UPSC NDA-1

11. Which one of the following cannot be the unit of frequency of a sound

Which one of the following cannot be the unit of frequency of a sound wave ?

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dB
s⁻¹
Hz
min⁻¹
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Frequency is defined as the number of occurrences of a repeating event per unit of time. Its standard SI unit is Hertz (Hz), which is equivalent to inverse seconds (s⁻¹). Other units based on time inverse are also valid units for frequency, such as per minute (min⁻¹), per hour, etc.
A) dB (decibel) is a logarithmic unit used to measure the ratio of a physical quantity (usually power or intensity) to a reference value. It is commonly used to quantify sound intensity or loudness, not frequency.
B) s⁻¹ (per second) is the standard unit of frequency, equivalent to Hertz.
C) Hz (Hertz) is the standard SI unit for frequency.
D) min⁻¹ (per minute) represents cycles or events per minute, which is a valid unit for frequency (though not the SI unit).
Therefore, decibel (dB) cannot be a unit of frequency.
Frequency measures how often something happens per unit time (cycles/time). Decibel measures relative intensity or amplitude on a logarithmic scale.
The frequency of a sound wave determines its pitch. Higher frequency means higher pitch. The amplitude or intensity of a sound wave is related to its loudness, often measured in decibels.

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12. Twinkling of stars is due to

Twinkling of stars is due to

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particular frequencies of the starlight.
reflection of starlight from the oceanic surface.
atmospheric refraction of starlight.
magnetic field of Earth.
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The twinkling of stars is caused by the Earth’s atmosphere. As starlight travels through the atmosphere, it passes through layers of air with varying densities and temperatures. This causes the light to be refracted (bent) multiple times. Since these layers are constantly moving and mixing due to atmospheric turbulence, the path of the starlight changes slightly from moment to moment. This results in fluctuations in the apparent brightness and position of the star, which we perceive as twinkling. This phenomenon is called atmospheric refraction.
Stars appear as point sources of light because they are very distant. The light from a point source is more susceptible to the bending effects of atmospheric turbulence compared to the light from extended sources like planets (which appear as discs). Planets do not typically twinkle for this reason.
Refraction is the bending of light as it passes from one medium to another or through a medium with varying properties. The refractive index of air changes with temperature and pressure, which vary throughout the atmosphere.

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13. Which one of the following is not a bio-mass energy source ?

Which one of the following is not a bio-mass energy source ?

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Wood
Nuclear reactor
Gobar gas
Coal
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Biomass energy sources are derived from organic matter, such as plants, animals, and their waste.
A) Wood is organic matter from trees and is a common biomass fuel.
C) Gobar gas (Biogas) is produced by the anaerobic decomposition of organic matter (like cow dung), making it a biomass energy source.
D) Coal is formed from ancient organic matter (plants) under heat and pressure over millions of years. While originated from biomass, it is classified as a fossil fuel and is distinct from renewable biomass sources used today.
B) A nuclear reactor generates energy through nuclear fission, typically using elements like uranium. This process involves the splitting of atomic nuclei and is not related to organic matter or biomass. Therefore, a nuclear reactor is not a biomass energy source.
Biomass energy is considered a renewable energy source (if harvested sustainably), whereas nuclear energy is non-renewable (as the fuel source, like uranium, is finite). Fossil fuels (coal, oil, natural gas) are also non-renewable energy sources derived from ancient biomass.
Biomass can be directly burned for heat/electricity (like wood), or converted into other forms of fuel like biogas (methane) or biofuels (ethanol, biodiesel). Nuclear energy harnesses the energy released from the strong nuclear force within atoms.

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14. Reaction of quick lime (CaO) with water to produce slaked lime (Ca(OH)

Reaction of quick lime (CaO) with water to produce slaked lime (Ca(OH)₂) is an example of

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Displacement reaction.
Endothermic reaction.
Decomposition reaction.
Exothermic reaction.
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The reaction between quick lime (calcium oxide, CaO) and water (H₂O) to form slaked lime (calcium hydroxide, Ca(OH)₂) is represented by the equation: CaO(s) + H₂O(l) → Ca(OH)₂(s). This reaction is well-known for releasing a significant amount of heat, making it a highly exothermic process.
Exothermic reactions are chemical reactions that release energy, typically in the form of heat or light. Endothermic reactions absorb energy. Displacement reactions involve one element displacing another from a compound. Decomposition reactions involve a single compound breaking down into simpler substances. The reaction of quick lime with water fits the definition of an exothermic reaction due to the large heat release, also sometimes classified as a combination reaction.
This reaction is used in the industrial production of slaked lime, which has various applications, including in mortar and plaster, water treatment, and agriculture. The heat released during the reaction is often sufficient to boil the water.

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15. Which one of the following conclusions could not be derived from Rut

Which one of the following conclusions could not be derived from Rutherford’s α-particle scattering experiment ?

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Most of the space in the atom is empty.
The radius of the atom is about 10⁵ times the radius of the nucleus.
Electrons move in a circular path of fixed energy called orbits.
Nearly all the mass of the atom resides in the nucleus.
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Rutherford’s alpha-particle scattering experiment (Gold foil experiment) led to the discovery of the atomic nucleus and the proposal of the nuclear model of the atom. The key conclusions were:
A) Most alpha particles passed straight through, indicating that most of the atom is empty space.
D) A small fraction of particles were deflected at large angles or bounced back, indicating the presence of a dense, positively charged nucleus containing most of the mass in a tiny volume.
B) Based on the trajectories, Rutherford estimated the size of the nucleus relative to the atom.
However, Rutherford’s model did not explain the stability of the atom (electrons orbiting a nucleus should lose energy and spiral into the nucleus) nor the observed atomic spectra. The idea that electrons move in specific circular paths of fixed energy (orbits) was a postulate introduced later by Niels Bohr to address these issues, building upon Rutherford’s model. Therefore, this conclusion was not derived from Rutherford’s experiment.
Rutherford’s model established the concept of a nucleus but did not explain electron behaviour in detail. Bohr’s model later quantized electron energy levels and introduced the concept of stable orbits.
The planetary model is often associated with Rutherford, but the idea of quantized orbits is specifically from Bohr. The scattering experiment involved firing alpha particles (He²⁺ ions) at a thin gold foil and observing their deflection patterns.

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16. Which one of the following acids is predominantly found in tomatoes ?

Which one of the following acids is predominantly found in tomatoes ?

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Acetic acid
Tartaric acid
Oxalic acid
Lactic acid
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Tomatoes contain several organic acids, including citric acid, malic acid, and oxalic acid. While citric acid is often the most abundant overall acid, tomatoes are a particularly significant source of oxalic acid compared to many other common fruits and vegetables. Among the options provided, oxalic acid is the predominant one found in notable quantities in tomatoes.
Oxalic acid is a dicarboxylic acid (COOH-COOH) found in many plants. Acetic acid is the primary component of vinegar. Tartaric acid is found in grapes and tamarind. Lactic acid is produced during fermentation (e.g., in yogurt) and in muscles during strenuous exercise.
The presence of oxalic acid contributes to the tangy taste of tomatoes. While important for understanding nutritional content and potential interactions (like with calcium), it’s worth noting that the relative abundance of acids can vary slightly depending on the tomato variety and ripeness.

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17. Which one of the following reactions does not result in the evolutio

Which one of the following reactions does not result in the evolution of hydrogen gas ?

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Reaction of zinc metal with dilute sulphuric acid solution
Mixing water to Plaster of Paris
Heating zinc metal with sodium hydroxide solution
Reaction of potassium metal with water
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Mixing water with Plaster of Paris (Calcium sulfate hemihydrate, CaSO₄·½H₂O) results in a hydration reaction where water is absorbed to form gypsum (Calcium sulfate dihydrate, CaSO₄·2H₂O). This reaction is exothermic and causes the mixture to set and harden, but it does not produce any gas, including hydrogen.
The reactions in options A, C, and D all involve the liberation of hydrogen gas:
A) Active metals like zinc react with dilute acids (Zn + H₂SO₄ → ZnSO₄ + H₂).
C) Amphoteric metals like zinc react with strong bases upon heating (Zn + 2NaOH → Na₂ZnO₂ + H₂).
D) Highly reactive metals like potassium react vigorously with water (2K + 2H₂O → 2KOH + H₂).
Plaster of Paris is commonly used in construction, casting, and medical applications (like casts for broken bones) due to its ability to quickly set after mixing with water. The setting process involves the crystallization of gypsum.

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18. Which one of the following materials is not an allotrope of carbon ?

Which one of the following materials is not an allotrope of carbon ?

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Diamond
Graphene
Fly ash
Fullerene
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Fly ash is a residue produced from the combustion of pulverized coal in thermal power plants. It is primarily composed of fine particles of various oxides and minerals, such as silicon dioxide (SiO₂), aluminium oxide (Al₂O₃), iron oxide (Fe₂O₃), and calcium oxide (CaO), along with some unburnt carbon. It is not a pure form of carbon existing in a specific structural modification like diamond, graphene, or fullerene.
Allotropes are different forms of the same element in the same physical state. Carbon exists in many allotropic forms due to its ability to bond in various ways. Common allotropes include diamond (tetrahedral sp³ bonding), graphite (layered sp² bonding), graphene (single layer of graphite), fullerenes (spherical or elliptical sp² bonding, like C₆₀), carbon nanotubes, and lonsdaleite.
Fly ash is often used as a pozzolanic material in concrete mixtures, improving its properties. While it contains some unburnt carbon, its composition is complex and includes many non-carbon components, classifying it as an industrial byproduct rather than an allotrope of carbon.

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19. Hydrogenation of vegetable oils using nickel catalyst is an example of

Hydrogenation of vegetable oils using nickel catalyst is an example of

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Substitution reaction.
Elimination reaction.
Addition reaction.
Free-radical polymerization.
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Hydrogenation is a chemical reaction that involves the addition of hydrogen gas (H₂) to a compound. Vegetable oils are unsaturated fats containing carbon-carbon double bonds (C=C). In the presence of a catalyst like nickel, hydrogen atoms add across these double bonds, converting them into single bonds (C-C) and making the oil saturated or partially saturated (like in the production of vanaspati ghee). This type of reaction, where atoms are added across a double or triple bond, is an addition reaction.
Addition reactions are characteristic of unsaturated compounds (alkenes, alkynes, etc.). Hydrogenation specifically involves the addition of hydrogen. Catalysts (like Ni, Pt, or Pd) are typically used to speed up the reaction rate.
Substitution reactions involve the replacement of one atom or group by another. Elimination reactions involve the removal of atoms or groups from a molecule to form a double or triple bond. Free-radical polymerization is a process where monomers join together to form a polymer chain via a free-radical mechanism.

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20. Which one of the following statements about the cleansing action of so

Which one of the following statements about the cleansing action of soap is not true ?

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The oil and dirt gets collected in the centre of the micelle.
Soap micelles scatter light.
Soaps are ammonium salts of long chain carboxylic acids.
Soap forms insoluble precipitates with the calcium and magnesium ions in hard water.
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Statement C is not true. Soaps are typically sodium or potassium salts of long chain carboxylic acids. Ammonium salts of long chain carboxylic acids are commonly used as detergents, which have similar cleansing properties but differ in their chemical composition and behaviour in hard water.
Soaps function through the formation of micelles, aggregates of soap molecules in water, where the hydrophobic tails face inwards, dissolving oils and dirt, and the hydrophilic heads face outwards, interacting with water. Micelles are large enough to scatter light, a phenomenon known as the Tyndall effect. Soaps react with calcium and magnesium ions present in hard water to form insoluble precipitates called scum, which reduces their effectiveness.
Detergents, unlike soaps, do not form insoluble precipitates with calcium and magnesium ions in hard water, making them more effective for washing in hard water. The general formula for a soap molecule is RCOO⁻Na⁺ or RCOO⁻K⁺, where R is a long alkyl chain.

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