Physical and Chemical Changes, Solution

31. Kerosene and petrol mixture can be best separated by

Kerosene and petrol mixture can be best separated by

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Sublimation
Separating funnel
Fractional distillation
Compressing and cooling
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UPSC NDA-2 – 2023
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The correct method to separate a mixture of kerosene and petrol is fractional distillation. Both kerosene and petrol are miscible liquids with different boiling points (petrol boils roughly between 40°C and 205°C, kerosene between 175°C and 325°C). Fractional distillation is used to separate miscible liquids that have close boiling points.
Fractional distillation is a technique used to separate miscible liquids with different boiling points, involving repeated vaporization and condensation steps within a fractionating column.
Sublimation is used for separating a solid that sublimes from a non-subliming solid or a liquid. A separating funnel is used to separate immiscible liquids. Compressing and cooling is a method used to liquefy gases.

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32. Which among the following is NOT true with respect to colloidal soluti

Which among the following is NOT true with respect to colloidal solution ?

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Particles are uniformly distributed throughout the solution
Colloidal solution is homogenous in nature
They show Tyndal effect
They do not settle down when kept undisturbed
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UPSC NDA-2 – 2023
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The statement “Colloidal solution is homogenous in nature” is NOT true.
Colloidal solutions are fundamentally heterogeneous mixtures. Although they may appear homogeneous to the naked eye because the dispersed particles do not settle out, they are heterogeneous at the microscopic level.
Properties of colloidal solutions include: they are heterogeneous; the particle size is larger than in true solutions but smaller than in suspensions (typically 1 nm to 1 $\mu$m); they exhibit the Tyndall effect (scattering of light); they undergo Brownian motion; they are generally stable and do not settle down when left undisturbed; they often carry an electric charge. Statement A is also generally not strictly true in the sense of uniform distribution like a true solution, but B is the most definitive and fundamental falsehood about the nature of colloids.

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33. Match List I with List II and select the correct answer using the code

Match List I with List II and select the correct answer using the code given below the Lists :

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List I
(Process)		 List II
(Separation method)
A. Separation of acetone and water from their mixture 1. Chromatography
B. Separation of water and kerosene oil from their mixture 2. Centrifugation
C. Separation of cream from milk 3. Distillation
D. Separation of pigments from plant extract 4. Separating Funnel
A-3, B-2, C-4, D-1
A-4, B-2, C-1, D-3
A-3, B-4, C-2, D-1
A-4, B-1, C-2, D-3
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UPSC NDA-2 – 2023
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C) The correct matching is A-3, B-4, C-2, D-1.
– This question requires matching separation processes with appropriate methods based on the properties of the mixtures and their components.
– A. Separation of acetone and water from their mixture: Acetone and water are miscible liquids with different boiling points. Distillation is used to separate miscible liquids based on differences in volatility. Acetone boils at a lower temperature (around 56°C) than water (100°C), allowing it to be vaporized and condensed separately. This matches with 3. Distillation.
– B. Separation of water and kerosene oil from their mixture: Water and kerosene oil are immiscible liquids with different densities. They form two distinct layers when left undisturbed. A separating funnel is used to separate immiscible liquids, allowing the denser liquid (water) to be drained out from the bottom. This matches with 4. Separating Funnel.
– C. Separation of cream from milk: Milk is a suspension or emulsion of fat globules (cream) in a watery phase. Centrifugation is used to separate components of a mixture based on density differences using centrifugal force. Spinning milk rapidly forces the denser skim milk to the periphery while the lighter cream concentrates towards the center. This matches with 2. Centrifugation.
– D. Separation of pigments from plant extract: Plant extracts contain a mixture of different colored pigments. Chromatography is a technique used to separate components of a mixture based on their differential partitioning between a stationary phase and a mobile phase. Different pigments travel at different rates, allowing them to be separated. This matches with 1. Chromatography.
– Distillation is used for miscible liquids with different boiling points.
– Separating funnel is used for immiscible liquids.
– Centrifugation is used to separate components based on density differences, often for solid-liquid suspensions or liquid-liquid emulsions.
– Chromatography is a versatile technique for separating components based on differential affinities to stationary and mobile phases, widely used for separating colored substances (pigments).

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34. Consider the following reaction : 2HgO ⎯⎯→ 2Hg + O₂ The respective sta

Consider the following reaction :
2HgO ⎯⎯→ 2Hg + O₂
The respective state of HgO, Hg and O₂ in the above reaction is

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Liquid, solid, gas
Solid, solid, gas
Liquid, solid, liquid
Solid, liquid, gas
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Mercuric oxide (HgO) is a solid compound at room temperature. Mercury (Hg) is the only metal that exists as a liquid at room temperature. Oxygen (O₂) is a gas at room temperature. Therefore, the respective states of HgO, Hg, and O₂ are solid, liquid, and gas.
The physical states of substances at standard temperature and pressure (or implied conditions) are important in chemical reactions. Mercury is unique among metals in being liquid at room temperature.
This reaction represents the thermal decomposition of mercuric oxide. Heating HgO breaks it down into its constituent elements, mercury and oxygen. The states indicated correspond to the stable forms of these substances under normal conditions after the reaction.

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35. Which one of the following processes involves chemical reaction ?

Which one of the following processes involves chemical reaction ?

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Evaporation of petrol
Dissolution of salt in water
Storing of oxygen gas under pressure in a gas cylinder
Burning of magnesium ribbon in air
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UPSC NDA-2 – 2023
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Burning of magnesium ribbon in air involves a chemical reaction.
A chemical reaction involves the rearrangement of atoms and molecules to form new substances with different chemical properties. Evaporation, dissolution, and storing gas under pressure are physical processes where the substance’s chemical composition remains unchanged, only its state or distribution changes. Burning (combustion) is a chemical process where a substance reacts rapidly with oxygen, typically producing heat and light and forming new chemical compounds. When magnesium burns in air, it reacts with oxygen and nitrogen to form magnesium oxide and magnesium nitride, respectively.
Evaporation is a phase transition from liquid to gas. Dissolution is the process of dissolving a solute in a solvent to form a solution; the solute particles disperse but do not chemically change. Storing gas under pressure is a physical change affecting density and volume. Burning involves chemical transformation.

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36. Match List I with List II and select the correct answer using the code

Match List I with List II and select the correct answer using the code given below the Lists :

List I
(Chemical process)		 List II
(Reaction)
A. Electrolysis of water 1. Double displacement reaction
B. Burning of coal 2. Combination reaction
C. Iron nail immersed in copper sulphate solution 3. Decomposition reaction
D. Addition of barium chloride solution to aluminium sulphate solution 4. Displacement reaction

Code :

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A B C D
(a) 3 4 2 1
(b) 4 2 1 3
(c) 3 2 4 1
(d) 4 1 2 3
(a) 3 4 2 1
(b) 4 2 1 3
(c) 3 2 4 1
(d) 4 1 2 3
This question was previously asked in
UPSC NDA-2 – 2023
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The correct matching of chemical processes with their reaction types is A-3, B-2, C-4, D-1.
A. Electrolysis of water involves breaking down water (a single compound) into hydrogen and oxygen (simpler substances) using electricity. This is a decomposition reaction. (H₂O → H₂ + O₂)
B. Burning of coal involves the reaction of carbon (and other components) with oxygen, usually forming carbon dioxide. This is a combination reaction (C + O₂ → CO₂) and also combustion.
C. Iron nail immersed in copper sulphate solution involves iron displacing copper from copper sulphate because iron is more reactive than copper. This is a displacement reaction. (Fe + CuSO₄ → FeSO₄ + Cu)
D. Addition of barium chloride solution to aluminium sulphate solution involves the exchange of ions between the two reactants, forming new compounds, one of which is insoluble (barium sulphate precipitate). This is a double displacement reaction. (3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄↓ + 2AlCl₃)
Matching these gives: A-3, B-2, C-4, D-1, which corresponds to option (c).

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37. Which of the following compounds undergoes/undergo thermal

Which of the following compounds undergoes/undergo thermal decomposition?

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Zinc oxide
Silver oxide and zinc oxide
Silver oxide
Magnesium oxide
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UPSC NDA-2 – 2022
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Silver oxide undergoes thermal decomposition.
Thermal decomposition is the breakdown of a compound by heating. Silver oxide (Ag2O) is relatively unstable and decomposes upon heating into silver metal and oxygen gas (2Ag2O(s) -> 4Ag(s) + O2(g)). Zinc oxide (ZnO) and Magnesium oxide (MgO) are stable oxides that do not decompose easily upon heating under normal conditions.
The ease of thermal decomposition of metal oxides is related to the reactivity of the metal. Oxides of less reactive metals (like silver, mercury, gold) decompose more easily than oxides of more reactive metals (like magnesium, zinc, calcium).

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38. Which one of the following solutions is not capable of conducting

Which one of the following solutions is not capable of conducting electricity?

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Copper sulphate
Sodium chloride
Sugar
Sodium hydroxide
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UPSC NDA-2 – 2022
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Sugar solution is not capable of conducting electricity.
For a solution to conduct electricity, it must contain free-moving ions. Solutions of ionic compounds (like Copper sulphate, Sodium chloride, Sodium hydroxide) dissociate into ions when dissolved in water, allowing them to conduct electricity. Sugar (sucrose) is a covalent compound; when it dissolves in water, it remains as neutral molecules and does not produce ions.
Electrolytes are substances that produce ions when dissolved in a solvent, making the solution electrically conductive. Non-electrolytes, like sugar, dissolve but do not ionize, and their solutions do not conduct electricity.

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39. Which one of the following methods can be used to separate anthracene

Which one of the following methods can be used to separate anthracene from a mixture of salt and anthracene ?

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Distillation
Sublimation
Evaporation
Chromatography
This question was previously asked in
UPSC NDA-2 – 2021
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Anthracene is a polycyclic aromatic hydrocarbon known for its property of sublimation, which is the process of transitioning directly from the solid to the gas phase upon heating, without passing through a liquid phase. Salt (e.g., sodium chloride) is an ionic compound with a very high melting point and does not sublime under normal conditions. Therefore, sublimation is an effective method to separate anthracene from a mixture containing salt. By heating the mixture, anthracene will turn into a gas and can be collected by cooling the vapour, leaving the non-sublimable salt behind.
– Anthracene sublimes easily upon heating.
– Salt does not sublime under typical conditions.
– Sublimation is used to separate a sublimable solid from non-sublimable solids.
Other methods like distillation and evaporation are generally used for separating liquids or separating dissolved solids from solvents. Chromatography is a technique for separating components of a mixture based on different affinities for a stationary and mobile phase, which could be used but is not as direct or efficient as sublimation for this specific type of mixture.

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40. Which one of the following is the best example of desiccant ?

Which one of the following is the best example of desiccant ?

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Silica gel
Polystyrene
Sodium chloride
Sodium carbonate
This question was previously asked in
UPSC NDA-2 – 2020
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Silica gel is the best example of a desiccant among the given options.
A desiccant is a substance that absorbs or adsorbs water, thus creating or maintaining a state of dryness. Silica gel is widely used for this purpose due to its porous structure, which allows it to adsorb a significant amount of water vapour.
Silica gel is often found in small packets in product packaging (like electronics, shoes, leather goods, etc.) to protect them from moisture damage. While other substances like calcium chloride or activated alumina are also used as desiccants, silica gel is a common and effective example. Polystyrene is a type of plastic. Sodium chloride (common salt) can absorb moisture (hygroscopic) but is not primarily used as a general industrial desiccant in the same way as silica gel. Sodium carbonate can absorb moisture to form hydrates but is also not the primary example of a desiccant.

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