Physical and Chemical Changes, Solution Archives

11. The method used in separating a mixture of two miscible liquids having

The method used in separating a mixture of two miscible liquids having sufficient difference in their boiling points is :

Filtration

Solvent Extraction

Centrifugation

Simple Distillation

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UPSC CAPF – 2023

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Simple distillation is a laboratory technique used to separate a mixture of two miscible liquids with significantly different boiling points. When the mixture is heated, the component with the lower boiling point vaporizes first, its vapor is condensed, and collected as a separate liquid (distillate). The other options are methods used for different types of separations: Filtration separates solids from liquids/gases, Solvent Extraction separates substances based on differential solubility in immiscible solvents, and Centrifugation separates components of a mixture based on density difference.

Simple distillation is effective for separating miscible liquids with a significant difference in boiling points (typically > 25°C).

Fractional distillation is used when the difference in boiling points is smaller. Azeotropes are mixtures of liquids that boil at a constant temperature and cannot be separated by simple or fractional distillation.

12. Calcium oxide reacts with water to produce slaked lime. It is an examp

Calcium oxide reacts with water to produce slaked lime. It is an example of

combination reaction

decomposition reaction

oxidation reaction

addition reaction

This question was previously asked in

UPSC CAPF – 2022

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The reaction described is between calcium oxide ($\text{CaO}$) and water ($\text{H}_2\text{O}$) to produce slaked lime, which is calcium hydroxide ($\text{Ca(OH)}_2$).
The balanced chemical equation for this reaction is:
$\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)}$

Let’s analyze the types of reactions given in the options:
A) Combination reaction: A reaction in which two or more reactants combine to form a single product. In this reaction, calcium oxide and water (two reactants) combine to form calcium hydroxide (a single product). This matches the definition of a combination reaction.
B) Decomposition reaction: A reaction in which a single compound breaks down into two or more simpler substances. This is the opposite of the given reaction.
C) Oxidation reaction: A reaction involving the loss of electrons or increase in oxidation state. While redox aspects might be present in the formation of bonds, the primary classification based on the change in the number of substances is combination. The oxidation states of Ca (+2), O (-2), and H (+1) do not change overall.
D) Addition reaction: A reaction in which atoms are added to a molecule across a multiple bond (like a double or triple bond). This term is mainly used in organic chemistry for reactions involving unsaturated hydrocarbons. While water is added to CaO, the term ‘combination reaction’ is the standard classification for this type of inorganic reaction.

The reaction fits perfectly the definition of a combination reaction. It is also a highly exothermic reaction, releasing a significant amount of heat, which is why it is also called the slaking of lime.

– Identifying the reactants and products of the reaction: Calcium oxide + Water -> Calcium hydroxide.
– Understanding the definition of different types of chemical reactions.
– Classifying the reaction based on the change in the number of reactants and products. Two reactants form one product, characteristic of a combination reaction.

This reaction is a classic example of an inorganic combination reaction and the process of slaking quicklime ($\text{CaO}$) to produce slaked lime ($\text{Ca(OH)}_2$). Slaked lime has many uses, including in mortars, plasters, and water treatment. The reaction is also an example of a hydrolysis reaction because water is a reactant. However, among the given choices, “combination reaction” is the most appropriate primary classification.

13. Which one of the following is an example of endothermic reaction?

Which one of the following is an example of endothermic reaction?

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

CaCO₃(s) → CaO(s) + CO₂(g)

C(s) + O₂(g) → CO₂(g)

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

This question was previously asked in

UPSC CAPF – 2021

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The correct answer is B) CaCO₃(s) → CaO(s) + CO₂(g).

An endothermic reaction is a reaction that absorbs heat from its surroundings.
A) CaO(s) + H₂O(l) → Ca(OH)₂(aq) (Slaking of lime) is a highly exothermic reaction, releasing significant heat.
B) CaCO₃(s) → CaO(s) + CO₂(g) (Decomposition of calcium carbonate) requires heating to occur, meaning it absorbs heat from the surroundings, making it an endothermic reaction.
C) C(s) + O₂(g) → CO₂(g) (Combustion of carbon) is an exothermic reaction, releasing heat and light.
D) CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) (Combustion of methane) is an exothermic reaction, releasing a large amount of heat.

Exothermic reactions have a negative enthalpy change (ΔH < 0), while endothermic reactions have a positive enthalpy change (ΔH > 0). Decomposition reactions often require energy input to break chemical bonds, making them endothermic. Combustion reactions always release energy, making them exothermic.

14. Which one of the following methods can be used to remove permanent har

Which one of the following methods can be used to remove permanent hardness of water ?

Boiling

Ion-exchange

Lime treatment

Bubbling sulphur dioxide gas

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UPSC CAPF – 2020

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The Ion-exchange method can be used to remove permanent hardness of water.

Permanent hardness in water is caused by dissolved sulfates and chlorides of calcium and magnesium, which do not precipitate out upon boiling. The ion-exchange method, using materials like zeolite or synthetic resin, replaces the hard ions (Ca2+, Mg2+) with soft ions (usually Na+). This process is effective in removing both temporary and permanent hardness.

A) Boiling removes only temporary hardness (caused by bicarbonates). C) Lime treatment primarily removes temporary hardness by precipitating calcium and magnesium bicarbonates. Adding lime and soda ash (lime-soda process) can remove both. D) Bubbling sulfur dioxide gas through water forms sulfurous acid, which does not effectively remove the ions responsible for permanent hardness and is not a standard water softening method.

15. The principle involved in paper chromatography is

The principle involved in paper chromatography is

Absorption

Partition

Solubility

Volatility

This question was previously asked in

UPSC CAPF – 2019

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Paper chromatography is a separation technique based on the principle of partition. The components of a mixture distribute themselves between two phases: a stationary phase (the water molecules adsorbed on the cellulose fibers of the paper) and a mobile phase (a solvent that moves up the paper). Separation occurs because different components have different solubilities in the mobile phase and different affinities for the stationary phase, leading to differential rates of movement up the paper.

The core principle of paper chromatography is the partition of solutes between the stationary phase (water on paper) and the mobile phase (solvent). While solubility in the mobile phase and interaction with the stationary phase are factors, the overall process is governed by the partition coefficient of each substance between the two phases.

Other chromatographic techniques exist, such as thin-layer chromatography (TLC), column chromatography, and gas chromatography, which utilize principles like adsorption, partition, size exclusion, or ion exchange for separation. Paper chromatography is a specific type of partition chromatography.

16. Which one of the following will NOT show Tyndall effect ?

Which one of the following will NOT show Tyndall effect ?

Chalk powder freshly mixed with water

Lime juice

Few drops of milk mixed with water

Starch solution

This question was previously asked in

UPSC CAPF – 2019

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The Tyndall effect is the scattering of light by colloidal particles in a colloid or a suspension. True solutions, where the solute particles are too small (individual ions or molecules) to scatter light significantly, do not exhibit the Tyndall effect. Among the given options, lime juice is primarily a true solution (containing dissolved sugars, citric acid, etc.), while chalk powder in water (suspension), milk in water (emulsion/colloid), and starch solution (colloid) contain particles large enough to scatter light.

– Tyndall effect is a phenomenon where light is scattered as it passes through a colloid or suspension.
– This scattering makes the path of the light beam visible.
– True solutions do not show the Tyndall effect because the dissolved particles are too small.

The Tyndall effect is a useful way to distinguish between a true solution and a colloid. If a beam of light passed through the mixture is visible, it is likely a colloid or suspension. If the beam is not visible, it is likely a true solution.

17. Which one of the following is not a colloid ?

Which one of the following is not a colloid ?

Milk

Mud

Butter

Boric acid

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UPSC CAPF – 2016

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The correct answer is Boric acid.

– Colloids are heterogeneous mixtures where the solute particles are dispersed evenly throughout the solvent as particles that are intermediate in size between those in true solutions and those in suspensions. Colloidal particles typically have a size between 1 nanometer and 1 micrometer.
– True solutions have particle sizes less than 1 nm. Suspensions have particle sizes greater than 1 micrometer, and the particles tend to settle out over time.
– Milk is an emulsion, which is a type of colloid where liquid droplets are dispersed in another liquid.
– Butter is also an emulsion, a type of colloid.
– Mud is primarily a suspension, where solid particles (soil, clay, silt) are dispersed in water. While mud can contain some clay particles of colloidal size, it is generally classified as a suspension because the larger particles settle out.
– Boric acid (H₃BO₃) is a molecular substance that dissolves in water to form a true solution. The particles in a boric acid solution are hydrated boric acid molecules, which are of molecular size, much smaller than colloidal particles.

The Tyndall effect (scattering of light) is a characteristic property of colloids, but not of true solutions. Suspensions may also scatter light, but they are unstable and settle. Boric acid solution will not show the Tyndall effect, unlike milk or muddy water (though settling occurs in mud).

18. A change of state directly from solid to gas without changing into liq

A change of state directly from solid to gas without changing into liquid state is called :

Fusion

Sublimation

Diffusion

Emulsion

This question was previously asked in

UPSC CAPF – 2016

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A change of state directly from solid to gas without changing into liquid state is called sublimation.

The transition of a substance directly from the solid to the gas state, without passing through the liquid state, is known as sublimation. This process occurs when the substance absorbs enough energy to overcome the intermolecular forces holding it in a solid state, but at a pressure and temperature where the liquid phase is unstable or does not exist.
– Fusion (Melting) is the transition from solid to liquid.
– Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration.
– Emulsion is a mixture of two or more liquids that are normally immiscible.

Sublimation is an endothermic process (requires heat). The reverse process, where a gas directly changes to a solid, is called deposition or desublimation. Common examples of substances that readily sublime include dry ice (solid CO2), iodine, and naphthalene.

19. The following item consists of two statements, Statement I and Stateme

The following item consists of two statements, Statement I and Statement II. Examine these two statements carefully and select the correct answer using the code given below :
Code :
Statement I : Catalytic hydrogenation is highest when the catalyst remains in the powdered form
Statement II : When a catalyst is in the powdered form, its surface area becomes highest

Both the statements are individually true and Statement II is the correct explanation of Statement I

Both the statements are individually true but Statement II is NOT the correct explanation of Statement I

Statement I is true but Statement II is false

Statement I is false but Statement II is true

This question was previously asked in

UPSC CAPF – 2015

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Statement I says that catalytic hydrogenation is highest when the catalyst is in powdered form. Statement II says that when a catalyst is in powdered form, its surface area becomes highest. Both statements are individually true, and Statement II correctly explains why Statement I is true.

Catalysis occurs on the surface of the catalyst. A larger surface area provides more active sites for the reaction to take place, thus increasing the reaction rate. Powdering a solid increases its total surface area significantly.

Heterogeneous catalysis involves the catalyst being in a different phase from the reactants, often a solid catalyst with gaseous or liquid reactants. In such systems, the surface area of the solid catalyst is a crucial factor affecting the reaction kinetics. Increasing the surface area (e.g., by powdering or using porous materials) generally increases the rate of heterogeneous catalytic reactions.

20. Which of the following are not chemical changes ? 1. Tempering of iron

Which of the following are not chemical changes ?
1. Tempering of iron
2. Conversion of iron piece into an electro-magnet by passing the current around the iron
3. Melting of iron
4. Rusting of iron
Select the correct answer using the code given below :

1 and 2 only

3 and 4 only

1, 2 and 3

1, 3 and 4

This question was previously asked in

UPSC CAPF – 2014

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A chemical change involves the formation of new substances with different chemical properties. A physical change alters the form or appearance of a substance but not its chemical composition.
1. Tempering of iron: This is a heat treatment process that changes the microstructure and properties (like hardness and toughness) of iron alloys (steel). While it involves heat, it primarily changes the physical state and crystalline structure, not the fundamental chemical composition of the iron or steel. It is considered a physical process.
2. Conversion of iron piece into an electro-magnet by passing the current: When electric current passes around an iron core, it becomes a temporary magnet. This is a physical phenomenon (magnetism) induced by the electric current. The iron itself does not undergo a chemical transformation.
3. Melting of iron: This is a change of state from solid to liquid. The chemical composition of iron remains Fe. This is a physical change.
4. Rusting of iron: Iron reacts with oxygen and water to form hydrated iron(III) oxides (rust, typically Fe2O3.nH2O). This is a chemical reaction that produces new substances with different chemical properties than iron. This is a chemical change.
The question asks which are *not* chemical changes. These are the physical changes: 1 (Tempering), 2 (Electromagnetism), and 3 (Melting).

Identify the difference between chemical changes (formation of new substances) and physical changes (change in form, state, or appearance without changing chemical composition).

Examples of chemical changes include burning, rusting, digestion, and cooking. Examples of physical changes include melting, freezing, boiling, cutting, and bending. Magnetism is a physical property. Heat treatments like tempering involve physical changes in material properties.