UPSC NDA-2 Archives

21. Why is potassium permanganate used for purifying drinking water ?

Why is potassium permanganate used for purifying drinking water ?

It kills germs

It dissolves the impurities

It is a reducing agent

It is an oxidizing agent

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The correct answer is It is an oxidizing agent.

Potassium permanganate (KMnO₄) is a strong oxidizing agent. Its oxidizing properties are utilized in water purification to kill bacteria and other microorganisms (germs), and to oxidize impurities like dissolved iron and manganese, causing them to precipitate and be easily removed.

While potassium permanganate does kill germs, this action is a result of its oxidizing property. It does not dissolve impurities; rather, it helps in their precipitation after oxidation. It is distinctly an oxidizing agent, not a reducing agent. Therefore, its role as an oxidizing agent is the fundamental reason for its use in purifying drinking water.

22. Which one among the following chemicals is used as washing soda ?

Which one among the following chemicals is used as washing soda ?

Calcium carbonate

Calcium bicarbonate

Sodium carbonate

Sodium bicarbonate

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The correct answer is Sodium carbonate.

Washing soda is the common name for sodium carbonate (Na₂CO₃). It is typically found as a hydrated salt, sodium carbonate decahydrate (Na₂CO₃·10H₂O).

Sodium bicarbonate (NaHCO₃) is commonly known as baking soda. Calcium carbonate (CaCO₃) is found in substances like limestone, marble, and chalk. Calcium bicarbonate is a temporary form of calcium carbonate that exists in aqueous solution and is not a stable solid chemical compound used as “washing soda”. Washing soda is used as a cleaning agent and water softener.

23. Which one of the following is a chemical change ?

Which one of the following is a chemical change ?

Cutting of hair

Graying of hair naturally

Swelling of resin in water

Cutting of fruit

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A chemical change results in the formation of new substances with different chemical properties.
A) Cutting of hair is a physical change; the hair material itself (keratin) remains chemically unchanged, only its physical shape and size are altered.
B) Graying of hair naturally involves chemical changes in the hair follicle where the production of melanin (the pigment that gives hair its color) decreases or stops. This is a biological process that alters the chemical composition of the hair shaft over time, leading to a change in color. It is an irreversible change at the level of pigment production.
C) Swelling of resin in water is a physical change; the resin absorbs water but its chemical composition does not fundamentally change. This is often a process of hydration or dissolution/dispersion.
D) Cutting of fruit is primarily a physical change; the fruit’s chemical composition remains the same, although the exposed surface might subsequently undergo chemical reactions like oxidation (browning), but the act of cutting is physical.
Therefore, graying of hair is a chemical change.

– Chemical change involves the formation of new substances.
– Physical change alters form or appearance without changing chemical composition.
– Graying of hair involves a change in pigment (melanin) production, a chemical/biological process.

Distinguishing between physical and chemical changes is a fundamental concept in chemistry. Physical changes are often easily reversible (e.g., melting ice), while chemical changes are typically irreversible under normal conditions (e.g., burning wood). Clues for chemical change include color change (not due to mixing), gas production, heat absorption or release, and precipitate formation.

24. Which one of the following gases is placed second in respect of abunda

Which one of the following gases is placed second in respect of abundance in the Earth’s atmosphere ?

Oxygen

Hydrogen

Nitrogen

Carbon dioxide

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The Earth’s atmosphere is primarily composed of nitrogen (N₂) and oxygen (O₂). By volume, dry air contains approximately 78% nitrogen and 21% oxygen. The next most abundant gas is Argon (Ar), which makes up about 0.9%. Carbon dioxide (CO₂) is present in much smaller concentrations, around 0.04%. Hydrogen is present in trace amounts. Therefore, nitrogen is the most abundant gas, and oxygen is the second most abundant gas in the Earth’s atmosphere.

– Nitrogen (N₂) is the most abundant gas (approx. 78%).
– Oxygen (O₂) is the second most abundant gas (approx. 21%).
– Argon is the third most abundant (~0.9%).

The exact composition of the atmosphere can vary slightly with altitude, location, and the presence of moisture (water vapor is a significant component in humid air, but its concentration is highly variable). The balance of these gases is crucial for life on Earth and for regulating climate.

25. Zinc is used to protect iron from corrosion because zinc is

Zinc is used to protect iron from corrosion because zinc is

more electropositive than iron

cheaper than iron

a bluish white metal

a good conductor of heat and electricity

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Zinc is used to protect iron from corrosion through a process called galvanization. Zinc protects iron because it is more electropositive (or more reactive) than iron. In the electrochemical series, zinc is placed above iron, meaning it loses electrons more easily and has a higher tendency to be oxidized. When both metals are present in the presence of an electrolyte (like moisture), zinc acts as the anode and corrodes sacrificially, while iron acts as the cathode and is protected from oxidation (rusting). Even if the zinc coating is scratched and iron is exposed, the zinc in contact with the electrolyte will still corrode preferentially, providing galvanic protection to the iron.

– Zinc is more electropositive/reactive than iron.
– Zinc acts as a sacrificial anode, corroding instead of iron.
– Provides protection even if the coating is scratched (galvanic protection).

Other methods of protecting iron from rusting include painting, greasing, plating with less reactive metals (like tin, although this doesn’t provide sacrificial protection if scratched), and alloying (like stainless steel). Galvanization is a very effective and widely used method due to zinc’s sacrificial nature.

26. How much CO₂ is produced on heating of 1 kg of carbon ?

How much CO₂ is produced on heating of 1 kg of carbon ?

11/3 kg

3/11 kg

4/3 kg

3/4 kg

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The balanced chemical equation for the complete combustion of carbon is:
C (s) + O₂ (g) → CO₂ (g)
The molar mass of Carbon (C) is approximately 12 g/mol.
The molar mass of Carbon Dioxide (CO₂) is approximately 12 g/mol (for C) + 2 * 16 g/mol (for O) = 44 g/mol.
According to the stoichiometry of the balanced equation, 1 mole of carbon reacts to produce 1 mole of carbon dioxide.
Therefore, 12 grams of carbon produce 44 grams of carbon dioxide.
To find the amount of CO₂ produced from 1 kg (1000 g) of carbon, we can use the ratio:
(Mass of CO₂ produced / Mass of C reacted) = (Molar mass of CO₂ / Molar mass of C)
Mass of CO₂ produced = (44 g / 12 g) * 1000 g
Mass of CO₂ produced = (11/3) * 1000 g = 11000/3 g
Converting grams to kilograms: 11000/3 g = (11000/3) / 1000 kg = 11/3 kg.

– Chemical reaction: C + O₂ → CO₂
– Molar mass ratio of CO₂ to C is 44:12, which simplifies to 11:3.
– The mass of CO₂ produced is 11/3 times the mass of carbon reacted.

This calculation assumes complete combustion of pure carbon. In real-world scenarios, combustion might be incomplete (producing CO) or the fuel might contain impurities, affecting the actual CO₂ yield. The calculation uses approximate standard atomic weights.

27. Which one among the following waves carries the maximum energy per pho

Which one among the following waves carries the maximum energy per photon ?

X-rays

Radio waves

Light waves

Microwaves

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The energy carried by a single photon is directly proportional to the frequency of the electromagnetic wave. The relationship is given by the equation E = hf, where E is the energy of the photon, h is Planck’s constant, and f is the frequency of the wave. Comparing the given options based on their typical frequencies in the electromagnetic spectrum (from lowest frequency/energy to highest): Radio waves < Microwaves < Light waves (visible light) < X-rays. Therefore, X-rays have the highest frequency and carry the maximum energy per photon among the listed options.

– Photon energy is directly proportional to wave frequency (E = hf).
– X-rays have higher frequency than radio waves, microwaves, and visible light.
– Higher frequency means higher energy per photon.

The order of the electromagnetic spectrum by increasing frequency and energy is: Radio waves, Microwaves, Infrared radiation, Visible light, Ultraviolet radiation, X-rays, Gamma rays. This spectrum spans a vast range of frequencies and energies, each part having different properties and applications.

28. Step-up transformers are used for

Step-up transformers are used for

increasing electrical power

decreasing electrical power

decreasing voltage

increasing voltage

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A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. Step-up transformers are specifically designed to increase (step up) the voltage from the primary coil to the secondary coil. They have more turns in the secondary coil than in the primary coil. Conversely, a step-down transformer decreases the voltage. Transformers are highly efficient, but they do not increase or decrease the total electrical power (ideally, power input equals power output, although small losses occur in real transformers).

– Step-up transformers increase voltage.
– They work based on electromagnetic induction.
– They are used in AC circuits.

Step-up transformers are commonly used in power transmission systems. Electricity is generated at a relatively low voltage at power plants. Step-up transformers increase the voltage to very high levels for transmission over long distances (reducing current and thus minimizing power loss due to resistance, P = I²R). At the receiving end, step-down transformers are used to reduce the voltage to safer levels for distribution and use in homes and industries.

29. Which one of the following statements about energy is correct ?

Which one of the following statements about energy is correct ?

Energy can be created as well as destroyed.

Energy can be created but not destroyed.

Energy can neither be created nor destroyed.

Energy cannot be created but can be destroyed.

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The statement that energy can neither be created nor destroyed is a fundamental principle known as the Law of Conservation of Energy (also the First Law of Thermodynamics). This law states that the total energy of an isolated system remains constant over time. Energy can be transformed from one form to another (e.g., potential energy to kinetic energy, chemical energy to heat and light), but the total amount of energy in the universe is conserved.

– Law of Conservation of Energy: Energy cannot be created or destroyed.
– Energy can be transformed between different forms.
– The total energy in a closed system remains constant.

In the context of mass-energy equivalence (E=mc²), mass can be converted into energy and vice versa, for example, in nuclear reactions. However, this is still considered a transformation of mass into energy, not the creation or destruction of the fundamental entity of “mass-energy”. In many common physical and chemical processes, mass conservation and energy conservation can be treated separately.

30. Which one of the following statements about a satellite orbiting aroun

Which one of the following statements about a satellite orbiting around the Earth is correct ?

Satellite is kept in orbit by remote control from ground station.

Satellite is kept in orbit by retro-rocket and solar energy keeps it moving around the Earth.

Satellite requires energy from solar panels and solid fuels for orbiting.

Satellite does not require any energy for orbiting.

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Once a satellite is placed into a stable orbit around the Earth, it does not require continuous energy input for orbiting itself. The motion is maintained by the balance between the satellite’s inertia (tendency to keep moving in a straight line) and the gravitational pull of the Earth, which continuously deflects it towards the Earth, causing it to follow a curved path (the orbit). It is essentially in a state of perpetual freefall around the Earth. Energy is required for launching the satellite into orbit and for maneuvers to adjust or maintain the orbit, but not for the fundamental motion of orbiting once the orbit is established.

– Satellites orbit due to the balance between inertia and Earth’s gravity.
– Orbital motion itself is a form of freefall.
– No continuous energy input is needed solely to *stay* in orbit.

While continuous energy is not needed for orbiting, satellites do require energy for various operations, such as powering scientific instruments, communication systems, onboard computers, and making small adjustments to maintain the orbit (orbital maintenance burns using fuel). Solar panels convert sunlight into electrical energy for these purposes and for charging batteries. Retro-rockets are primarily used for de-orbiting or changing orbits, not for sustaining the orbital motion itself.