Chemistry

51. Which of the following is/are the example/examples of chemical change?

Which of the following is/are the example/examples of chemical change?

  • 1. Crystallization of sodium chloride
  • 2. Melting of ice
  • 3. Souring of milk

Select the correct answer using the code given below.

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1 and 2 only
3 only
1, 2 and 3
None
This question was previously asked in
UPSC IAS – 2014
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The correct answer is B) 3 only. A chemical change results in the formation of new substances with different properties, while a physical change only alters the form or appearance of a substance, but not its chemical composition.
1. Crystallization of sodium chloride: This is a process where dissolved salt precipitates out of a solution as crystals. It is a physical change because the chemical composition (NaCl) remains the same.
2. Melting of ice: This is a change of state from solid water (ice) to liquid water. It is a physical change because the substance is still water (H₂O), just in a different form.
3. Souring of milk: This occurs when bacteria convert lactose (milk sugar) into lactic acid. This process creates new chemical substances (lactic acid) and changes the milk’s properties (taste, smell, texture). This is a chemical change.
Indicators of a chemical change often include the formation of a gas, a change in color, a change in temperature, the formation of a precipitate, or the production of light or sound. Souring of milk is a form of fermentation, a biochemical process leading to chemical transformation.

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52. Which one among the following is used in the manufacture of ultra-viol

Which one among the following is used in the manufacture of ultra-violet protective glasses ?

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Aluminium oxide
Tungsten oxide
Molybdenum oxide
Cerium oxide
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The correct option is D. Cerium oxide is commonly used as an additive in glass manufacturing to absorb ultraviolet radiation, thus producing ultra-violet protective glasses.
– Certain metal oxides, when added to glass, can impart specific optical properties, including the absorption of UV light.
– Cerium oxide (CeO₂) is particularly effective at absorbing UV radiation across the spectrum relevant for protecting eyes and materials from UV damage.
While other oxides can affect glass properties, cerium oxide is specifically known and widely used for its UV absorption capabilities in glasses for spectacles, laboratory ware, and architectural uses.

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53. Which one among the following is an example of endothermic process ?

Which one among the following is an example of endothermic process ?

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Combustion of carbon
Mixing acid and alkali
Photosynthesis
Respiration
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The correct option is C. Photosynthesis is an example of an endothermic process because it requires energy input (light energy) from the surroundings to proceed.
– An endothermic process absorbs heat or energy from the surroundings. The temperature of the surroundings typically decreases.
– An exothermic process releases heat or energy into the surroundings. The temperature of the surroundings typically increases.
– Photosynthesis converts light energy into chemical energy stored in glucose: $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Light Energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$.
Combustion of carbon, mixing acid and alkali (neutralization), and respiration are all exothermic processes, releasing energy.

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54. Which one among the following statements with respect to the atomic nu

Which one among the following statements with respect to the atomic number of an atom is correct ?

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The number of neutrons is same as atomic number
The sum of electrons and neutrons is same as atomic number
The number of protons is same as atomic number
The sum of protons and neutrons is same as atomic number
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The correct option is C. The atomic number of an atom is defined as the number of protons in the nucleus of that atom.
– Atomic number (Z) is a fundamental property of an element that uniquely identifies it.
– It represents the number of positive charges (protons) in the nucleus.
– In a neutral atom, the number of electrons is equal to the number of protons.
The mass number (A) of an atom is the sum of the number of protons and neutrons in the nucleus. Isotopes of an element have the same atomic number (same number of protons) but different mass numbers (different numbers of neutrons).

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55. Which among the following are the main ingredients for manufacturing o

Which among the following are the main ingredients for manufacturing of glass ?

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Silica, sodium carbonate, borax, alumina and cullets
Iron oxide, lead, sodium bicarbonate, alumina and cullets
Magnesium carbonate, alumina, silica and cullets
Iron oxide, sodium, alumina, silica and cullets
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The correct option is A. The main ingredients for manufacturing common glass (soda-lime glass) are silica, sodium carbonate (soda ash), and calcium carbonate (limestone). Option A includes silica and sodium carbonate, along with other common additives and components used in glass manufacturing.
– Silica (sand) is the primary component, forming the glass structure.
– Sodium carbonate (soda ash) is added to lower the melting point of silica. It decomposes to sodium oxide (Na₂O) during heating.
– Calcium carbonate (limestone) is added to improve durability and prevent the glass from dissolving in water. It decomposes to calcium oxide (CaO) during heating.
– Alumina (aluminium oxide) is a common additive to increase strength, chemical resistance, and reduce devitrification.
– Borax (sodium borate) is used in the manufacture of borosilicate glass, which has high thermal resistance. Its presence in option A suggests a broader view of glass manufacturing ingredients beyond just standard soda-lime glass.
– Cullets are recycled glass added to the batch mixture. They act as a flux, reducing the melting temperature and saving energy.
While standard soda-lime glass primarily uses silica, sodium carbonate, and calcium carbonate, option A provides a list that includes key primary components and common additives like alumina and borax (for specific glass types), along with cullets which are universally used in modern glass production. Options B, C, and D contain inaccuracies like elemental sodium or less common primary components.

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56. Which among the following is correct with respect to bond formation in

Which among the following is correct with respect to bond formation in an ethyne molecule ?

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Carbon-Carbon single bond
Carbon-Carbon double bond
Carbon-Carbon triple bond
Carbon-Hydrogen double bond
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Ethyne is an organic compound with the chemical formula C₂H₂. It belongs to the alkyne family of hydrocarbons, which are characterized by the presence of a carbon-carbon triple bond. The structure of ethyne is H-C≡C-H. Each carbon atom is bonded to one hydrogen atom via a single covalent bond and to the other carbon atom via a triple covalent bond. Alkanes have single bonds (C-C), alkenes have double bonds (C=C), and alkynes have triple bonds (C≡C) between carbon atoms.
Hydrocarbons are classified based on the types of bonds between carbon atoms. Alkanes have only single bonds, alkenes have at least one double bond, and alkynes have at least one triple bond. Ethyne is the simplest alkyne.
A carbon-carbon triple bond consists of one sigma bond and two pi bonds. The carbon atoms in ethyne are sp-hybridized, resulting in a linear geometry around the triple bond. Ethyne is also commonly known as acetylene and is used as a fuel in welding and cutting.

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57. Water is a good solvent for dissolving ionic compounds because

Water is a good solvent for dissolving ionic compounds because

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It has a high specific heat
It has no colour
It has a high dipole moment
It has a high boiling point
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Water is an excellent solvent for ionic compounds because it is a polar molecule with a high dipole moment. An ionic compound is held together by strong electrostatic forces between positive and negative ions in a crystal lattice. Water molecules, being polar, have a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom. When an ionic compound is placed in water, the positive ends of water molecules are attracted to the negative ions, and the negative ends are attracted to the positive ions. These attractive forces between water molecules and ions (ion-dipole interactions) are strong enough to overcome the lattice energy of the ionic compound, causing the ions to separate and become surrounded by water molecules (hydration). A high dipole moment indicates strong polarity, which is crucial for this process. High specific heat, no colour, and high boiling point are characteristics of water but are not the primary reason for its ability to dissolve ionic compounds.
Water’s ability to dissolve ionic compounds stems primarily from its strong polarity and high dipole moment, which allows it to hydrate (surround and separate) the positive and negative ions.
The process of dissolving an ionic compound in water is called dissociation and hydration. The hydrated ions are dispersed throughout the solution. Water is often referred to as the “universal solvent” because of its ability to dissolve a wide range of substances, including many ionic and polar covalent compounds.

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58. The isotope of which one among the following elements is used in the t

The isotope of which one among the following elements is used in the treatment of cancer ?

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Uranium
Cobalt
Iodine
Fluorine
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Several radioactive isotopes are used in the treatment of cancer, primarily in radiotherapy. Cobalt-60 is a widely used radioisotope that emits gamma rays, which are used in external beam radiotherapy machines (sometimes called “cobalt therapy”). Iodine-131 is used for internal radiotherapy, specifically in the treatment of thyroid cancer because the thyroid gland preferentially absorbs iodine. Uranium is used in nuclear power and weapons, not typically for direct cancer treatment. Fluorine-18 is a positron-emitting isotope used as a tracer in PET scans (Positron Emission Tomography), which is a diagnostic imaging technique, not a treatment. While Iodine-131 is used, Cobalt-60 is perhaps the most general and well-known example of a radioisotope used in radiotherapy for a wide range of cancers. Therefore, Cobalt is a correct answer.
Radioisotopes of certain elements are used in cancer treatment, particularly radiotherapy, to target and destroy cancer cells using emitted radiation. Cobalt-60 is a common example for external radiotherapy.
Other isotopes used in cancer therapy include Iridium-192 and Cesium-137 (brachytherapy), Gold-198 (colloidal gold therapy), and Phosphorus-32 (for polycythemia vera and chronic myeloid leukemia). Targeted alpha therapy uses isotopes that emit alpha particles (e.g., Radium-223, Actinium-225) linked to targeting molecules.

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59. Which one among the following statements about matter is NOT correct ?

Which one among the following statements about matter is NOT correct ?

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On increasing the temperature of solids, the kinetic energy of particles increases
The maximum temperature at which a solid melts to become a liquid at atmospheric pressure is called its melting point
Particles of steam have more energy than water at 100°C
Direct change of gas to solid is called deposition
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UPSC CAPF – 2024
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Let’s examine each statement:
A) On increasing the temperature of solids, the kinetic energy of particles increases. This is correct. Temperature is a measure of the average kinetic energy of the particles.
B) The maximum temperature at which a solid melts to become a liquid at atmospheric pressure is called its melting point. For a pure crystalline solid at constant pressure, melting occurs *at* a specific temperature called the melting point. Saying it is the *maximum* temperature at which it melts is imprecise or incorrect. Above the melting point, the substance is in the liquid state. For impure or amorphous solids, melting can occur over a range of temperatures, but the term ‘melting point’ usually refers to the start of the melting range or the specific temperature for pure substances. The phrasing “maximum temperature at which a solid melts” is misleading for a pure substance and not a standard definition for the melting point.
C) Particles of steam have more energy than water at 100°C. This is correct. When water boils at 100°C, it absorbs latent heat of vaporization to change state into steam at the same temperature. This latent heat increases the internal energy (primarily potential energy related to intermolecular forces) of the particles in the gaseous state compared to the liquid state at the same temperature.
D) Direct change of gas to solid is called deposition. This is correct. The phase transition from gas directly to solid is known as deposition or desublimation. The reverse process, solid to gas, is called sublimation.
Therefore, statement B is the incorrect statement due to its inaccurate phrasing regarding the melting point of a solid.
Understanding the states of matter, phase transitions, and the relationship between temperature, energy, and particle motion. Melting point is the specific temperature at which a pure crystalline solid melts at a given pressure.
Phase transitions like melting, boiling, sublimation, condensation, freezing, and deposition occur at specific temperatures (at constant pressure) for pure substances, involving the absorption or release of latent heat. The kinetic energy of particles is related to temperature within a single phase, while potential energy changes significantly during phase transitions.

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60. An aqueous solution of a salt is known as brine. The salt is :

An aqueous solution of a salt is known as brine. The salt is :

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Sodium chloride
Potassium chloride
Calcium chloride
Sodium nitrate
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UPSC CAPF – 2023
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Brine is the term used for a highly concentrated aqueous solution of sodium chloride (NaCl). While other salt solutions exist, “brine” specifically refers to sodium chloride solution, often much more concentrated than seawater. Seawater itself is a form of brine, but artificial brines are typically prepared by dissolving large amounts of NaCl in water for industrial or culinary purposes.
Brine is a concentrated aqueous solution of sodium chloride (common salt).
Brine has various uses, including food preservation, as a heat-transfer fluid, for de-icing roads, and in industrial processes like chlor-alkali production.

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