UPSC CDS-1

31. Which one of the following statements is correct regarding the definit

Which one of the following statements is correct regarding the definition of a literate person for Census 2011?

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a person aged seven and above who can both read and write with understanding in any language is treated as a literate
a person aged eight and above who can both read and write with understanding in any language is treated as a literate
a person aged nine and above who can both read and write with understanding in any language is treated as a literate
a person aged ten and above who can both read and write with understanding in any language is treated as a literate
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For Census 2011, a person aged seven and above who can both read and write with understanding in any language is treated as a literate.
The definition of literacy adopted for the Census of India 2011 includes two main criteria: age (seven years and above) and the ability to read and write with understanding in any language. The ability to only read but not write does not qualify a person as literate.
This definition has been consistent across recent Indian censuses (since 1991). Children below the age of seven are treated as illiterates, even if they attend school and can read and write.

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32. What is the number of atoms in 46 g of sodium-23 (N = Avogadro

What is the number of atoms in 46 g of sodium-23 (N = Avogadro constant)?

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N/2
N
2N
23N
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The number of atoms in 46 g of sodium-23 is 2N.
The atomic mass of sodium-23 is approximately 23 g/mol. One mole of any substance contains Avogadro’s number (N) of particles (atoms, molecules, etc.). Therefore, 46 g of sodium-23 constitutes (46 g / 23 g/mol) = 2 moles of sodium atoms. The number of atoms is then 2 moles * N atoms/mole = 2N atoms.
Avogadro’s constant (N) is approximately 6.022 x 10^23 mol^-1. This constant represents the number of constituent particles, usually atoms or molecules, that are contained in one mole of a substance.

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33. Who proposed the ‘Plum Pudding Model’ for an atom?

Who proposed the ‘Plum Pudding Model’ for an atom?

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Antoine Lavoisier
Robert Boyle
Ernest Rutherford
J.J. Thomson
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J.J. Thomson proposed the ‘Plum Pudding Model’ for an atom.
The Plum Pudding Model suggested that the atom was a sphere of positive charge with negatively charged electrons embedded within it, much like plums in a pudding.
Ernest Rutherford’s gold foil experiment later disproved the Plum Pudding Model, leading to the development of the nuclear model of the atom. Antoine Lavoisier is known as the ‘father of modern chemistry’ for his work on the law of conservation of mass. Robert Boyle is known for Boyle’s Law relating to gases.

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34. Calcium (atomic number 20) and argon (atomic number 18) have a mass nu

Calcium (atomic number 20) and argon (atomic number 18) have a mass number of 40. What are they known as?

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Isotones
Isochores
Isobars
Isotopes
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The correct answer is C) Isobars.
Atoms of different elements that have the same mass number (sum of protons and neutrons) but different atomic numbers (number of protons) are called isobars. Calcium has atomic number 20 and mass number 40. Argon has atomic number 18 and mass number 40. Since they have different atomic numbers (20 vs 18) but the same mass number (40), they are isobars.
– Isotopes are atoms of the same element (same atomic number) with different mass numbers (different number of neutrons). Example: Carbon-12 and Carbon-14.
– Isotones are atoms of different elements (different atomic numbers) with the same number of neutrons. For Calcium-40, neutrons = 40-20 = 20. For Argon-40, neutrons = 40-18 = 22. They are not isotones.
– Isochores relate to constant volume processes in thermodynamics, not atomic structure.

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35. Which one of the following is the most characteristic property of an

Which one of the following is the most characteristic property of an element?

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Density
Boiling point
Mass number
Atomic number
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The correct answer is D) Atomic number.
The atomic number (Z) is the number of protons in the nucleus of an atom. This number is unique to each element and determines its chemical properties. For example, any atom with 6 protons is Carbon, and any atom with 8 protons is Oxygen. No two elements have the same atomic number.
Mass number (A) is the total number of protons and neutrons in the nucleus. Isotopes of an element have the same atomic number but different mass numbers (due to different numbers of neutrons). Density and boiling point are physical properties that can vary depending on the specific isotope, physical state, temperature, pressure, and purity of the substance. While characteristic, they are not the fundamental defining property of an element itself.

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36. The pH value of a sample of multiple-distilled water is

The pH value of a sample of multiple-distilled water is

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zero
14
very near to zero
very near to seven
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The correct answer is D) very near to seven.
pH is a measure of the acidity or alkalinity of a solution. Pure water is neutral because the concentration of hydrogen ions (H⁺) equals the concentration of hydroxide ions (OH⁻) produced by the auto-ionization of water (H₂O ⇌ H⁺ + OH⁻). At standard temperature (25°C), in pure water, [H⁺] = 10⁻⁷ mol/L. The pH is defined as -log₁₀[H⁺]. Thus, the pH of pure water is -log₁₀(10⁻⁷) = 7. Multiple-distilled water is highly purified water with most impurities removed. While it’s difficult to achieve a perfect pH of 7 in practice (e.g., due to dissolved CO₂ from the air forming carbonic acid, making it slightly acidic, or dissolved substances from the container), the theoretical and practical pH of pure, neutral water is very close to 7.
A pH of zero indicates a highly acidic solution (e.g., 1M HCl). A pH of 14 indicates a highly alkaline solution (e.g., 1M NaOH). Values very near zero would indicate strong acidity, which is not characteristic of pure water. Values very near seven indicate neutrality, which is the property of pure water.

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37. Which one of the following is a physical change?

Which one of the following is a physical change?

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Burning of coal
Burning of wood
Heating of a platinum crucible
Heating of potassium chlorate
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The correct answer is C) Heating of a platinum crucible.
A physical change is a change in the form of a substance, but not in its chemical identity. Chemical changes involve a chemical reaction where new substances are formed.
– Burning of coal and wood are combustion reactions where the substance reacts with oxygen to form ash, carbon dioxide, etc. These are chemical changes.
– Heating of potassium chlorate (KClO₃) typically refers to its thermal decomposition, which produces potassium chloride (KCl) and oxygen (O₂). This is a chemical change.
– Heating a platinum crucible: Platinum is a very noble (unreactive) metal. Heating it, even to high temperatures in air, usually only changes its temperature or potentially causes thermal expansion or changes in its physical state (if heated to melting point). It does not readily undergo a chemical reaction or decomposition under normal heating conditions in a lab. Therefore, heating a platinum crucible is primarily a physical change.
Physical changes are often reversible (e.g., melting/freezing water). Chemical changes are usually irreversible without further chemical reactions. Recognizing chemical changes often involves looking for signs like color change, gas evolution, precipitate formation, or significant energy change (heat/light).

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38. For which one of the following does the centre of mass lie outside the

For which one of the following does the centre of mass lie outside the body?

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A fountain pen
A cricket ball
A ring
A book
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The correct answer is C) A ring.
The center of mass of an object is a point where the weighted average of the positions of all parts of the object is located. While for uniform solid objects with simple geometry (like a sphere or cube), the center of mass is often at the geometric center and inside the object, for objects with holes or non-uniform density, the center of mass can lie outside the physical boundaries of the object. A ring is a hollow circular structure. Its mass is distributed along the circumference, but the geometric center (and thus the center of mass for a uniform ring) is in the empty space at the center of the circle.
For a fountain pen, cricket ball, and book, the mass is distributed throughout their volume, and their center of mass will typically be located somewhere within the material of the object itself. Examples of other objects where the center of mass is outside the physical body include a hollow sphere or a boomerang.

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39. A parallel-plate capacitor, with air in between the plates, has capaci

A parallel-plate capacitor, with air in between the plates, has capacitance C. Now the space between the two plates of the capacitor is filled with a dielectric of dielectric constant 7. Then the value of the capacitance will become

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C
C/7
7C
14C
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The correct answer is C) 7C.
The capacitance of a parallel-plate capacitor is given by C = ε * A / d, where ε is the permittivity of the material between the plates, A is the area of the plates, and d is the distance between them. When there is air (or vacuum) between the plates, the permittivity is approximately ε₀, and the capacitance is C = ε₀ * A / d. When the space is filled with a dielectric material of dielectric constant κ, the permittivity changes to ε = κ * ε₀. Therefore, the new capacitance C’ becomes C’ = (κ * ε₀) * A / d = κ * (ε₀ * A / d). Given that the dielectric constant κ = 7, the new capacitance is C’ = 7 * C.
Introducing a dielectric material between the plates of a capacitor increases the electric field strength between the plates for the same voltage, or reduces the voltage across the plates for the same charge. This results in the capacitor being able to store more charge at the same voltage, effectively increasing its capacitance.

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40. Which one of the following is not a semiconductor?

Which one of the following is not a semiconductor?

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Silicon
Germanium
Quartz
Gallium arsenide
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The correct answer is C) Quartz.
Semiconductors are materials with electrical conductivity between that of a conductor and an insulator. Silicon (Si) and Germanium (Ge) are elemental semiconductors. Gallium arsenide (GaAs) is a compound semiconductor. Quartz is primarily silicon dioxide (SiO₂), which is an excellent electrical insulator.
Semiconductors have a band gap that is smaller than insulators but larger than conductors, allowing their conductivity to be significantly altered by temperature, doping, and other factors. Insulators like quartz have a large band gap, making it very difficult for electrons to move and conduct electricity.

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