UPSC NDA-2 Archives

31. Which one among the following is an example of chemical change ?

Which one among the following is an example of chemical change ?

[amp_mcq option1=”Sublimation of camphor” option2=”Melting of ice” option3=”Heating of an iron rod” option4=”Mixing of NaOH and HCl” correct=”option4″]

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A chemical change results in the formation of new substances with different chemical properties. Mixing sodium hydroxide (NaOH, a strong base) and hydrochloric acid (HCl, a strong acid) causes a neutralization reaction: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l). Sodium chloride (NaCl) and water (H₂O) are produced, which have entirely different chemical properties compared to the reactants. Sublimation, melting, and simple heating of a solid are physical changes, altering the state or temperature but not the chemical identity of the substance.

Chemical changes involve the breaking and forming of chemical bonds, resulting in new substances. Physical changes involve alterations in form, state, or appearance without changing chemical composition.

Neutralization reactions are a common type of chemical change where an acid and a base react to form a salt and water. These reactions are often exothermic, releasing heat.

32. The elemental composition of diamond is

The elemental composition of diamond is

[amp_mcq option1=”Carbon and Hydrogen” option2=”Carbon and Oxygen” option3=”Pure Carbon” option4=”Pure Silicon” correct=”option3″]

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Diamond is an allotrope of carbon. This means it is a form of pure carbon where the carbon atoms are arranged in a specific crystal structure (a tetrahedral lattice). Therefore, diamond consists solely of carbon atoms and no other elements. Graphite is another well-known allotrope of pure carbon.

Allotropes are different structural modifications of an element. Diamond is a crystalline allotrope of carbon.

In diamond, each carbon atom is covalently bonded to four other carbon atoms in a rigid, three-dimensional tetrahedral network. This structure makes diamond extremely hard and gives it a high melting point.

33. For Aluminium (Al) (atomic number : 13, mass number : 27), which one a

For Aluminium (Al) (atomic number : 13, mass number : 27), which one among the following statements is NOT correct ?

[amp_mcq option1=”Number of electrons present in Al is 13″ option2=”Number of protons present in Al is 13″ option3=”Number of neutrons present in Al is 14″ option4=”The valency of Al is 2″ correct=”option4″]

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For a neutral atom, the number of protons is equal to the atomic number (Z), and the number of electrons is also equal to the atomic number. Given the atomic number of Aluminium (Al) is 13, it has 13 protons and 13 electrons. The mass number (A) is the sum of protons and neutrons. Given the mass number is 27 and the number of protons is 13, the number of neutrons is 27 – 13 = 14. Aluminium is in Group 13 of the periodic table, with electronic configuration 1s² 2s² 2p⁶ 3s² 3p¹. It readily loses its 3 valence electrons to form a stable Al³⁺ ion. Therefore, the typical valency of Aluminium is 3, not 2.

Atomic number = Number of protons. For a neutral atom, Number of electrons = Number of protons. Mass number = Number of protons + Number of neutrons. Valency is related to the number of electrons an atom gains, loses, or shares to form chemical bonds.

Elements in Group 13 (like Aluminium) typically exhibit a valency of +3, forming ions with a +3 charge by losing their three valence electrons.

34. Which one among the following fertilizers is a source of nitrogen ?

Which one among the following fertilizers is a source of nitrogen ?

[amp_mcq option1=”Superphosphate” option2=”Potassium sulfate” option3=”Bone meal” option4=”Urea” correct=”option4″]

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Urea, with the chemical formula (NH₂)₂CO, is a widely used nitrogenous fertilizer. It contains a high percentage of nitrogen by weight. When applied to soil, urea is hydrolyzed by enzymes to produce ammonia (NH₃), which is then converted to ammonium ions (NH₄⁺) and nitrate ions (NO₃⁻) that plants can absorb as sources of nitrogen. Superphosphate is a phosphorus fertilizer, potassium sulfate is a potassium fertilizer, and bone meal is primarily a source of phosphorus and calcium.

Plants require macronutrients like nitrogen (N), phosphorus (P), and potassium (K) for growth. Fertilizers provide these nutrients. Nitrogen fertilizers specifically supply nitrogen compounds to the soil.

Urea is an organic compound and is often considered the most important nitrogen fertilizer globally due to its high nitrogen content and ease of synthesis. Other common nitrogen fertilizers include ammonium nitrate (NH₄NO₃) and ammonium sulfate ((NH₄)₂SO₄).

35. Which one among the following is an example of oxidation reaction ?

Which one among the following is an example of oxidation reaction ?

[amp_mcq option1=”Freezing of water” option2=”Dissolving sugar in water” option3=”Rusting of iron” option4=”Boiling of petrol” correct=”option3″]

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Oxidation is a chemical reaction where a substance loses electrons or its oxidation state increases. Rusting of iron is the process where iron reacts with oxygen and water to form hydrated iron(III) oxides, commonly known as rust. This is a chemical change where iron is oxidized from its elemental state (oxidation state 0) to an oxidation state of +3 in the iron oxides. Freezing of water, dissolving sugar, and boiling of petrol are physical changes or processes, not chemical reactions involving oxidation in the typical sense tested in such questions.

An oxidation reaction involves the loss of electrons or an increase in oxidation state. Chemical changes result in the formation of new substances. Physical changes alter the form or appearance but not the chemical composition.

Rusting is a specific type of corrosion, which is an electrochemical process. The chemical equation for the formation of rust is complex, but the overall process involves the oxidation of iron: Fe → Fe³⁺ + 3e⁻, and the reduction of oxygen.

36. Which one among the following has valency of 3 ?

Which one among the following has valency of 3 ?

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

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Nitrogen commonly has a valency of 3, particularly in compounds like ammonia (NH3).

– Valency is the measure of the combining capacity of an element. It is typically equal to the number of electrons an atom gains, loses, or shares to achieve a stable electron configuration (usually a full outermost electron shell).
– Carbon (Atomic number 6): Electron configuration is 2, 4. It needs 4 electrons to complete its outer shell, so its common valency is 4 (e.g., CH4, CO2).
– Oxygen (Atomic number 8): Electron configuration is 2, 6. It needs 2 electrons, so its common valency is 2 (e.g., H2O, O2).
– Nitrogen (Atomic number 7): Electron configuration is 2, 5. It needs 3 electrons to complete its outer shell. Thus, its common valency is 3 (e.g., NH3, NCl3). Nitrogen can also exhibit other valencies (e.g., +5 in nitrates), but 3 is a very common valency.
– Sodium (Atomic number 11): Electron configuration is 2, 8, 1. It tends to lose 1 electron to achieve a stable configuration. Its valency is 1 (e.g., NaCl, Na2O).

The concept of valency is simplified and primarily applies to covalent bonding and simple ionic compounds. More rigorous descriptions of bonding use oxidation states and formal charges, especially for elements in higher periods or in complex compounds. However, for basic elements like N, O, C, and Na, their typical valencies are characteristic.

37. At which temperature does liquid water show maximum density ?

At which temperature does liquid water show maximum density ?

[amp_mcq option1=”299 K” option2=”277 K” option3=”285 K” option4=”373 K” correct=”option2″]

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Liquid water exhibits its maximum density at 4°C. Converting this to Kelvin, we get 4 + 273.15 = 277.15 K. The option closest to this value is 277 K.

– Unlike most substances which become denser as they are cooled, water behaves unusually in the temperature range of 0°C to 4°C.
– As liquid water cools from higher temperatures, its density increases until it reaches a maximum density at approximately 4°C (3.98°C to be precise).
– Below 4°C, the density of liquid water starts to decrease as it approaches its freezing point (0°C). When it freezes to form ice at 0°C, its density decreases significantly, which is why ice floats on water.
– This anomalous expansion of water is crucial for aquatic life in cold climates, as the densest water (at 4°C) sinks to the bottom of lakes, allowing life to survive below the surface ice.

The Kelvin scale is an absolute thermodynamic temperature scale where 0 K is absolute zero. The relationship between Celsius (°C) and Kelvin (K) is K = °C + 273.15. The boiling point of water at standard atmospheric pressure is 100°C or 373.15 K.

38. What is the maximum number of electrons that can occupy the first shel

What is the maximum number of electrons that can occupy the first shell of an atom ?

[amp_mcq option1=”2″ option2=”4″ option3=”8″ option4=”16″ correct=”option1″]

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The maximum number of electrons that can occupy the first electron shell of an atom is 2.

– Electron shells (also called energy levels) surround the nucleus of an atom. They are denoted by the principal quantum number, n (n=1, 2, 3, …), where n=1 is the shell closest to the nucleus.
– The maximum number of electrons that can occupy a given shell is determined by the formula 2n², where n is the shell number.
– For the first shell (n=1), the maximum number of electrons is 2 * (1)² = 2 * 1 = 2.
– For the second shell (n=2), the maximum number of electrons is 2 * (2)² = 2 * 4 = 8.
– For the third shell (n=3), the maximum number of electrons is 2 * (3)² = 2 * 9 = 18, and so on.

The first shell (n=1) contains only one subshell, the ‘s’ subshell, which can hold a maximum of 2 electrons. The electron configuration follows the Aufbau principle, Hund’s rule, and the Pauli exclusion principle, filling lower energy levels before higher ones.

39. Which one among the following processes is NOT part of chemical weathe

Which one among the following processes is NOT part of chemical weathering ?

[amp_mcq option1=”Solution” option2=”Hydration” option3=”Reduction” option4=”Thawing” correct=”option4″]

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Thawing is the process of ice melting into water. While it is part of the freeze-thaw cycle, which is a significant process of *physical* weathering, thawing itself is not a chemical reaction that alters the composition of the rock.

– Chemical weathering involves the alteration of the chemical composition of rocks and minerals through processes like dissolution, oxidation, reduction, hydrolysis, hydration, and carbonation.
– Solution: Minerals dissolving in water (e.g., salts, limestone in acidic water).
– Hydration: Minerals absorbing water molecules into their structure (e.g., anhydrite to gypsum).
– Reduction: Gaining electrons; often involves changes in oxidation states of elements like iron, common in anaerobic environments.
– Thawing: The physical change of state from solid (ice) to liquid (water). Freeze-thaw weathering occurs because water expands when it freezes in cracks, exerting pressure that widens the cracks. Thawing allows more water to enter, continuing the cycle, but it is the freezing/expansion that causes mechanical breakdown, not a chemical change during thawing.

Physical weathering breaks down rocks into smaller pieces without changing their chemical composition. Other physical weathering processes include abrasion, exfoliation, granular disintegration, and biological activity (like root wedging). Chemical weathering is more effective in warm and humid climates, while physical weathering is prominent in areas with significant temperature fluctuations and frost action.

40. Which one of the following is correct ?

Which one of the following is correct ?

[amp_mcq option1=”Pyroxene is commonly found in meteorites” option2=”Amphiboles form about 20% of the Earth’s crust” option3=”Quartz is soluble in water” option4=”The word ‘sedimentary’ is derived from the Latin word kelimentum” correct=”option1″]

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The statement that Pyroxene is commonly found in meteorites is correct. The other statements are incorrect.

– Pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. They are also abundant minerals in stony meteorites, particularly in chondrites and achondrites like basalts and dunites originating from asteroids.
– Amphiboles are also common rock-forming silicate minerals, but they constitute a smaller percentage of the Earth’s crust compared to feldspars and quartz. Their abundance is estimated to be around 5-7% of the continental crust.
– Quartz (silicon dioxide, SiO2) is highly insoluble in ordinary water under surface conditions, which contributes to its resistance to chemical weathering. It can be dissolved under extreme conditions (e.g., high temperature and pressure, or presence of specific chemicals), but not in typical rainwater or groundwater.
– The word ‘sedimentary’ is derived from the Latin word ‘sedimentum’, which means “a settling” or “a depositing”.

Feldspars are the most abundant mineral group in the Earth’s crust, making up over 50% by weight. Quartz is the second most abundant mineral in the continental crust. The study of minerals found in meteorites provides valuable insights into the composition and formation of the early solar system and other planetary bodies.