UPSC CAPF Archives

1. Which one among the following is the correct order of west flowing riv

Which one among the following is the correct order of west flowing rivers in terms of decreasing size of their river basins ?

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Narmada — Tapi — Sabarmati — Mahi

Tapi — Narmada — Mahi — Sabarmati

Sabarmati — Narmada — Tapi — Mahi

Narmada — Tapi — Mahi — Sabarmati

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UPSC CAPF – 2015

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The major west-flowing rivers in peninsular India are the Narmada and Tapi, which flow into the Arabian Sea. Other significant west-flowing rivers include the Mahi, Sabarmati, Luni (inland drainage), Periyar, Bharatpuzha, etc. Comparing the basin sizes of the rivers listed in the options:
– Narmada River Basin: Approximately 98,796 sq km. It is the largest west-flowing river basin in Peninsular India.
– Tapi River Basin: Approximately 65,145 sq km.
– Mahi River Basin: Approximately 34,842 sq km.
– Sabarmati River Basin: Approximately 21,674 sq km.
Arranging these in decreasing order of basin size: Narmada (98,796) > Tapi (65,145) > Mahi (34,842) > Sabarmati (21,674).
The correct order is Narmada — Tapi — Mahi — Sabarmati.

Narmada and Tapi are the largest west-flowing rivers of Peninsular India, flowing through rift valleys. Basin size refers to the total area drained by a river and its tributaries.

Most major peninsular rivers in India flow eastwards into the Bay of Bengal, forming large deltas (e.g., Ganga-Brahmaputra, Mahanadi, Godavari, Krishna, Cauvery). The west-flowing rivers, particularly Narmada and Tapi, flow through rift valleys and form estuaries instead of deltas.

2. Which one of the following compounds is NOT considered an acid ?

Which one of the following compounds is NOT considered an acid ?

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BF₃

AlCl₃

NH₃

C₆H₅OH

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A) BF₃ (Boron Trifluoride) is a Lewis acid because the Boron atom has an incomplete octet and can accept an electron pair.
B) AlCl₃ (Aluminum Chloride) is also a Lewis acid as the Aluminum atom has an incomplete octet and can accept an electron pair.
C) NH₃ (Ammonia) is a Brønsted-Lowry base because it can accept a proton (H⁺) to form ammonium ion (NH₄⁺). It is also a Lewis base because the nitrogen atom has a lone pair of electrons that it can donate. It is not considered an acid in standard definitions.
D) C₆H₅OH (Phenol) is a weak acid because the hydroxyl group (-OH) attached to the benzene ring can donate a proton, forming the phenoxide ion. The acidity is enhanced by the stability of the phenoxide ion through resonance.
Therefore, NH₃ is the compound that is NOT considered an acid; it is a base.

Acids can be defined in various ways (Arrhenius, Brønsted-Lowry, Lewis). Brønsted-Lowry acids are proton donors. Lewis acids are electron pair acceptors. Bases are proton acceptors (Brønsted-Lowry) or electron pair donors (Lewis). Ammonia is a common example of a base.

Lewis acidity is particularly relevant for species like BF₃ and AlCl₃, which lack a hydrogen ion to donate but can accept electron pairs. Phenol is a classic example of an organic acid, albeit a weaker one compared to carboxylic acids, with a pKa of around 10.

3. The hydrogen atoms present in acetylene molecule are :

The hydrogen atoms present in acetylene molecule are :

acidic

basic

both acidic and basic

neutral

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UPSC CAPF – 2015

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Acetylene has the chemical formula C₂H₂ and a linear structure H-C≡C-H. The carbon atoms in acetylene are sp hybridized. sp hybridized orbitals have higher s-character (50%) compared to sp² (33.3%) or sp³ (25%) hybridized orbitals. Higher s-character means the electrons in the C-H bond are held more closely to the carbon nucleus due to the greater penetration of the s orbital. This makes the carbon atom more electronegative than sp² or sp³ hybridized carbons, and consequently, it pulls electron density away from the hydrogen atom. This makes the C-H bond weaker and the hydrogen slightly positive, allowing it to be removed as a proton (H⁺) by a strong base. Therefore, the hydrogen atoms in acetylene are acidic.

The acidity of terminal alkynes (like acetylene) is due to the high electronegativity of the sp-hybridized carbon atom to which the hydrogen is attached. This polarizes the C-H bond, making the proton relatively easy to remove.

Acetylene is a weak acid, much weaker than carboxylic acids or mineral acids, but significantly more acidic than alkanes or alkenes. It can react with very strong bases, such as sodium amide (NaNH₂), to form acetylide salts (e.g., Na⁺ [C≡CH]⁻ or Na⁺ [C≡C]⁻ Na⁺). This acidic property is utilized in certain organic synthesis reactions.

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4. Regur soil is most suitable for the cultivation of :

Regur soil is most suitable for the cultivation of :

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Groundnut

Cotton

Tobacco

Sugarcane

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UPSC CAPF – 2015

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Regur soil is the local name for Black soil found in India, particularly in the Deccan Trap region. These soils are known for their high clay content and ability to retain moisture, making them very fertile for certain crops, especially cotton. Black soil is often referred to as “black cotton soil” due to its suitability for cotton cultivation.

Regur soil is synonymous with Black soil in India and is derived from weathering of volcanic rocks. Its key characteristic is its moisture-retentive capacity and high fertility for cotton.

While black soils can also be used for other crops like sugarcane, groundnut, jowar, etc., cotton is the most prominently associated crop with Regur soil because of the soil’s unique properties that suit cotton cultivation well, such as its ability to retain moisture during dry spells.

5. Which one among the following plains is associated with Limestone Topo

Which one among the following plains is associated with Limestone Topography ?

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Bajada plain

Alluvial plain

Karst plain

Pene plain

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Limestone topography is also known as Karst topography. It is a landscape formed from the dissolution of soluble rocks, such as limestone, dolomite, and gypsum. Characteristics of karst topography include sinkholes (dolines), caves, underground drainage systems, and the absence of surface rivers or lakes over large areas. A plain developed in such a region would be termed a Karst plain, often featuring numerous sinkholes or other dissolution features.

Karst landscapes are specifically associated with the chemical weathering (dissolution) of soluble bedrock, most commonly limestone.

Bajada plains are formed by the coalescing of several alluvial fans along the base of a mountain front in arid or semi-arid regions. Alluvial plains are level landform areas built by deposition of sediment by rivers. Peneplains are low-relief plains that form over long periods as a result of erosion wearing down highlands, often approaching a base level. None of these are primarily defined by limestone dissolution processes.

6. How many numbers from 1 to 1000 are there which are NOT divisible by a

How many numbers from 1 to 1000 are there which are NOT divisible by any of the digits 2, 3 and 5 ?

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166

266

357

366

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UPSC CAPF – 2015

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We need to find the number of integers from 1 to 1000 that are not divisible by 2, 3, or 5. This is equivalent to finding the total number of integers minus the number of integers that are divisible by at least one of 2, 3, or 5.
Using the Principle of Inclusion-Exclusion:
Total numbers = 1000
Let A be the set of numbers divisible by 2, B by 3, C by 5.
|A| = floor(1000/2) = 500
|B| = floor(1000/3) = 333
|C| = floor(1000/5) = 200
|A ∩ B| (divisible by 6) = floor(1000/6) = 166
|A ∩ C| (divisible by 10) = floor(1000/10) = 100
|B ∩ C| (divisible by 15) = floor(1000/15) = 66
|A ∩ B ∩ C| (divisible by 30) = floor(1000/30) = 33
Number of elements divisible by at least one of 2, 3, or 5 is:
|A U B U C| = |A| + |B| + |C| – (|A ∩ B| + |A ∩ C| + |B ∩ C|) + |A ∩ B ∩ C|
= 500 + 333 + 200 – (166 + 100 + 66) + 33
= 1033 – 332 + 33 = 701 + 33 = 734
The number of numbers NOT divisible by any of the digits 2, 3, and 5 is:
Total Numbers – |A U B U C| = 1000 – 734 = 266.

The problem requires finding numbers relatively prime to the product of the given digits (2*3*5=30) within a range. The Principle of Inclusion-Exclusion is a standard method for counting elements in the union of sets.

Alternatively, this can be viewed as finding numbers whose prime factors are not 2, 3, or 5. The fraction of such numbers up to N is approximately N * (1 – 1/p₁) * (1 – 1/p₂) * …, where p₁, p₂, … are the prime factors. For 30, the prime factors are 2, 3, 5. So the fraction is (1 – 1/2)(1 – 1/3)(1 – 1/5) = (1/2)(2/3)(4/5) = 8/30 = 4/15. Approximate count = 1000 * (4/15) ≈ 266.67. The inclusion-exclusion method gives the exact integer count.

7. 29th February of the year 2000 was Tuesday. After this date how many t

29th February of the year 2000 was Tuesday. After this date how many times 29th February falls on Tuesday in the whole century ?

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February 29th occurs only in leap years. The day of the week for a given date repeats every 400 years exactly (due to the Gregorian calendar rules for leap years), but for specific dates like Feb 29th in leap years, the cycle of days repeats every 28 years (7 days * 4 years per cycle = 28 years, modified by the 100/400 year rule which is relevant over longer periods, but within a single century like 2001-2099, the 28-year cycle holds because 2100 is not a leap year and 2000 was a leap year).
Feb 29, 2000 was a Tuesday.
Leap years after 2000 in that century are 2004, 2008, …, 2096.
The day shifts by 5 days (3 normal years + 1 leap year = 365*3 + 366 = 1461 days; 1461 mod 7 = 5) every 4 years.
Starting from Tuesday (in 2000, reference):
2004: Tuesday + 5 days = Sunday
2008: Sunday + 5 days = Friday
2012: Friday + 5 days = Wednesday
2016: Wednesday + 5 days = Monday
2020: Monday + 5 days = Saturday
2024: Saturday + 5 days = Thursday
2028: Thursday + 5 days = Tuesday (1st occurrence after 2000)
The cycle of days for Feb 29 is Tuesday, Sunday, Friday, Wednesday, Monday, Saturday, Thursday, Tuesday…
This repeats every 7 leap years, which is 28 years.
The leap years after 2000 where Feb 29 is Tuesday will be 2000 + 28n.
For n=1: 2000 + 28 = 2028.
For n=2: 2000 + 56 = 2056.
For n=3: 2000 + 84 = 2084.
For n=4: 2000 + 112 = 2112 (outside the century 2001-2100 or 2001-2099).
Thus, Feb 29 falls on Tuesday after 2000 in the years 2028, 2056, and 2084. This is a total of 3 times.

For any given date (except possibly those affected by the 100/400 year rule transitions), the day of the week repeats every 28 years. The year 2000 was a leap year (divisible by 400), and 2100 is not a leap year (divisible by 100 but not 400).

The Gregorian calendar rule states that a year is a leap year if it is divisible by 4, unless it is divisible by 100 but not by 400. This rule causes the full calendar cycle (including day of the week for a given date) to repeat every 400 years. However, for Feb 29 within a period that doesn’t cross a non-leap century year boundary (like 2100), the 28-year sub-cycle for the day of the week holds.

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8. Ozone layer depletion is a major phenomenon in :

Ozone layer depletion is a major phenomenon in :

Troposphere

Stratosphere

Thermosphere

Exosphere

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UPSC CAPF – 2015

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The ozone layer is a region of Earth’s stratosphere that absorbs most of the Sun’s ultraviolet (UV) radiation. Ozone (O₃) concentration is highest in the lower portion of the stratosphere, from about 15 to 35 kilometers above Earth’s surface, although it is present in small quantities throughout the atmosphere. Ozone depletion, primarily caused by human-produced ozone-depleting substances like chlorofluorocarbons (CFCs), is a significant phenomenon occurring within the stratosphere.

The Earth’s atmosphere is divided into several layers: Troposphere (lowest), Stratosphere, Mesosphere, Thermosphere, and Exosphere (highest). The ozone layer resides primarily within the stratosphere.

While the stratosphere is where beneficial ozone is found and depleted, ozone is also present in the troposphere. Tropospheric ozone is a key component of smog and is considered a harmful air pollutant and a greenhouse gas, but it does not form a “layer” in the same way as stratospheric ozone and is not where the major depletion phenomenon occurs.

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9. Which one among the following substances is NOT a ‘green house’ gas ?

Which one among the following substances is NOT a ‘green house’ gas ?

Water vapour

Chlorofluorocarbon

Methane

Nitrogen

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UPSC CAPF – 2015

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Nitrogen (N₂) is the most abundant gas in the Earth’s atmosphere, making up about 78%. However, it is not considered a greenhouse gas because its symmetrical diatomic molecule does not absorb and re-emit infrared radiation effectively, unlike gases with three or more atoms (like water vapour, methane) or asymmetrical diatomic molecules (like carbon monoxide) or complex molecules like CFCs.

Greenhouse gases absorb and emit infrared radiation, trapping heat in the atmosphere. Major greenhouse gases include carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), water vapour (H₂O), and fluorinated gases (like CFCs, HFCs, PFCs, SF₆).

While nitrogen does not directly contribute to the greenhouse effect, nitrogen oxides (like N₂O, which is a powerful greenhouse gas, and NO₂) are significant air pollutants and can be formed from nitrogen and oxygen under high temperatures. However, the question specifically asks about ‘Nitrogen’ (referring to N₂).

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10. 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 :
Statement I : All the energy received by the earth is from the Sun through electro-magnetic radiation
Statement II : The earth also radiates back all the received energy through various ways to maintain the heat budget of the planet

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

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The correct answer is C. Statement I is true, but Statement II is false based on a strict interpretation.

Statement I: All the energy received by the earth is from the Sun through electro-magnetic radiation. This statement is true. The Sun is Earth’s primary energy source, and solar energy reaches Earth as electromagnetic radiation (visible light, infrared, UV, etc.). Internal heat and tidal energy sources are negligible in comparison to the solar input in the context of the overall energy budget driving climate.
Statement II: The earth also radiates back all the received energy through various ways to maintain the heat budget of the planet. This statement is false as worded. The Earth does radiate energy back into space (primarily infrared radiation) to maintain a thermal balance. However, it does not radiate *all* the received energy. A significant portion of incoming solar radiation (about 30%) is reflected back to space by clouds, aerosols, and the Earth’s surface (this is known as albedo). The Earth absorbs the remaining energy (about 70%) and then radiates an equivalent amount of energy as infrared radiation on average to achieve radiative equilibrium. The statement “all the received energy” is inaccurate because it ignores the reflected portion. Furthermore, the current climate change indicates that the heat budget is currently not perfectly maintained; the Earth is absorbing slightly more energy than it radiates back due to the greenhouse effect.

The concept described in Statement II is related to Earth’s energy balance and radiative equilibrium. For a stable temperature, the energy absorbed by Earth must equal the energy radiated back into space. However, Statement II incorrectly states that “all the received energy” is radiated back, neglecting the reflected portion of solar radiation which is also part of balancing the energy budget. The primary way Earth radiates energy is through longwave (infrared) electromagnetic radiation. While Statement II captures the general idea of radiative balance, its specific wording makes it false.

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