcanic material. The resulting solid contains many gas-filled cavities and is a solid foam. Clouds and fog contain liquid droplets dispersed in air, whereas ordinary froth is gas dispersed in liquid.
8. What happens to the boiling point of pure water when salt is added?
[WBCS Preliminary, 2018]
(A) It increases
(B) It decreases
(C) It remains unchanged
(D) It becomes zero
Answer: (A) It increases
Explanation: Dissolving a nonvolatile solute such as sodium chloride lowers the solvent vapour pressure. The solution consequently needs a higher temperature to reach atmospheric pressure and boil. This boiling-point elevation is a colligative property governed by the concentration of dissolved particles.
7. Fog is a colloid of which dispersed phase and medium?
[RPSC Senior Teacher Grade II Science, 01 Jul 2017]
(A) Liquid particles dispersed in gas
(B) Gas particles dispersed in liquid
(C) Solid particles dispersed in liquid
(D) Solid particles dispersed in gas
Answer: (A) Liquid particles dispersed in gas
Explanation: Fog is made of very fine liquid water droplets suspended in air. Thus the dispersed phase is liquid and the dispersion medium is gaseous. Such colloids are called liquid aerosols; smoke, by comparison, is largely solid particles dispersed in gas.
6. Pasteurisation is related to which of the following?
[MPPSC State Service Preliminary, General Studies, 2016]
(A) Sterilisation of milk
(B) Dehydration of milk
(C) Fermentation of milk
(D) Distillation of milk
Answer: (A) Sterilisation of milk
Explanation: Pasteurisation uses controlled heating followed by cooling to reduce harmful microorganisms in milk and improve safety. Among the given exam options, sterilisation is the intended response. Strictly, however, pasteurisation is not complete sterilisation because it does not eliminate every resistant microbial spore.
5. Water gas is a mixture of which gases?
[CGPSC Preliminary General Studies, 2016]
(A) Carbon monoxide and hydrogen
(B) Carbon monoxide and water vapour
(C) Carbon dioxide and hydrogen
(D) Carbon dioxide and water vapour
Answer: (A) Carbon monoxide and hydrogen
Explanation: Water gas is chiefly a mixture of carbon monoxide and hydrogen. It can be generated by passing steam over hot carbon: C + H2O → CO + H2. The product mixture has served historically as a combustible industrial gas.
4. Which of these protective colloids has the greatest protective power based on its gold number?
[RPSC Senior Teacher Grade II Science, 2015]
(A) Gelatin (0.005–0.01)
(B) Haemoglobin (0.03–0.07)
(C) Potato starch (25)
(D) Gum arabic (0.15–0.25)
Answer: (A) Gelatin (0.005–0.01)
Explanation: The gold number measures the amount of protective colloid required to prevent coagulation of a standard gold sol. A lower gold number indicates stronger protective ability. Gelatin has the smallest listed range and therefore offers the highest protection among these choices.
3. Which of the following are chemical changes?
1. Crystallisation of sodium chloride
2. Melting of ice
3. Souring of milk
[UPSC Civil Services Preliminary, General Studies Paper I, 24 Aug 2014]
(A) 1 and 2 only
(B) 3 only
(C) 1, 2 and 3
(D) None of the above
Answer: (B) 3 only
Explanation: Crystallisation of sodium chloride and melting of ice change physical state without producing a new substance. Souring of milk is different because bacteria convert lactose into lactic acid, altering composition. Therefore only the third process represents chemical change.
2. Which of the following is an example of a chemical change?
[RAS/RTS Preliminary, 2003]
(A) White light separating into seven colours through a prism
(B) Vegetables becoming soft when cooked
(C) A wet lump of soil becoming fragile after drying
(D) Salt dissolving in water
Answer: (B) Vegetables becoming soft when cooked
Explanation: Cooking vegetables involves numerous heat-driven chemical reactions that alter cell-wall components, pigments and other molecules. Dispersion of light is optical; drying soil and dissolving salt are predominantly physical processes. The cooking option is therefore classified as a chemical change.
1. Which statement about aqueous solutions is NOT true?
[UPSC Civil Services Preliminary, 2002]
(A) Boiling point of any water solution is higher than pure water
(B) Addition of solute to a solution increases its water potential
(C) Water vapour pressure of any solution is lower than vapour pressure of pure water
(D) When a solution is separated with a semipermeable membrane, water flow can be stopped by giving pressure to the solution
Answer: (B) Addition of solute to a solution increases its water potential
Explanation: Adding solute decreases the chemical potential and water potential of a solvent rather than increasing them. For typical nonvolatile solutes, boiling-point elevation and vapour-pressure lowering occur. Applying the appropriate osmotic pressure can prevent net solvent flow across a semipermeable membrane.