UPSC CDS-2

11. Which one among the following is responsible for determining the chemi

Which one among the following is responsible for determining the chemical properties of an element ?

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Protons
Electrons
Neutrons
Nucleus
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Electrons are responsible for determining the chemical properties of an element.
An element’s chemical properties dictate how it reacts and forms bonds with other elements. This behavior is governed by the configuration of its electrons, particularly the valence electrons in the outermost shell, which are involved in chemical interactions.
Protons, located in the nucleus, determine the atomic number and thus the identity of the element. Neutrons, also in the nucleus, affect the atomic mass and form isotopes but have little direct impact on chemical reactivity. The nucleus (containing protons and neutrons) determines the atom’s mass and stability, but the electron cloud outside the nucleus dictates its chemical behavior.

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12. Which one among the following statements is not true for glass ?

Which one among the following statements is not true for glass ?

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Sodium silicate is known as water glass.
Glass is also called super cooled liquid.
Crystallinity develops on cooling of the molten mass of the glass.
Coloured glasses can be made by adding different colouring materials.
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Statement C, “Crystallinity develops on cooling of the molten mass of the glass,” is not true for glass.
Glass is defined as an amorphous solid. The process of glass formation (vitrification) involves cooling a molten material rapidly enough to prevent the atoms or molecules from arranging themselves into a regular, repeating crystalline structure. If crystallization occurs during cooling, the material becomes a crystalline solid, not glass.
Statement A is true: Sodium silicate solution (Na₂SiO₃) is widely known as water glass or liquid glass. Statement B is true: Glass is often described as a supercooled liquid because it lacks a long-range crystalline order, its structure resembles that of a liquid, although its viscosity is extremely high, giving it the mechanical properties of a solid. Statement D is true: Coloured glasses are made by adding small amounts of various metal oxides or other colouring agents to the glass composition before melting, which absorb or scatter specific wavelengths of light.

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13. Which one among the following statements for the gases mentioned below

Which one among the following statements for the gases mentioned below is not correct ?

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Hydrogen has 3 isotopes.
Helium is a commonly used gas in electric bulbs.
Oxygen is used to burn fuel in rocket engines in spacecraft.
Nitrogen can be used in gaseous form to replace air in food packaging.
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Statement B, “Helium is a commonly used gas in electric bulbs,” is not correct.
Standard incandescent electric bulbs commonly use inert gases like Argon or Nitrogen to reduce filament evaporation and blackening of the bulb. While Helium can be used in some specialized discharge lamps or gas mixtures, it is not the gas commonly found in typical household electric bulbs.
Statement A is correct: Hydrogen has three common isotopes: Protium (¹H), Deuterium (²H), and Tritium (³H). Statement C is correct: Oxygen (or an oxidizer like Liquid Oxygen, LOX) is essential for burning fuel in rocket engines through combustion. Statement D is correct: Nitrogen is an inert gas used in Modified Atmosphere Packaging (MAP) for food to displace oxygen, preventing oxidation and inhibiting the growth of aerobic bacteria, thus extending shelf life.

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14. The surface of the concrete structure of a new construction is covered

The surface of the concrete structure of a new construction is covered with straw/gunny bags by wetting. This is done to :

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prevent fast evaporation, until hydration has proceeded well
protect the concrete structure from contamination by dust particles
prevent development of any fungus on the surface
give smoother and cleaner surface over cement structure
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The curing of concrete involves maintaining adequate moisture and temperature for a period after placing to allow the cement to properly hydrate and gain strength. Covering the concrete surface with wet straw or gunny bags helps to prevent the rapid evaporation of water from the concrete mix. This ensures that enough water is available for the complete hydration of cement, leading to the development of the concrete’s intended strength, durability, and surface properties.
– Hydration is the chemical reaction between cement and water that hardens concrete.
– Proper curing requires maintaining moisture content.
– Rapid evaporation leads to incomplete hydration, reduced strength, increased permeability, and potential cracking.
Other curing methods include ponding (flooding the surface), spraying, using wet coverings like burlap, or applying curing compounds that form a membrane to seal in moisture. Curing should continue for a sufficient period, typically several days to weeks, depending on the type of cement, mix design, strength requirements, and environmental conditions.

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15. Potassium nitrate is not used :

Potassium nitrate is not used :

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as a fertilizer
in medicine
in making gunpowder
as a preservative of meat products
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Potassium nitrate (KNO3), also known as saltpeter or nitre, has several common uses. It is widely used as a fertilizer (source of potassium and nitrogen), as an oxidizing agent in black powder (gunpowder), and as a food additive (E252) for curing meat products. While potassium nitrate has historical medicinal mentions (e.g., as a diuretic), it is not commonly used as a primary medicine in modern pharmaceutical practice compared to its industrial and food preservation roles.
– Potassium nitrate is a source of potassium and nitrogen for plants.
– It is a strong oxidizing agent used in explosives.
– It acts as a preservative and curing agent in meat.
Modern medicine primarily relies on different compounds for the roles potentially associated with potassium nitrate historically. Its uses in fertilizer, gunpowder, and food preservation are well-established and current.

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16. Which one among the following metals is not stored under oil ?

Which one among the following metals is not stored under oil ?

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Sodium
Rubidium
Potassium
Lithium
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Alkali metals (Group 1) are highly reactive and readily react with oxygen and moisture in the air. Sodium (Na), Potassium (K), and Rubidium (Rb) are particularly reactive and are stored under inert liquids like kerosene or mineral oil to prevent reactions with air and water. Lithium (Li), while also an alkali metal, is less reactive than Na, K, and Rb. It reacts readily with nitrogen in the air (unlike other alkali metals) but is less reactive with oxygen and water compared to the others. While it is often stored under oil or an inert atmosphere, it is sometimes the exception discussed when comparing storage methods due to its different reactivity profile. Among the given options, Lithium is the metal *least necessarily* stored under oil compared to the others, or has alternative common storage methods.
– Alkali metals are highly reactive.
– Storage under oil or inert atmosphere prevents reaction with air and moisture.
– Lithium’s reactivity and reaction products differ somewhat from heavier alkali metals.
The reactivity of alkali metals increases down the group. Potassium and Rubidium are more reactive than Sodium, which is more reactive than Lithium. Cesium (Cs) is even more reactive than Rubidium. These heavier alkali metals are often stored in evacuated glass ampoules or under inert gas rather than oil due to their extreme reactivity and lower melting points.

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17. The two isotopes of elemental chlorine are :

The two isotopes of elemental chlorine are :

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$^{35}_{17}$C and $^{36}_{17}$C
$^{34}_{17}$C and $^{36}_{18}$C
$^{35}_{17}$C and $^{37}_{18}$C
$^{35}_{17}$C and $^{37}_{17}$C
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Isotopes of an element are atoms that have the same number of protons (atomic number) but different numbers of neutrons (leading to different mass numbers). The atomic number is represented by the subscript, and the mass number by the superscript before the element symbol. Chlorine (Cl) has an atomic number of 17. Assuming the element symbol ‘C’ in the options is a typo for ‘Cl’, option D, $^{35}_{17}$C and $^{37}_{17}$C, correctly shows the same atomic number (17) but different mass numbers (35 and 37), fitting the definition of isotopes of Chlorine-17.
– Atomic number (number of protons) defines the element.
– Mass number is the sum of protons and neutrons.
– Isotopes of the same element have the same atomic number but different mass numbers.
Naturally occurring chlorine is a mixture of two main isotopes: $^{35}_{17}$Cl (approximately 75.77% abundance) and $^{37}_{17}$Cl (approximately 24.23% abundance). The average atomic mass of chlorine is approximately 35.45 u.

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18. In which of the following colloids is the dispersing medium a liquid ?

In which of the following colloids is the dispersing medium a liquid ?

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Mist
Foam
Jelly
Shaving cream
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A colloid consists of a dispersed phase (the substance distributed) and a dispersing medium (the substance in which the dispersed phase is distributed). In shaving cream, gas bubbles are dispersed within a liquid medium, forming a foam. Therefore, the dispersing medium in shaving cream is a liquid.
– Colloids are mixtures with particles intermediate in size between solutions and suspensions.
– Identifying the dispersed phase and dispersing medium is key to classifying colloids.
– Mist is an aerosol where liquid droplets are dispersed in a gas (air). The dispersing medium is gas.
– Jelly is a gel where a liquid is dispersed in a solid network. The dispersing medium is effectively solid.
– Foam is a colloid where gas is dispersed in a liquid (liquid foam) or solid (solid foam). While ‘Foam’ could be ambiguous, Shaving cream is a specific example of a liquid foam.

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19. Photosynthesis that converts light energy into chemical energy by the

Photosynthesis that converts light energy into chemical energy by the plants, involves the use of green pigments of leaves called chlorophyll. The only structure in a leaf that hosts chlorophyll is :

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Nucleus
Ribosome
Chloroplast
Chromosome
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Photosynthesis, the process by which plants convert light energy into chemical energy, primarily takes place in specialized organelles called chloroplasts. Chloroplasts contain the green pigment chlorophyll, which absorbs light energy necessary for the photosynthetic reactions. While other cell structures are involved in the overall life of the leaf cell, chlorophyll is specifically located within the chloroplasts.
– Chloroplasts are the site of photosynthesis in plant cells.
– Chlorophyll, the main light-absorbing pigment, is located on the thylakoid membranes within chloroplasts.
The nucleus contains the cell’s genetic material. Ribosomes are involved in protein synthesis. Chromosomes are structures within the nucleus that carry genetic information. None of these structures host chlorophyll or are the primary site of photosynthesis.

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20. Which organelle in animal cells can digest foreign materials ?

Which organelle in animal cells can digest foreign materials ?

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Endoplasmic reticulum
Lysosome
Golgi body
Mitochondria
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Lysosomes are membrane-bound organelles found in animal cells containing hydrolytic enzymes. These enzymes are capable of breaking down various biological molecules, including waste materials, cellular debris, and foreign particles (like bacteria or viruses) that are taken into the cell through processes like phagocytosis. This digestive function helps in cellular waste removal, recycling, and defense against pathogens.
– Lysosomes contain powerful digestive enzymes.
– They are involved in phagocytosis (digestion of foreign particles) and autophagy (digestion of cellular components).
The Endoplasmic Reticulum is involved in protein and lipid synthesis. Golgi apparatus modifies, sorts, and packages proteins and lipids. Mitochondria are responsible for cellular respiration and energy production. None of these organelles primarily function in digesting foreign materials within the cell.

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