UPSC CDS-2 Archives

21. In the human body, blood flows through a process of double circulation

In the human body, blood flows through a process of double circulation. Which one of the following statements is true in this regard ?

Oxygenated blood reaches the left side of the heart from the lungs.

Blood in the left side of the heart is poor in oxygen and is brought to the right side of the heart.

Deoxygenated blood from the left side of the heart is brought to the lungs for oxygenation.

Oxygenated blood from the right side of the heart is sent around the body.

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Statement A is true regarding double circulation in the human body. Oxygenated blood returns from the lungs via the pulmonary veins to the left atrium, then enters the left ventricle before being pumped to the rest of the body.

Double circulation involves two separate loops: the pulmonary circulation (heart to lungs and back) and the systemic circulation (heart to the rest of the body and back). The right side of the heart handles deoxygenated blood, while the left side handles oxygenated blood.

Option B is false because blood in the left side of the heart is rich in oxygen. Option C is false because deoxygenated blood from the right side of the heart goes to the lungs. Option D is false because oxygenated blood is pumped from the left side of the heart to the body.

22. While studying vegetation of an area, terms like ‘population’ and ‘com

While studying vegetation of an area, terms like ‘population’ and ‘community’ are often used. Which one of the following statements best describes a population ?

A group of organisms of one species, living in the same area at the same time.

A group of organisms of one species living in different areas during different seasons.

A unit consisting of biotic and abiotic components.

A group of organisms of more than one species, living in the same area at the same time.

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Option A correctly defines a population in an ecological context. A population is a group of individuals belonging to the same species that live in the same area at the same time and are capable of interbreeding.

The defining characteristics of a biological population are that the organisms must be of the same species, live in the same geographical area, and exist at the same time.

Option C describes an ecosystem, which includes both living (biotic) and non-living (abiotic) components. Option D describes a community, which consists of populations of different species living and interacting in the same area. Option B describes a distribution pattern or migration rather than a population at a specific point in time and space.

23. Which one of the following statements about variations is not true ?

Which one of the following statements about variations is not true ?

Variation is minimum in asexual reproduction.

All variations in a species have equal chances of survival.

Changes in genetic constitution result in variation.

Variants can be selected by environmental factors.

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A) Variation is minimum in asexual reproduction: Asexual reproduction involves a single parent producing genetically identical offspring (clones) through mitosis. The only source of variation is random mutation. Sexual reproduction, involving two parents and meiosis with crossing over and independent assortment, creates much greater genetic variation through new combinations of genes. So, variation is indeed minimal in asexual reproduction compared to sexual reproduction. This statement is true.
B) All variations in a species have equal chances of survival: This statement is false. Survival is not random with respect to variations. Variations confer different traits on individuals. Depending on the environmental conditions, some traits (and thus the variations that cause them) may be advantageous, increasing an individual’s chances of survival and reproduction. Other variations may be disadvantageous, decreasing the chances of survival. This differential survival and reproduction based on variations is the basis of natural selection.
C) Changes in genetic constitution result in variation: Variations are heritable differences between individuals. These differences arise from changes in the genetic material (DNA), such as mutations (changes in nucleotide sequence) or rearrangements of chromosomes, as well as the combination of genes inherited from parents in sexually reproducing organisms. So, this statement is true.
D) Variants can be selected by environmental factors: This is a fundamental principle of evolution by natural selection. Environmental conditions act as selective pressures, favouring individuals with certain variations that make them better adapted to that environment. These individuals are more likely to survive and pass on their advantageous variations to their offspring. This statement is true.
The statement that is NOT true is B.

– Variation arises from changes or combinations of genes.
– Sexual reproduction generates more variation than asexual reproduction.
– Natural selection acts on variations; survival is not equally likely for all variations.
– Environmental factors drive the selection of variants.

Variations are the raw material for evolution. Without variation, natural selection cannot occur, and populations would not be able to adapt to changing environments.

24. In pea, a pure tall plant (TT) is crossed with a short plant (tt). Wha

In pea, a pure tall plant (TT) is crossed with a short plant (tt). What will be the ratio of pure tall plants to short plants in the F₂ generation ?

1:1

1:3

3:1

2:1

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The initial cross is between a pure tall plant (TT) and a short plant (tt).
Parental Generation (P): TT x tt
The gametes produced by the TT parent are all T. The gametes produced by the tt parent are all t.
First Filial Generation (F1): When TT is crossed with tt, the offspring (F1) are all heterozygous tall (Tt).
F1 selfing: Tt x Tt
The gametes produced by a Tt plant are T and t, in equal proportions (1:1).
To find the F2 generation, we cross the F1 gametes:
Punnett Square:
| T | t
—|—-|—-
T | TT | Tt
t | Tt | tt
The genotypes in the F2 generation are TT, Tt, and tt in the ratio 1:2:1.
The phenotypes are Tall (TT and Tt) and Short (tt).
The ratio of Tall to Short phenotypes is 3:1.
The question asks for the ratio of **pure tall plants (TT)** to **short plants (tt)** in the F2 generation.
From the genotype ratio (1 TT : 2 Tt : 1 tt), the number of pure tall plants (TT) is 1 unit, and the number of short plants (tt) is 1 unit.
Therefore, the ratio of pure tall plants to short plants in the F2 generation is 1:1.

– Pure tall genotype is TT, short genotype is tt.
– F1 generation from TT x tt is always Tt (heterozygous tall).
– F2 generation from Tt x Tt has genotypes TT, Tt, tt in ratio 1:2:1.
– Pure tall is TT, short is tt.

Mendel’s experiments with pea plants established the principles of dominance and segregation. This question illustrates the segregation of alleles during gamete formation and their re-combination in subsequent generations. The phenotypic ratio is often 3:1 for a monohybrid cross involving a dominant trait, but the question asks for specific genotypes (pure tall and short), not phenotypes.

25. Which of the following limits the number of trophic levels in a food c

Which of the following limits the number of trophic levels in a food chain ?

Deficient food supply

Polluted air

Decrease in the available energy at higher trophic levels

Parasitic organisms

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In a food chain, energy flows from producers (e.g., plants) to primary consumers (herbivores), secondary consumers (carnivores that eat herbivores), tertiary consumers (carnivores that eat other carnivores), and so on. At each successive trophic level, a significant amount of energy is lost to the environment as heat during metabolic processes, respiration, and excretion. Only about 10% of the energy from one trophic level is typically transferred to the next (the 10% law of energy transfer). Due to this substantial energy loss at each step, the amount of available energy decreases rapidly as you move up the food chain. Eventually, there is not enough energy remaining at higher levels to support a viable population, which limits the number of trophic levels in most ecosystems, usually to four or five.

– Energy transfer between trophic levels is inefficient.
– Approximately 90% of energy is lost at each step.
– The decrease in available energy limits the length of food chains.

Other factors like ecosystem stability, size of organisms, and difficulty in hunting or finding prey can also play a role, but the fundamental physical limit on the number of transfers is imposed by the decreasing energy availability. Deficient food supply can limit population size, but not necessarily the number of potential levels. Polluted air affects health but not the energy transfer mechanism directly limiting trophic levels. Parasitic organisms are consumers but part of the overall complexity, not the primary limiter on the number of main levels.

26. What are the constituents of alloy solder ?

What are the constituents of alloy solder ?

Pb and Zn

Pb and Sn

Pb and Si

Pb and Co

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Solder is a fusible metal alloy used for joining materials by melting the solder and causing it to flow into the joint. The most common type of solder used in electronics and plumbing is a eutectic alloy of tin (Sn) and lead (Pb). While lead-free solders using tin alloys with silver or copper are now common due to environmental concerns regarding lead, traditional and historically significant solders are alloys of lead and tin. Among the given options, Pb and Sn are the constituents of common solder.

– Solder is a metal alloy used for joining metals.
– Common solder alloys primarily consist of tin and lead.

The melting point of solder is lower than that of the metals being joined, allowing it to melt and bond without melting the base metals. The ratio of tin to lead affects the melting point and properties of the solder. Eutectic solder (63% Sn, 37% Pb) has a single melting point, whereas non-eutectic solders melt over a range of temperatures.

27. What is the name of the process that converts sulphide ores into oxide

What is the name of the process that converts sulphide ores into oxides by heating strongly in the presence of excess air ?

Calcination

Roasting

Smelting

Incineration

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The process described involves heating a sulphide ore strongly in the presence of excess air to convert it into an oxide. This process is known as roasting. The general reaction is:
2MS(s) + 3O₂(g) → 2MO(s) + 2SO₂(g) (where M is the metal)
For example, roasting of zinc sulphide: 2ZnS(s) + 3O₂(g) → 2ZnO(s) + 2SO₂(g).
Calcination involves heating a carbonate or hydroxide ore strongly in limited or no air to decompose it into an oxide.
Smelting is a process of using heat to extract a metal from its ore, often involving a reducing agent, and doesn’t specifically focus on converting sulphide to oxide in air.
Incineration is the burning of waste.
Therefore, roasting is the correct term for converting sulphide ores to oxides by heating in excess air.

– Roasting is specifically applied to sulphide ores.
– It requires heating in the presence of excess air.
– It converts the sulphide into a metal oxide and releases sulphur dioxide.

Oxide ores are generally easier to reduce to metals than sulphide ores. Hence, sulphide ores are often first converted to oxides by roasting before reduction processes are applied. Calcination is commonly used for carbonate ores (e.g., CaCO₃ to CaO).

28. When copper reacts with moist carbon dioxide (CO₂) in air, it forms a

When copper reacts with moist carbon dioxide (CO₂) in air, it forms a green coating of which one of the following compounds ?

Cupric carbonate

Cuprous oxide

Cupric oxide

Copper sulphate

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When copper is exposed to the atmosphere, it reacts with components in the air, including moisture (H₂O), oxygen (O₂), and carbon dioxide (CO₂). This reaction leads to the formation of a green layer known as patina. The primary chemical compound responsible for this green colour is basic copper carbonate, with the formula generally represented as Cu₂(OH)₂CO₃ (malachite) or Cu₃(OH)₂(CO₃)₂ (azurite, blue, often found mixed with malachite) or a mixture of Cu(OH)₂ and CuCO₃. The term “cupric carbonate” refers to Copper(II) carbonate, CuCO₃. While the patina is technically *basic* copper carbonate, Cu₂(OH)₂CO₃, which is a compound containing both hydroxide and carbonate, among the given options, “Cupric carbonate” is the closest description of the carbonate component that gives the green colour, often formed via the reaction:
2Cu(s) + H₂O(g) + CO₂(g) + O₂(g) → Cu₂(OH)₂CO₃(s)
Cupric means Copper(II). Option A correctly identifies the compound as a form of copper carbonate.

– The green coating on copper exposed to air is called patina.
– Patina is formed by the reaction of copper with moisture, oxygen, and carbon dioxide.
– The main component of green patina is basic copper carbonate.

Other compounds listed: Cuprous oxide (Cu₂O) is reddish-brown. Cupric oxide (CuO) is black. Copper sulphate (CuSO₄) is a blue crystalline solid or white powder and is soluble in water; it wouldn’t typically form a persistent green coating from atmospheric exposure alone unless sulphur dioxide is also present, forming basic copper sulfate.

29. What is the approximate percentage of carbon in the Earth’s crust ?

What is the approximate percentage of carbon in the Earth’s crust ?

0.045%

0.025%

0.015%

0.005%

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The Earth’s crust is primarily composed of silicate minerals. While carbon is abundant on Earth as a whole (present in the atmosphere, oceans, biosphere, and mantle), its percentage in the *crust* specifically (excluding the organic component and sedimentary rocks like carbonates which are often considered part of the crust but distinct in geological abundance statistics) is relatively low compared to elements like oxygen, silicon, aluminium, iron, etc. Carbon exists in the crust mainly in carbonate rocks (limestone, dolomite), coal, petroleum, and organic matter. Estimates for the abundance of carbon in the Earth’s crust typically fall in the range of 0.03% to 0.05% by weight. Looking at the options provided, 0.045% falls within this commonly cited range, making it the most likely correct answer.

– The Earth’s crust is dominated by silicates.
– Carbon in the crust is mainly found in carbonates and organic matter.
– Its percentage by weight is relatively low compared to the major rock-forming elements.

The lithosphere (crust and upper mantle) contains significantly more carbon than the atmosphere, oceans, and biosphere combined, primarily stored in carbonate rocks formed over geological time. The total amount of carbon on Earth is vast, but its distribution across different reservoirs is uneven.

30. Which of the following carbon allotropes is/are good conductor(s) of e

Which of the following carbon allotropes is/are good conductor(s) of electricity ?

1. Diamond
2. Graphite
3. Fullerene

Select the correct answer using the code given below :

1 only

1 and 2 only

2 only

1 and 3 only

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1. Diamond: In diamond, each carbon atom is covalently bonded to four other carbon atoms in a rigid tetrahedral structure. All valence electrons are involved in strong covalent bonds, and there are no free electrons. Hence, diamond is a very poor conductor of electricity (an insulator).
2. Graphite: In graphite, carbon atoms are arranged in layers of hexagonal rings. Within each layer, atoms are covalently bonded (sp2 hybridisation). Each carbon atom has one valence electron that is not involved in sigma bonding and is delocalized across the layers. These delocalized electrons are free to move, making graphite a good conductor of electricity.
3. Fullerene: Fullerenes, such as C60, have carbon atoms arranged in spherical or cage-like structures involving pentagons and hexagons. The bonding is predominantly sp2 hybridisation with some delocalized electrons. However, in their pure form, most fullerenes are semiconductors or insulators due to the localisation of the pi electrons compared to graphite. While some doped fullerenes can become conductors or even superconductors, pure fullerenes are not considered good conductors in the same way as graphite.
Therefore, among the listed common allotropes, only graphite is a good conductor of electricity.

– Electrical conductivity in carbon allotropes depends on the presence of delocalized or free electrons.
– Diamond has no free electrons.
– Graphite has delocalized pi electrons allowing it to conduct electricity.
– Pure fullerenes are typically semiconductors or insulators.

The differences in electrical conductivity (and other properties like hardness) among carbon allotropes arise from the different ways carbon atoms are bonded and arranged in their crystal structures. Graphite’s layered structure and delocalized electrons make it suitable for applications like electrodes and lubricants. Diamond’s strong 3D network makes it extremely hard.