UPSC NDA-2 Archives

31. An important cause of tooth decay by the bacterium Streptococcus mutan

An important cause of tooth decay by the bacterium Streptococcus mutans is because of their ability to attach to teeth surface by producing slime layer from

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Sugar

Alcohol

Saliva

Teichoic acid

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Streptococcus mutans metabolizes dietary sugars, particularly sucrose, to produce acids that demineralize tooth enamel (causing decay) and to synthesize extracellular polysaccharides (glucans and fructans). These sticky polysaccharides form a ‘slime layer’ or matrix which allows the bacteria to adhere firmly to the tooth surface and form dental plaque (biofilm).

Streptococcus mutans is a key bacterium in the initiation of dental caries (tooth decay). It produces both acid and a sticky matrix from fermentable carbohydrates, primarily sugars.

While saliva plays a role in buffering acids and clearing food debris, it is not used by the bacteria to produce the sticky layer. Teichoic acid is a component of the bacterial cell wall, contributing to cell shape and adhesion, but the bulk of the slime layer is composed of polysaccharides synthesized from dietary sugars. Alcohol is generally not relevant to this process in the context of dietary intake leading to decay.

32. Eukaryotic cells are much more complex as compared to prokaryotes. Whi

Eukaryotic cells are much more complex as compared to prokaryotes. Which one of the following structures is exclusively present in a eukaryotic cell ?

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Cell wall

Plasma membrane

Nucleic acid

Mitochondria

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Mitochondria are membrane-bound organelles responsible for cellular respiration in eukaryotic cells. They are absent in prokaryotic cells. The other options are present in both types of cells: Cell walls are present in plant and fungal eukaryotes as well as bacterial prokaryotes. Plasma membrane is the outer boundary of all cells. Nucleic acids (DNA and RNA) are the genetic material found in all cells.

Eukaryotic cells are characterized by the presence of a nucleus and membrane-bound organelles like mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and chloroplasts (in plants and algae), which are absent in prokaryotes.

Prokaryotic cells lack a true nucleus and complex membrane-bound organelles. Their genetic material is located in a nucleoid region. Eukaryotic cells compartmentalize cellular functions within organelles, contributing to their complexity.

33. The two important features of sexual reproduction in higher organisms

The two important features of sexual reproduction in higher organisms that create genetic diversity in offspring are

Mitosis and fertilization

Meiosis and fertilization

Mitosis and binary fission

Meiosis and conjugation

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Meiosis and fertilization are the two key processes in sexual reproduction that generate genetic diversity. Meiosis involves crossing over and independent assortment of chromosomes, creating unique combinations of alleles in the haploid gametes. Fertilization is the fusion of two distinct gametes (usually from different individuals), combining their unique genetic material to form a diploid zygote with a novel genetic makeup.

Genetic diversity is primarily generated through recombination (crossing over) and segregation of homologous chromosomes during meiosis, and the random combination of gametes during fertilization.

Mitosis is involved in growth and repair but produces genetically identical cells. Binary fission is a form of asexual reproduction producing clones. Conjugation is a method of genetic transfer in some prokaryotes and protists but is not the fundamental process for generating diversity in sexual reproduction of higher organisms like the combination of meiosis and fertilization.

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34. Consider the following reaction : 2HgO ⎯⎯→ 2Hg + O₂ The respective sta

Consider the following reaction :
2HgO ⎯⎯→ 2Hg + O₂
The respective state of HgO, Hg and O₂ in the above reaction is

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Liquid, solid, gas

Solid, solid, gas

Liquid, solid, liquid

Solid, liquid, gas

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Mercuric oxide (HgO) is a solid compound at room temperature. Mercury (Hg) is the only metal that exists as a liquid at room temperature. Oxygen (O₂) is a gas at room temperature. Therefore, the respective states of HgO, Hg, and O₂ are solid, liquid, and gas.

The physical states of substances at standard temperature and pressure (or implied conditions) are important in chemical reactions. Mercury is unique among metals in being liquid at room temperature.

This reaction represents the thermal decomposition of mercuric oxide. Heating HgO breaks it down into its constituent elements, mercury and oxygen. The states indicated correspond to the stable forms of these substances under normal conditions after the reaction.

35. Which one of the following metals does NOT react with cold water to li

Which one of the following metals does NOT react with cold water to liberate hydrogen gas ?

Potassium

Iron

Calcium

Sodium

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Iron (Fe) is a less reactive metal compared to Potassium (K), Sodium (Na), and Calcium (Ca). Highly reactive metals like K and Na react vigorously with cold water, producing hydrogen gas and metal hydroxides. Calcium also reacts with cold water, though less vigorously, producing hydrogen gas and calcium hydroxide. Iron, however, does not react with cold water. It reacts slowly with hot water and vigorously with steam to produce hydrogen gas and metal oxide.

The reactivity of metals determines their reaction with water. More reactive metals displace hydrogen from cold water; moderately reactive metals displace hydrogen from hot water or steam; less reactive metals do not react with water at all.

The general order of reactivity of these metals with water is K > Na > Ca > Fe. Potassium and Sodium react violently with cold water. Calcium reacts steadily with cold water. Iron reacts with steam (very hot water) to form iron oxides and hydrogen gas.

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36. Which one of the following processes involves chemical reaction ?

Which one of the following processes involves chemical reaction ?

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Evaporation of petrol

Dissolution of salt in water

Storing of oxygen gas under pressure in a gas cylinder

Burning of magnesium ribbon in air

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Burning of magnesium ribbon in air involves a chemical reaction.

A chemical reaction involves the rearrangement of atoms and molecules to form new substances with different chemical properties. Evaporation, dissolution, and storing gas under pressure are physical processes where the substance’s chemical composition remains unchanged, only its state or distribution changes. Burning (combustion) is a chemical process where a substance reacts rapidly with oxygen, typically producing heat and light and forming new chemical compounds. When magnesium burns in air, it reacts with oxygen and nitrogen to form magnesium oxide and magnesium nitride, respectively.

Evaporation is a phase transition from liquid to gas. Dissolution is the process of dissolving a solute in a solvent to form a solution; the solute particles disperse but do not chemically change. Storing gas under pressure is a physical change affecting density and volume. Burning involves chemical transformation.

37. Match List I with List II and select the correct answer using the code

Match List I with List II and select the correct answer using the code given below the Lists :

List I (Chemical process)	List II (Reaction)
A. Electrolysis of water	1. Double displacement reaction
B. Burning of coal	2. Combination reaction
C. Iron nail immersed in copper sulphate solution	3. Decomposition reaction
D. Addition of barium chloride solution to aluminium sulphate solution	4. Displacement reaction

Code :

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	A	B	C	D
(a)	3	4	2	1
(b)	4	2	1	3
(c)	3	2	4	1
(d)	4	1	2	3

(a) 3 4 2 1

(b) 4 2 1 3

(d) 4 1 2 3

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The correct matching of chemical processes with their reaction types is A-3, B-2, C-4, D-1.

A. Electrolysis of water involves breaking down water (a single compound) into hydrogen and oxygen (simpler substances) using electricity. This is a decomposition reaction. (H₂O → H₂ + O₂)
B. Burning of coal involves the reaction of carbon (and other components) with oxygen, usually forming carbon dioxide. This is a combination reaction (C + O₂ → CO₂) and also combustion.
C. Iron nail immersed in copper sulphate solution involves iron displacing copper from copper sulphate because iron is more reactive than copper. This is a displacement reaction. (Fe + CuSO₄ → FeSO₄ + Cu)
D. Addition of barium chloride solution to aluminium sulphate solution involves the exchange of ions between the two reactants, forming new compounds, one of which is insoluble (barium sulphate precipitate). This is a double displacement reaction. (3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄↓ + 2AlCl₃)

Matching these gives: A-3, B-2, C-4, D-1, which corresponds to option (c).

38. Consider the following reaction : Fe 2 O 3 (s) + 2Al(s) → 2Fe(s) + Al

Consider the following reaction :
Fe₂O₃(s) + 2Al(s) → 2Fe(s) + Al₂O₃(s)
Which of the following statements about the given reaction is NOT correct ?

It is an example of displacement reaction

It is highly endothermic reaction

Fe<sub>2</sub>O<sub>3</sub> acts as oxidizing agent

This reaction is applicable to joining of railway tracks

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The statement that is NOT correct about the given reaction is that it is highly endothermic reaction.

The reaction Fe₂O₃(s) + 2Al(s) → 2Fe(s) + Al₂O₃(s) is known as the thermite reaction. It is a highly exothermic reaction, releasing a large amount of heat. Aluminum is more reactive than iron and displaces iron from its oxide, making it a displacement reaction. Fe₂O₃ is reduced (loses oxygen), so it acts as an oxidizing agent. The large amount of heat produced makes this reaction useful for welding, including joining railway tracks.

The thermite reaction releases so much heat that the iron produced is in a molten state, which is essential for its application in welding. An endothermic reaction absorbs heat from the surroundings, which is the opposite of what happens in the thermite process.

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39. The green plants in a terrestrial eco- system absorbs sunlight that fa

The green plants in a terrestrial eco- system absorbs sunlight that falls on their leaves and convert it into food energy. The percentage of solar radia- tion absorbed by all green plants for the process of photosynthesis is about

10%

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The percentage of solar radiation absorbed by all green plants for the process of photosynthesis is about 1%.

Ecological studies show that only a small fraction of the total solar energy incident on the Earth’s surface is captured by producers (green plants) and converted into chemical energy through photosynthesis. This efficiency is typically stated as being around 1%. This absorbed energy forms the base of most ecosystems, supporting all subsequent trophic levels.

While plants absorb some portion of the sunlight (especially in the red and blue wavelengths), much of the incident light is reflected or transmitted. Furthermore, not all absorbed energy is converted into biomass; a significant portion is lost as heat or used by the plant for its own metabolic processes (respiration). The 1% value refers to the efficiency of conversion of incident solar energy into net primary production.

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40. Which one of the following graphs correctly represents the current (I)

Which one of the following graphs correctly represents the current (I) – voltage (V) variation for a rectangular piece of a semiconductor wafer ?

Graph (a)

Graph (b)

Graph (c)

Graph (d)

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Graph (a) correctly represents the current (I) – voltage (V) variation for a rectangular piece of a semiconductor wafer under conditions where it behaves ohmically.

While semiconductors are known for non-linear behavior in devices like diodes and transistors, a simple rectangular piece of bulk semiconductor material with ohmic contacts behaves like a resistor. According to Ohm’s Law, the current (I) flowing through a resistor is directly proportional to the applied voltage (V) (V = IR or I = V/R), provided the temperature is constant. This relationship is represented by a straight line passing through the origin on an I-V graph. Graph (a) shows this linear relationship, indicating constant resistance.

Graphs (b), (c), and (d) represent non-linear current-voltage characteristics typical of semiconductor devices (like diodes or transistors) or bulk materials under specific conditions (e.g., high electric fields causing velocity saturation). However, for a basic piece of material operating within its ohmic region, the linear relationship is the expected behavior.

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