UPSC NDA-1 Archives

11. Which one of the following statements is not correct?

Which one of the following statements is not correct?

Most carbon compounds are good conductors of electricity.

Bonding in organic compounds is covalent.

Graphite is used as a lubricant.

Diamond is an allotrope of carbon.

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The correct answer is A) Most carbon compounds are good conductors of electricity. This statement is incorrect; most organic (carbon) compounds are covalent and do not conduct electricity well in their solid or liquid state.

– Conductivity of electricity requires the presence of free-moving charged particles (electrons or ions).
– Most carbon compounds (organic compounds) are formed by covalent bonds between nonmetal atoms (C, H, O, N, S, halogens, etc.). Covalent compounds typically do not form free ions or mobile electrons, so they are poor conductors or insulators.
– Statement B is correct: The primary bonding in organic compounds is covalent.
– Statement C is correct: Graphite, an allotrope of carbon, has a layered structure with delocalized electrons within the layers, which makes it a conductor and a good solid lubricant. However, graphite is an *exception* among carbon compounds regarding conductivity.
– Statement D is correct: Diamond is another well-known crystalline allotrope of carbon.
– The statement claims *most* carbon compounds are good conductors, which is false. The vast majority of organic molecules (e.g., sugars, plastics, oils, alcohols, proteins) are electrical insulators.

Exceptions to the non-conductivity of carbon compounds include graphite, some conjugated polymers, and ionic carbides (though ionic carbides are not typically considered “organic” compounds in the traditional sense). However, the general rule for the vast class of organic compounds is that they are poor conductors of electricity.

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12. Which one of the following statements about fertilizers is not

Which one of the following statements about fertilizers is not correct?

Urea is a phosphorus-containing fertilizer.

Application of fertilizer to the soil increases fertility of the soil.

Urea can be prepared by action of ammonia and carbon dioxide under high pressure and at high temperature.

Urea contains more nitrogen than other fertilizers.

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The correct answer is A) Urea is a phosphorus-containing fertilizer. This statement is incorrect because urea ((NH₂)₂CO) is a nitrogen-containing fertilizer.

– Fertilizers are substances that provide nutrients to plants, primarily nitrogen (N), phosphorus (P), and potassium (K), which are macronutrients.
– Urea has the chemical formula (NH₂)₂CO. It is a rich source of nitrogen, with approximately 46% nitrogen by weight.
– Phosphorus-containing fertilizers typically contain compounds like phosphates (e.g., calcium phosphate in superphosphate).
– Statement B is correct: Fertilizers add essential nutrients to the soil, thus increasing its fertility and supporting plant growth.
– Statement C is correct: Urea is synthesized industrially from ammonia (NH₃) and carbon dioxide (CO₂) under high pressure and temperature (Bosch–Meiser process).
– Statement D is generally correct in the context of solid nitrogen fertilizers; urea has one of the highest nitrogen contents among commonly used solid fertilizers.

Fertilizers can be organic (derived from plant/animal matter) or inorganic (synthetically produced or mined). Urea is an organic compound but is typically synthesized industrially and used as an inorganic fertilizer in agriculture. Different plants require different ratios of N, P, and K, and various fertilizers are available to meet these needs.

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13. Which one of the following is not a pigment?

Which one of the following is not a pigment?

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Zinc oxide

Chalk

White lead

Silica

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The correct answer is Silica. While finely divided silica can appear white and contribute to opacity, it is not primarily classified or used as a pigment in the same way as Zinc oxide, Chalk, and White lead, which are well-established white pigments used for imparting colour (whiteness) and opacity.

– A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. Pigments are typically solid and ground into fine powder.
– Zinc oxide (ZnO) is a widely used white pigment known as Zinc White.
– Chalk is primarily Calcium carbonate (CaCO₃), which is used as a white pigment and filler.
– White lead (basic lead carbonate, (PbCO₃)₂·Pb(OH)₂) is a dense white pigment historically used in paints, although its use has declined due to toxicity.
– Silica (Silicon dioxide, SiO₂) occurs in many forms. While very fine forms like fumed silica can act as opacifiers and contribute to whiteness and are used as fillers in paints, its primary applications often lie outside of being a dedicated coloring pigment, such as abrasives, drying agents, fillers for controlling rheology or structure, and as a component of glass or ceramics. Compared to the others, its role as a pigment is less central.

The term “pigment” usually refers to a substance added to a material to change its color. While silica is white when powdered, its applications are broader than just coloration, and other options are classic examples of white pigments specifically used for their tinting strength and opacity.

14. Which one of the following is the colour of hydrogen gas?

Which one of the following is the colour of hydrogen gas?

Light yellow

Orange

Black

Colourless

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The correct answer is Colourless. Hydrogen gas (H₂) is a colourless gas.

– Hydrogen is the lightest and most abundant element in the universe.
– At standard temperature and pressure, hydrogen exists as a diatomic molecule (H₂) and is a gas.
– Pure hydrogen gas is colourless, odourless, and tasteless.
– It is also highly flammable.

Many common gases like nitrogen (N₂), oxygen (O₂), noble gases (He, Ne, Ar, etc.), and carbon dioxide (CO₂) are also colourless. Some gases do have colour, such as chlorine (greenish-yellow), bromine vapour (reddish-brown), iodine vapour (violet), and nitrogen dioxide (reddish-brown).

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15. Which one of the following allotropes of carbon is isomorphous with cr

Which one of the following allotropes of carbon is isomorphous with crystalline silicon?

Coke

Diamond

Graphite

Coal

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The correct answer is Diamond. Diamond is a crystalline allotrope of carbon that is isomorphous with crystalline silicon because they share the same crystal structure.

– Isomorphism refers to the phenomenon where two or more substances crystallize in the same crystal structure.
– Crystalline silicon has a diamond cubic crystal structure, where each silicon atom is covalently bonded to four other silicon atoms in a tetrahedral arrangement.
– Diamond, which is also a crystalline allotrope of carbon, also has a diamond cubic structure, with carbon atoms replacing silicon atoms in the same arrangement.
– Coke, Graphite, and Coal are other forms of carbon, but they do not share the same crystal structure as crystalline silicon. Graphite has a layered hexagonal structure, while Coke and Coal are largely amorphous or contain various carbonaceous materials without a single, defined crystal structure.

The similarity in structure between diamond and silicon is due to both carbon and silicon belonging to Group 14 of the periodic table and forming strong covalent bonds in a tetrahedral geometry. This structural similarity has implications for their properties, although the bond strength (C-C vs Si-Si) leads to significant differences in hardness and melting point.

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16. Which one of the following is not an example of a redox reaction?

Which one of the following is not an example of a redox reaction?

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AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl

2NaH → 2Na + H₂

4Fe + 3O₂ → 2Fe₂O₃

CuSO₄ + Zn → Cu + ZnSO₄

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The correct answer is A) AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl. This reaction is a double displacement (or hydrolysis) reaction, where no changes in oxidation states occur, unlike redox reactions.

– A redox reaction is a chemical reaction that involves the transfer of electrons, resulting in changes in the oxidation states of atoms.
– Oxidation is an increase in oxidation state (loss of electrons).
– Reduction is a decrease in oxidation state (gain of electrons).
– Let’s analyze the oxidation states in each reaction:
– A) AlCl₃ (+3, -1) + H₂O (+1, -2) → Al(OH)₃ (+3, -2, +1) + HCl (+1, -1). Oxidation states of all elements remain unchanged.
– B) 2NaH (+1, -1) → 2Na (0) + H₂ (0). Na changes from +1 to 0 (reduction), H changes from -1 to 0 (oxidation). Redox.
– C) 4Fe (0) + 3O₂ (0) → 2Fe₂O₃ (+3, -2). Fe changes from 0 to +3 (oxidation), O changes from 0 to -2 (reduction). Redox.
– D) CuSO₄ (+2, -2) + Zn (0) → Cu (0) + ZnSO₄ (+2, -2). Cu changes from +2 to 0 (reduction), Zn changes from 0 to +2 (oxidation). Redox.

Double displacement reactions, such as precipitation, neutralization, and some hydrolysis reactions, are typically non-redox reactions as they involve the exchange of ions without changes in oxidation states.

17. For an element with atomic number 35, which one of the following will

For an element with atomic number 35, which one of the following will be the correct number of electrons in its valence shell based on Bohr’s model of an atom?

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The correct answer is 7. An element with atomic number 35 is Bromine (Br), which is a halogen in Group 17 of the periodic table and typically has 7 valence electrons.

– Bohr’s model describes electrons orbiting the nucleus in specific energy levels or shells (K, L, M, N, etc.).
– The maximum number of electrons a shell can hold is generally given by 2n², where n is the shell number (n=1 for K, n=2 for L, etc.).
– For atomic number 35 (Bromine), we fill the shells sequentially:
– K shell (n=1): 2 electrons (capacity 2)
– L shell (n=2): 8 electrons (capacity 8)
– M shell (n=3): 18 electrons (capacity 18)
– N shell (n=4): The remaining electrons: 35 – (2 + 8 + 18) = 35 – 28 = 7 electrons.
– The outermost shell is the valence shell, which is the N shell in this case.
– The number of electrons in the valence shell is 7.

While the simple Bohr model doesn’t fully explain the filling order of electrons in multi-electron atoms due to subshells, the concept of valence electrons in the outermost shell is fundamental. Elements in the same group of the periodic table generally have the same number of valence electrons (e.g., Halogens in Group 17 all have 7 valence electrons), which determines their chemical properties.

18. A sound wave has a frequency of 1 kHz and wavelength 50 cm. How long w

A sound wave has a frequency of 1 kHz and wavelength 50 cm. How long will it take to travel 1 km?

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5 s

4 s

3 s

2 s

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The correct answer is 2 seconds. The time taken to travel a certain distance can be calculated if the velocity of the wave is known.

– The relationship between the velocity (v), frequency (f), and wavelength (λ) of a wave is given by the formula v = fλ.
– Given frequency f = 1 kHz = 1000 Hz.
– Given wavelength λ = 50 cm = 0.50 m.
– Velocity v = (1000 Hz) × (0.50 m) = 500 m/s.
– Distance to be travelled = 1 km = 1000 m.
– Time taken (t) = Distance / Velocity.
– t = 1000 m / 500 m/s = 2 seconds.

Frequency is the number of wave cycles passing a point per second, measured in Hertz (Hz). Wavelength is the distance between two consecutive corresponding points on a wave. Velocity is the speed at which the wave propagates through the medium.

19. The energy possessed by a body due to its change in position or shape

The energy possessed by a body due to its change in position or shape is called

thermal energy

potential energy

kinetic energy

electric energy

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The correct answer is potential energy. Potential energy is the energy stored by an object due to its position or its shape.

– Potential energy is a form of stored energy.
– Gravitational potential energy depends on an object’s height above a reference point.
– Elastic potential energy is stored in a material when it is deformed (stretched or compressed), like a spring or a stretched rubber band.
– The definition provided in the question (“energy possessed by a body due to its change in position or shape”) precisely describes potential energy.

– Thermal energy is related to the internal energy of a system due to its temperature.
– Kinetic energy is the energy of motion.
– Electric energy is energy associated with the flow of electric charge or the presence of electric fields.

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20. A boy of mass 52 kg jumps with a horizontal velocity of 2 m/s onto a s

A boy of mass 52 kg jumps with a horizontal velocity of 2 m/s onto a stationary cart of mass 3 kg. The cart is fixed with frictionless wheels. Which one of the following would be the speed of the cart?

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2.15 m/s

1.89 m/s

1.51 m/s

2.51 m/s

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The correct answer is 1.89 m/s. This is calculated using the principle of conservation of linear momentum for the inelastic collision between the boy and the cart.

– The system consists of the boy and the stationary cart.
– Before the jump, the boy has momentum (mass × velocity) and the cart has zero momentum.
– After the boy jumps onto the cart, they move together as a single system with a common velocity.
– According to the law of conservation of momentum, the total momentum of the system before the collision is equal to the total momentum after the collision, provided no external forces act on the system (frictionless wheels are mentioned).
– Initial momentum = (mass of boy × velocity of boy) + (mass of cart × velocity of cart) = (52 kg × 2 m/s) + (3 kg × 0 m/s) = 104 kg m/s.
– Let the final velocity of the combined system (boy + cart) be ‘v’. The combined mass is 52 kg + 3 kg = 55 kg.
– Final momentum = (combined mass × final velocity) = 55 kg × v.
– By conservation of momentum: 104 kg m/s = 55 kg × v.
– v = 104 / 55 ≈ 1.89 m/s.

This is an example of a perfectly inelastic collision because the two objects stick together after colliding. Kinetic energy is not conserved in an inelastic collision, but momentum is always conserved in the absence of external forces.