Physical and Chemical Changes, Solution

51. Ammonia (NH 3 ) obtained from different sources always has same propor

Ammonia (NH3) obtained from different sources always has same proportion of Nitrogen and Hydrogen. It proves the validity of law of:

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Reciprocal proportion
Constant proportion
Multiple proportions
None of the above
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The statement “Ammonia (NH3) obtained from different sources always has same proportion of Nitrogen and Hydrogen” is a direct application of the Law of Constant Proportions (also known as the Law of Definite Proportions). This law states that a given chemical compound always contains its component elements in fixed proportion by mass, regardless of the source or method of preparation.
The Law of Constant Proportions explains why the elemental composition of a specific compound is always the same.
The Law of Reciprocal Proportion relates the masses of two elements that combine separately with a fixed mass of a third element. The Law of Multiple Proportions states that if two elements can combine to form more than one compound, then the masses of one element that combine with a fixed mass of the other element are in the ratio of small whole numbers. The given statement specifically addresses the fixed composition of a single compound, which is the domain of the Law of Constant Proportions.

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52. An iron nail dipped in copper sulphate solution turns brown. This is d

An iron nail dipped in copper sulphate solution turns brown. This is due to which one of the following types of reactions ?

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Addition reaction
Decomposition reaction
Substitution reaction
Displacement reaction
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The correct answer is D) Displacement reaction.
This reaction involves a more reactive metal (iron, Fe) displacing a less reactive metal (copper, Cu) from its salt solution (copper sulphate, CuSO4). The general form of a displacement reaction involving metals is A + BX → AX + B, where A is more reactive than B.
The reaction is represented as: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s). The iron nail gets coated with brown copper metal. This is a classic example illustrating the reactivity series of metals, where iron is higher than copper. It is also a redox reaction where iron is oxidized and copper ions are reduced. While substitution reactions also involve replacement, “displacement reaction” is the specific term used for this type of reaction involving elements and compounds.

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53. Which one of the following apparatus is used for separating benzene an

Which one of the following apparatus is used for separating benzene and water mixture ?

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Round bottom flask
Conical flask
Separating funnel
Dean and Stark apparatus
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UPSC NDA-1 – 2023
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The correct answer is C) Separating funnel.
A separating funnel is a piece of laboratory glassware used to separate immiscible liquids, i.e., liquids that do not mix and form distinct layers, based on their densities. Benzene and water are immiscible; benzene is less dense than water.
When a mixture of benzene and water is placed in a separating funnel, the two liquids separate into layers (water at the bottom, benzene at the top). The stopper is then opened, and the lower layer (water) is drained out through the stopcock until the interface between the two layers reaches the stopcock. The stopcock is then closed, and the upper layer (benzene) is collected in a separate container. Round bottom flasks and conical flasks are used for mixing, heating, or storage, not for separating immiscible liquids. A Dean and Stark apparatus is used for determining moisture content by distillation.

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54. Which one of the following reactions is an example of decomposition

Which one of the following reactions is an example of decomposition reaction?

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CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
2AgCl(s) Sunlight → 2Ag(s) + Cl₂(g)
CuO + H₂ Heat → Cu + H₂O
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
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The correct option is B.
A decomposition reaction is a reaction in which a single compound breaks down into two or more simpler substances.
Option A is a combustion reaction, where a substance reacts rapidly with oxygen to produce heat and light.
Option B, $2\text{AgCl(s)} \text{ Sunlight} \rightarrow 2\text{Ag(s)} + \text{Cl}_2\text{(g)}$, shows silver chloride (a single compound) breaking down into silver and chlorine (simpler substances) upon exposure to sunlight. This is a classic example of photolytic decomposition.
Option C is a reduction reaction, specifically the reduction of copper(II) oxide by hydrogen.
Option D is a displacement reaction, where iron displaces copper from copper sulphate solution.

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55. Which one of the following is a chemical change?

Which one of the following is a chemical change?

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Dissolving sugar in water
Melting of ice
Crystallization
Milk turning sour
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A chemical change involves the formation of new substances with different chemical properties. Milk turning sour is a process where lactose is converted into lactic acid by bacteria, resulting in a change in chemical composition.
Physical changes alter the form or appearance of a substance but not its chemical identity (e.g., dissolving, melting, freezing, boiling, crystallization). Chemical changes result in the formation of one or more new substances with different chemical properties (e.g., burning, rusting, cooking, souring of milk).
Dissolving sugar in water, melting of ice, and crystallization are all physical changes. In milk turning sour, bacteria perform fermentation, converting lactose (a sugar) into lactic acid, which changes the taste and properties of the milk. This is a clear example of a chemical reaction producing new substances.

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56. Refining of petroleum is carried out using which one of the following

Refining of petroleum is carried out using which one of the following techniques?

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Evaporation
Fractional distillation
Separating funnel
Sublimation
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Refining of petroleum (crude oil) is done by separating its various components based on their different boiling points. This process is called fractional distillation.
Crude oil is a complex mixture of hydrocarbons with varying chain lengths and boiling points. Fractional distillation involves heating the crude oil and separating the vapours into different fractions at various levels of a fractionating column based on their condensation temperatures. Lighter fractions with lower boiling points rise higher in the column, while heavier fractions with higher boiling points condense lower down.
Evaporation is used to separate a solvent from a non-volatile solute. A separating funnel is used to separate immiscible liquids. Sublimation is used to separate a substance that changes directly from solid to gas from substances that do not sublime. Fractional distillation is the standard industrial method for separating crude oil into useful products like gasoline, diesel, kerosene, etc.

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57. Which one of the following is not a solution?

Which one of the following is not a solution?

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Alloy
Milk
Air
Sugar
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A solution is a homogeneous mixture of two or more substances. Among the given options, milk is a heterogeneous mixture (specifically a colloid).
An alloy is a homogeneous mixture of metals or a metal and a non-metal (e.g., brass, steel), so it is a solution (solid solution). Air is a homogeneous mixture of gases (primarily nitrogen and oxygen), so it is a solution (gaseous solution). Milk contains dispersed particles of fat and protein which are not uniformly distributed at the microscopic level, making it a heterogeneous mixture (colloid). Sugar itself is a pure substance (compound), but if dissolved in water, it forms a homogeneous mixture (sugar solution). However, given the options as listed, Milk is the only one that is definitively not a solution.
Mixtures can be homogeneous (solutions) or heterogeneous (suspensions and colloids). Solutions have uniform composition throughout. Colloids have particles dispersed uniformly, but these particles are larger than molecules and cause light scattering (Tyndall effect); they are considered heterogeneous. Suspensions have even larger particles that settle out over time.

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58. Reaction of quick lime (CaO) with water to produce slaked lime (Ca(OH)

Reaction of quick lime (CaO) with water to produce slaked lime (Ca(OH)₂) is an example of

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Displacement reaction.
Endothermic reaction.
Decomposition reaction.
Exothermic reaction.
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The reaction between quick lime (calcium oxide, CaO) and water (H₂O) to form slaked lime (calcium hydroxide, Ca(OH)₂) is represented by the equation: CaO(s) + H₂O(l) → Ca(OH)₂(s). This reaction is well-known for releasing a significant amount of heat, making it a highly exothermic process.
Exothermic reactions are chemical reactions that release energy, typically in the form of heat or light. Endothermic reactions absorb energy. Displacement reactions involve one element displacing another from a compound. Decomposition reactions involve a single compound breaking down into simpler substances. The reaction of quick lime with water fits the definition of an exothermic reaction due to the large heat release, also sometimes classified as a combination reaction.
This reaction is used in the industrial production of slaked lime, which has various applications, including in mortar and plaster, water treatment, and agriculture. The heat released during the reaction is often sufficient to boil the water.

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59. Which one of the following substances is NOT a mixture ?

Which one of the following substances is NOT a mixture ?

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Ice
Ice-cream
Air
Honey
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UPSC NDA-1 – 2019
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Ice is NOT a mixture.
A mixture is a substance comprising two or more components that are not chemically bonded.
Ice is the solid state of water (H₂O). Water is a chemical compound, meaning it is formed from different elements (Hydrogen and Oxygen) chemically combined in a fixed ratio. A compound is considered a pure substance.
Ice-cream is a mixture of water, milk solids, sugars, fats, flavorings, and air.
Air is a mixture of gases, primarily nitrogen, oxygen, argon, and carbon dioxide.
Honey is a mixture consisting mainly of sugars and water, along with other components like enzymes, minerals, etc.
Pure substances have a fixed composition and definite physical and chemical properties (like melting point, boiling point). Mixtures have variable compositions, and their properties depend on the proportions of the components. Ice (pure water) has a definite melting point (0°C at standard pressure), whereas mixtures like ice-cream, air, and honey do not melt/freeze or boil at a single fixed temperature but over a range.

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60. Which one of the following statements is NOT correct for the given rea

Which one of the following statements is NOT correct for the given reaction ? Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

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Iron is the reducing agent
The solution turns green in colour after the reaction
Copper is a more reactive metal than iron
The reaction is an example of a redox reaction
This question was previously asked in
UPSC NDA-1 – 2019
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The reaction shown is a single displacement reaction where iron (Fe) displaces copper (Cu) from copper sulphate (CuSO₄) solution: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). This occurs because iron is more reactive than copper. In the reactivity series of metals, a more reactive metal can displace a less reactive metal from its salt solution.
– Statement A is correct: Iron (oxidation state 0) is oxidized to Fe²⁺ (oxidation state +2) by losing electrons. A substance that causes reduction by being oxidized itself is called a reducing agent.
– Statement B is correct: Copper sulphate solution is typically blue in colour. Iron(II) sulphate (FeSO₄) solution, formed after the reaction, is green in colour. So, the solution’s colour changes from blue to green.
– Statement C is incorrect: The displacement of copper by iron indicates that iron is more reactive than copper. Copper is less reactive than iron.
– Statement D is correct: Iron is oxidized (Fe → Fe²⁺) and copper ions are reduced (Cu²⁺ → Cu). Reactions involving both oxidation and reduction are called redox reactions.
The reactivity series of metals (partially): K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au. A metal higher in the series can displace a metal lower in the series from its salt solution.

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