UPSC NDA-1

41. Which one of the following is the correct molecular formula of ammoniu

Which one of the following is the correct molecular formula of ammonium carbonate if the valency of ammonium ion is (+1) and carbonate anion is (-2) ?

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(NH₄)₂CO₃
NH₄(CO₃)₂
(NH₃)₂CO₃
NH₄CO₃
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To form a neutral compound, the total positive charge from the cations must balance the total negative charge from the anions. Given the valencies, the correct ratio of ammonium ions to carbonate ions needs to be determined.
The valency (or charge) of the ammonium ion (NH₄⁺) is +1. The valency (or charge) of the carbonate anion (CO₃²⁻) is -2. To form a neutral compound, we need a total positive charge equal in magnitude to the total negative charge. If we take two ammonium ions, the total positive charge is 2 * (+1) = +2. If we take one carbonate ion, the total negative charge is 1 * (-2) = -2. The charges +2 and -2 balance to give a neutral compound. Therefore, the formula is (NH₄)₂CO₃, indicating two ammonium ions for every one carbonate ion.
When writing formulas for ionic compounds, the cation is written first, followed by the anion. Subscripts indicate the number of ions required to achieve charge neutrality. If the ion is polyatomic (like NH₄⁺ or CO₃²⁻) and more than one is needed, the ion formula is enclosed in parentheses before the subscript.

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

Which one of the following is a chemical change?

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Dissolving sugar in water
Melting of ice
Crystallization
Milk turning sour
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A chemical change involves the formation of new substances with different chemical properties. Milk turning sour is a process where lactose is converted into lactic acid by bacteria, resulting in a change in chemical composition.
Physical changes alter the form or appearance of a substance but not its chemical identity (e.g., dissolving, melting, freezing, boiling, crystallization). Chemical changes result in the formation of one or more new substances with different chemical properties (e.g., burning, rusting, cooking, souring of milk).
Dissolving sugar in water, melting of ice, and crystallization are all physical changes. In milk turning sour, bacteria perform fermentation, converting lactose (a sugar) into lactic acid, which changes the taste and properties of the milk. This is a clear example of a chemical reaction producing new substances.

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43. Refining of petroleum is carried out using which one of the following

Refining of petroleum is carried out using which one of the following techniques?

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Evaporation
Fractional distillation
Separating funnel
Sublimation
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Refining of petroleum (crude oil) is done by separating its various components based on their different boiling points. This process is called fractional distillation.
Crude oil is a complex mixture of hydrocarbons with varying chain lengths and boiling points. Fractional distillation involves heating the crude oil and separating the vapours into different fractions at various levels of a fractionating column based on their condensation temperatures. Lighter fractions with lower boiling points rise higher in the column, while heavier fractions with higher boiling points condense lower down.
Evaporation is used to separate a solvent from a non-volatile solute. A separating funnel is used to separate immiscible liquids. Sublimation is used to separate a substance that changes directly from solid to gas from substances that do not sublime. Fractional distillation is the standard industrial method for separating crude oil into useful products like gasoline, diesel, kerosene, etc.

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44. Which one of the following is not a solution?

Which one of the following is not a solution?

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Alloy
Milk
Air
Sugar
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A solution is a homogeneous mixture of two or more substances. Among the given options, milk is a heterogeneous mixture (specifically a colloid).
An alloy is a homogeneous mixture of metals or a metal and a non-metal (e.g., brass, steel), so it is a solution (solid solution). Air is a homogeneous mixture of gases (primarily nitrogen and oxygen), so it is a solution (gaseous solution). Milk contains dispersed particles of fat and protein which are not uniformly distributed at the microscopic level, making it a heterogeneous mixture (colloid). Sugar itself is a pure substance (compound), but if dissolved in water, it forms a homogeneous mixture (sugar solution). However, given the options as listed, Milk is the only one that is definitively not a solution.
Mixtures can be homogeneous (solutions) or heterogeneous (suspensions and colloids). Solutions have uniform composition throughout. Colloids have particles dispersed uniformly, but these particles are larger than molecules and cause light scattering (Tyndall effect); they are considered heterogeneous. Suspensions have even larger particles that settle out over time.

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45. The electric field lines from an isolated positively charged conductin

The electric field lines from an isolated positively charged conducting sphere are

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tangential to the conducting surface
at right angles to the conducting surface and towards the centre of the sphere
at any angle to the conducting surface
at right angles to the conducting surface and outwards from the centre of the sphere
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For a conductor in electrostatic equilibrium, the electric field lines are always perpendicular to the surface, and for a positively charged object, they point outwards.
In electrostatic equilibrium, the electric field inside a conductor is zero, and any net charge resides on the surface. If the electric field had a component parallel to the surface, charges would move along the surface, and the conductor would not be in equilibrium. Therefore, the electric field lines must be perpendicular (at right angles) to the surface of the conductor. For a positively charged object, electric field lines originate from the positive charges and point away from them. For a sphere, this direction is radially outwards from the centre.
For a negatively charged conducting sphere, the electric field lines would also be at right angles to the surface but would point inwards towards the centre of the sphere, terminating on the negative charges on the surface.

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46. An electric wire of resistance 50 ohm is cut into five equal wires. Th

An electric wire of resistance 50 ohm is cut into five equal wires. These wires are then connected in parallel. What is the equivalent resistance of this combination?

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2 ohm
10 ohm
0.5 ohm
5 ohm
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When a wire of resistance 50 ohm is cut into five equal parts, each part will have one-fifth of the original resistance. Connecting these five equal resistances in parallel results in a combined resistance calculated using the parallel resistance formula.
The resistance of a uniform wire is directly proportional to its length. So, if the wire is cut into five equal parts, the resistance of each part is R’ = R/5 = 50 ohm / 5 = 10 ohm. When n equal resistors (each of resistance r) are connected in parallel, the equivalent resistance (Req) is given by Req = r/n. In this case, r = 10 ohm and n = 5. Thus, Req = 10 ohm / 5 = 2 ohm.
Alternatively, the formula for resistors in parallel can be used: 1/Req = 1/R1 + 1/R2 + … + 1/Rn. For five equal resistances of 10 ohm in parallel: 1/Req = 1/10 + 1/10 + 1/10 + 1/10 + 1/10 = 5/10 = 1/2. Therefore, Req = 2 ohm. Connecting resistors in parallel decreases the overall resistance.

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47. Which of the following statements correctly explains/explain the exist

Which of the following statements correctly explains/explain the existence of a positive force between two electric charges?

  • Both the charges are positive.
  • Both the charges are negative.
  • Both the charges are oppositely charged.

Select the correct answer using the code given below.

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1 only
2 only
1 and 2 only
1, 2 and 3
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A positive force between two electric charges indicates a repulsive force. Repulsive forces exist between like charges, meaning both charges are either positive or both are negative.
According to Coulomb’s Law, the force between two point charges is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them. The force is repulsive if the charges are of the same sign (both positive or both negative) and attractive if they are of opposite signs (one positive and one negative). A positive force value in physics convention often represents repulsion, while a negative force value represents attraction.
Statement 1 describes the case of two positive charges, which repel each other. Statement 2 describes the case of two negative charges, which also repel each other. Statement 3 describes the case of oppositely charged particles (e.g., positive and negative), which attract each other, resulting in an attractive (negative) force. Therefore, only statements 1 and 2 correctly explain the existence of a positive (repulsive) force.

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48. Evaporation from the surface of a given liquid takes place more rapidl

Evaporation from the surface of a given liquid takes place more rapidly when

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the temperature is high and the surface area of the liquid is large
the temperature is low and the surface area of the liquid is large
the temperature is low and the surface area of the liquid is small
the temperature is high and the surface area of the liquid is small
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Evaporation is a surface phenomenon where liquid turns into gas. The rate of evaporation is influenced by several factors, including temperature and surface area.
Higher temperature increases the kinetic energy of liquid molecules, making it easier for them to escape the liquid surface and become vapour. A larger surface area exposes more liquid molecules to the air, thus increasing the rate at which they can evaporate. Therefore, a combination of high temperature and large surface area leads to more rapid evaporation.
Other factors that increase the rate of evaporation include increased wind speed (removes saturated air above the surface) and decreased humidity (lower concentration of water vapour in the surrounding air allows more liquid to evaporate). Conversely, factors like low temperature, small surface area, still air, and high humidity decrease the rate of evaporation.

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49. The specific latent heat of vaporization of a substance is the quantit

The specific latent heat of vaporization of a substance is the quantity of heat needed to change unit mass from

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liquid to vapour with a change of temperature
liquid to vapour without a change of temperature
vapour to liquid without a change of temperature
vapour to liquid with a change of temperature
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The specific latent heat of vaporization is defined as the heat energy required to change the state of a unit mass of a substance from liquid to vapour at a constant temperature.
Latent heat is the heat energy absorbed or released during a phase transition (like melting, freezing, vaporization, condensation) at a constant temperature and pressure. Specific latent heat refers to the amount of heat per unit mass. Vaporization specifically refers to the transition from liquid to gas. This phase change occurs without a change in temperature, as the energy is used to break intermolecular bonds.
There are two main types of specific latent heat: specific latent heat of fusion (solid to liquid or liquid to solid) and specific latent heat of vaporization (liquid to gas or gas to liquid). Each substance has characteristic values for these latent heats.

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50. What is the mass of a material, whose specific heat capacity is 400 J/

What is the mass of a material, whose specific heat capacity is 400 J/(kg °C) for a rise in temperature from 15 °C to 25 °C, when heat received is 20 kJ?

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0.1 kg
1 kg
10 kg
5 kg
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The mass of the material can be calculated using the formula relating heat absorbed, specific heat capacity, mass, and temperature change.
The formula used is Q = mcΔT, where Q is the heat energy absorbed (20 kJ = 20,000 J), m is the mass, c is the specific heat capacity (400 J/(kg °C)), and ΔT is the change in temperature (25 °C – 15 °C = 10 °C). Rearranging the formula to find mass: m = Q / (cΔT). Plugging in the values: m = 20000 J / (400 J/(kg °C) * 10 °C) = 20000 / 4000 kg = 5 kg.
Specific heat capacity is a material property that quantifies the amount of heat required to raise the temperature of one unit mass of the substance by one degree Celsius (or Kelvin). The unit J/(kg °C) or J/(kg K) is commonly used. This calculation assumes no phase change occurs within the given temperature range.

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