Chemistry Archives

61. The gas generated on reacting zinc with dilute sulphuric acid is :

The gas generated on reacting zinc with dilute sulphuric acid is :

[amp_mcq option1=”Argon” option2=”Helium” option3=”Hydrogen” option4=”Nitrogen” correct=”option3″]

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When a reactive metal like zinc reacts with a dilute acid like sulphuric acid, it displaces hydrogen from the acid. The reaction is: Zn(s) + H₂SO₄(aq) -> ZnSO₄(aq) + H₂(g). This reaction produces zinc sulphate (a salt) and hydrogen gas.

Active metals react with dilute acids to produce hydrogen gas and a salt.

This is a standard method for preparing hydrogen gas in the laboratory. The hydrogen gas can be tested by bringing a burning splint near it; it will burn with a pop sound.

62. The common element in graphite, fullerene, diamond and graphene is :

The common element in graphite, fullerene, diamond and graphene is :

[amp_mcq option1=”Oxygen” option2=”Nitrogen” option3=”Hydrogen” option4=”Carbon” correct=”option4″]

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Graphite, fullerene, diamond, and graphene are all different structural forms of the element carbon. They are known as allotropes of carbon. Diamond has a tetrahedral structure, graphite has a layered hexagonal structure, fullerenes are cage-like molecules (e.g., C60), and graphene is a single layer of carbon atoms arranged in a hexagonal lattice. Therefore, the common element in all of them is carbon.

Allotropes are different structural modifications of an element; the atoms are bonded together in a different manner. Diamond, graphite, fullerenes, and graphene are allotropes of carbon.

The different structures of carbon allotropes result in vastly different physical and chemical properties. For instance, diamond is extremely hard and an electrical insulator, while graphite is soft and a good electrical conductor.

63. Which one of the following statements is correct?

Which one of the following statements is correct?

[amp_mcq option1=”Alloys are mixtures.” option2=”Alloys are compounds.” option3=”Alloys are always made up of metals.” option4=”All alloys contain carbon as one of their components.” correct=”option1″]

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An alloy is a metallic substance composed of two or more elements, at least one of which is a metal. It is typically formed by mixing molten components. Alloys are considered mixtures because their constituent elements are not chemically bonded in a fixed ratio throughout the material; their properties are a combination of the properties of the constituents, and they can often be separated by physical means. Option B is incorrect as alloys are mixtures, not compounds. Option C is incorrect as alloys can include non-metals (like carbon in steel). Option D is incorrect as many alloys do not contain carbon (e.g., brass is copper and zinc).

Alloys are physical mixtures of metals or metals and non-metals.

The properties of alloys often differ significantly from those of their constituent elements. For example, steel (iron and carbon) is much stronger than pure iron, and brass (copper and zinc) is harder than either copper or zinc.

64. 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 :

[amp_mcq option1=”Filtration” option2=”Solvent Extraction” option3=”Centrifugation” option4=”Simple Distillation” correct=”option4″]

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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.

65. Which one of the following metals does not react with oxygen directly?

Which one of the following metals does not react with oxygen directly?

[amp_mcq option1=”Ti” option2=”Fe” option3=”Pt” option4=”Zn” correct=”option3″]

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Platinum (Pt) is a noble metal known for its low reactivity. It does not react directly with oxygen under normal conditions (room temperature). While it can form oxides under extreme conditions or indirectly, direct oxidation like rusting of iron or tarnishing of silver does not occur easily.

Noble metals, such as Platinum, Gold (Au), and Silver (Ag), are resistant to oxidation and corrosion, which is why they are considered precious metals. Titanium (Ti), Iron (Fe), and Zinc (Zn) are reactive metals that readily react with oxygen to form stable oxides. Titanium forms a passive oxide layer that protects it from further corrosion, but the reaction with oxygen does occur. Iron readily forms rust (iron oxides/hydroxides). Zinc forms a protective layer of zinc oxide/carbonate.

The reactivity of metals with oxygen generally decreases across the periodic table from left to right and down within a group. Noble metals are found towards the right side of the d-block and have high ionization energies and positive standard electrode potentials, making them less likely to lose electrons and react with oxidizing agents like oxygen.

66. Which one is the most abundant of all the elements on Earth?

Which one is the most abundant of all the elements on Earth?

[amp_mcq option1=”Silicon” option2=”Aluminium” option3=”Carbon” option4=”Oxygen” correct=”option4″]

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Among the given options, Oxygen (O) is the most abundant element on Earth, particularly in the Earth’s crust, which is the most accessible part of the Earth’s solid sphere. By weight, Oxygen makes up about 46.6% of the Earth’s crust.

The elemental composition of the Earth’s crust differs from that of the entire Earth. For the Earth’s crust, the most abundant elements by weight are: Oxygen (O) > Silicon (Si) > Aluminium (Al) > Iron (Fe) > Calcium (Ca) > Sodium (Na) > Potassium (K) > Magnesium (Mg). If considering the entire Earth, Iron is the most abundant element, followed by Oxygen, Silicon, and Magnesium, as the core is primarily composed of iron and nickel.

Silicon is the second most abundant element in the Earth’s crust (about 27.7%). Aluminium is the third most abundant (about 8.1%). Carbon is a crucial element for life but is far less abundant overall compared to Oxygen, Silicon, and Aluminium in the Earth’s crust. Given the options, Oxygen is clearly the most abundant.

67. Which one of the following statements is not correct regarding the set

Which one of the following statements is not correct regarding the setting of cement?

[amp_mcq option1=”The addition of a small percentage of gypsum (CaSO₄) lengthens the setting period of cement.” option2=”According to the colloidal theory, gels of hydrated silicates are formed and when these gels harden, the set cement loses strength.” option3=”Tricalcium silicate is responsible for initial setting of cement.” option4=”Dicalcium silicate and tricalcium silicate are responsible for the final strength which occurs in about a year.” correct=”option2″]

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The incorrect statement is that according to the colloidal theory, gels harden and the set cement loses strength. When the gels formed during the hydration of cement silicates harden, they form a rigid structure that gives the set cement its strength. Therefore, hardening of gels leads to an *increase* in strength, not a loss.

The setting and hardening of cement are complex processes involving the hydration of its components. The colloidal theory describes the formation of hydration products as gels that eventually solidify. Gypsum (CaSO₄·2H₂O) is added to cement clinker to retard the setting time, preventing flash setting and allowing sufficient time for mixing, placing, and finishing. Tricalcium silicate (C₃S) hydrates relatively quickly and is primarily responsible for the initial set and early strength. Dicalcium silicate (C₂S) hydrates slowly and contributes significantly to the long-term strength gain of concrete.

The main components of Portland cement clinker are tricalcium silicate (alite), dicalcium silicate (belite), tricalcium aluminate (celite), and tetracalcium aluminoferrite (ferrite). The hydration reactions of these compounds produce hydrated calcium silicates (C-S-H gel) and calcium hydroxide, which are the main products responsible for binding the aggregates together and providing strength. The C-S-H gel is a colloidal substance that hardens over time.

68. Consider the following redox reaction: 2Cu2O (s) + Cu2S (s) —— 6Cu (s)

Consider the following redox reaction:
2Cu2O (s) + Cu2S (s) —— 6Cu (s) + SO2 (g)
Identify the species among the following acting as oxidant and reductant, respectively:

[amp_mcq option1=”Cu(I) and S of Cu2S” option2=”Cu and S of SO2″ option3=”Cu and O of SO2″ option4=”Cu(I) and O of SO2″ correct=”option1″]

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In the reaction 2Cu₂O (s) + Cu₂S (s) → 6Cu (s) + SO₂ (g), the oxidation states are as follows:
In Cu₂O, Cu is +1, O is -2.
In Cu₂S, Cu is +1, S is -2.
In Cu, Cu is 0.
In SO₂, S is +4, O is -2.
Cu changes from +1 to 0, so it is reduced. The species causing reduction (oxidant) contains Cu(+1). Both Cu₂O and Cu₂S contain Cu(+1).
S changes from -2 (in Cu₂S) to +4 (in SO₂), so it is oxidized. The species causing oxidation (reductant) is Cu₂S, specifically the S atom within it.
Thus, Cu(I) acts as the oxidant and S of Cu₂S acts as the reductant.

An oxidant (oxidizing agent) is a substance that accepts electrons and is itself reduced. A reductant (reducing agent) is a substance that donates electrons and is itself oxidized. In this reaction, Cu(I) gains electrons to become Cu(0), hence Cu(I) is the oxidant. S(-2) loses electrons to become S(+4), hence S(-2) is the reductant.

This is a self-reduction reaction where the same element (Copper) is present in both reactants and is reduced, while another element (Sulfur) is oxidized. Specifically, Copper from both Cu₂O and Cu₂S is reduced. The sulfur from Cu₂S is oxidized. Therefore, Cu(I) collectively from the reactants acts as the oxidant, and S in Cu₂S acts as the reductant.

69. Pure, demineralized water, free from all soluble mineral salts is obta

Pure, demineralized water, free from all soluble mineral salts is obtained by which of the following method?

[amp_mcq option1=”Passing water through microfiltration membrane” option2=”Calgon’s method” option3=”Passing water through a cation exchange and an anion exchange resin bed” option4=”By boiling” correct=”option3″]

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UPSC CAPF – 2023

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Pure, demineralized water, free from all soluble mineral salts, is obtained by passing water through a cation exchange and an anion exchange resin bed. This process removes dissolved ions (cations and anions) that constitute mineral salts.

Demineralization or deionization is the process of removing mineral salts from water. Ion exchange is a highly effective method for achieving this. Cation exchange resins replace positive ions (like Ca²⁺, Mg²⁺, Na⁺) with H⁺ ions, and anion exchange resins replace negative ions (like Cl⁻, SO₄²⁻, HCO₃⁻) with OH⁻ ions. The H⁺ and OH⁻ ions then combine to form water (H₂O).

Other options are not suitable for obtaining pure, demineralized water. Microfiltration removes suspended solids but not dissolved salts. Calgon’s method softens water by sequestering hardness ions but doesn’t remove all salts. Boiling removes temporary hardness and dissolved gases but leaves behind permanent hardness salts and other dissolved minerals. Distillation is another method to obtain pure water by separating it from dissolved substances through evaporation and condensation. Reverse osmosis can also produce demineralized water.

70. The atomic radius of hydrogen atom is

The atomic radius of hydrogen atom is

[amp_mcq option1=”37 nanometer” option2=”37 picometer” option3=”17 picometer” option4=”57 picometer” correct=”option2″]

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UPSC CAPF – 2022

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The atomic radius of a hydrogen atom is approximately 37 picometers (pm) when considered as a covalent radius, or about 52.9 pm for the Bohr radius. Given the options, 37 picometer is the closest and a commonly cited value for the covalent radius of hydrogen.

– The size of an atom is often described by its radius.
– Different definitions of atomic radius exist (e.g., covalent radius, van der Waals radius, Bohr radius) which yield different values.
– 1 picometer (pm) = 10⁻¹² meters.
– 1 nanometer (nm) = 10⁻⁹ meters = 1000 picometers.
– A radius of 37 nanometers would be extremely large, equivalent to 37,000 picometers, which is not the size of a hydrogen atom.

The Bohr radius (a₀) for the ground state of hydrogen is approximately 52.9 pm. The covalent radius of hydrogen, often determined from the H-H bond length in H₂ (about 74 pm), is taken as half of this length, i.e., 37 pm. The van der Waals radius of hydrogen is significantly larger, around 120 pm. The question does not specify which type of radius, but 37 pm is a valid and commonly used value for its covalent radius.