Chemistry

1. The original copies of the Constitution of India are kept in the Parli

The original copies of the Constitution of India are kept in the Parliament library in cases filled with which one of the following inert gases?

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Helium
Nitrogen
Argon
Hydrogen
This question was previously asked in
UPSC Combined Section Officer – 2021-22
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The correct answer is A, Helium.
The original handwritten copies of the Constitution of India are preserved in the Parliament Library in New Delhi. They are stored in special cases filled with Helium gas to prevent degradation due to oxidation and humidity. Helium is a noble gas and is inert, making it ideal for long-term preservation of such valuable documents.
Besides the Constitution, original manuscripts of other important documents and books are also preserved using similar techniques in libraries and archives worldwide. The practice ensures that these historical documents remain intact for future generations.

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2. Which one among the following elements has the highest catenation

Which one among the following elements has the highest catenation property?

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Silicon
Carbon
Nitrogen
Oxygen
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UPSC CISF-AC-EXE – 2024
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Catenation is the ability of an element to form bonds with atoms of the same element. Carbon has the strongest tendency for catenation among all elements. This property allows carbon atoms to link together to form long chains, branched structures, and rings, forming the backbone of the vast number of organic compounds.
Carbon exhibits the highest catenation property among elements, forming diverse organic structures.
Silicon also shows catenation, forming silanes (Si-Si bonds), but the Si-Si bond is weaker than the C-C bond, limiting the length and stability of silicon chains compared to carbon chains. Nitrogen can form chains (e.g., in hydrazine, N₂H₄), but N-N bonds are relatively weak. Oxygen primarily forms chains of two atoms (O-O bond in peroxides), and longer chains are generally unstable. The high strength of the C-C bond and carbon’s ability to form multiple bonds (single, double, triple) contribute to its exceptional catenation ability.

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3. A solution has a pH value around 13.0. The solution may be called

A solution has a pH value around 13.0. The solution may be called

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a strong acid
a strong base
neutral
a weak acid
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UPSC CISF-AC-EXE – 2024
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The pH scale measures the acidity or basicity of an aqueous solution. The scale ranges typically from 0 to 14. Solutions with a pH less than 7 are acidic, a pH of 7 is neutral, and solutions with a pH greater than 7 are basic (alkaline). A pH value around 13.0 is significantly greater than 7 and is close to the maximum value on the scale, indicating a highly basic solution.
A pH value of 13.0 indicates a strong base because it is high on the pH scale (greater than 7).
Strong acids have very low pH values (close to 0 or 1), weak acids have pH values between 1 and 7, neutral solutions have a pH of 7, weak bases have pH values between 7 and 14, and strong bases have very high pH values (close to 13 or 14). Examples of strong bases include sodium hydroxide (NaOH) and potassium hydroxide (KOH).

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4. Which one of the following is an example of a physical change?

Which one of the following is an example of a physical change?

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Ripening of banana
Souring of milk
Melting of ice
Passing CO₂ gas into water
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Melting of ice is a physical change because it involves a change of state from solid water (ice) to liquid water. The chemical composition of the substance remains H₂O throughout the process; only its physical form changes. Physical changes are typically reversible.
A physical change alters the form or appearance of a substance but not its chemical composition.
Ripening of banana and souring of milk are examples of chemical changes because they involve chemical reactions that result in the formation of new substances with different chemical properties (e.g., sugars converting to acids, proteins and sugars breaking down). Passing CO₂ gas into water leads to the formation of carbonic acid (H₂CO₃), which is also a chemical change. Chemical changes are often irreversible.

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5. The chemical formula of washing soda is

The chemical formula of washing soda is

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NaHCO₃
Na₂CO₃
Na₂CO₃·10H₂O
NaOCl
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UPSC CISF-AC-EXE – 2024
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Washing soda is the common name for hydrated sodium carbonate. Its chemical formula is Na₂CO₃·10H₂O, which represents sodium carbonate decahydrate. This compound is widely used as a cleaning agent and in various industrial processes.
Washing soda is chemically known as sodium carbonate decahydrate (Na₂CO₃·10H₂O).
Option A, NaHCO₃, is the chemical formula for baking soda (sodium bicarbonate). Option B, Na₂CO₃, is anhydrous sodium carbonate, also known as soda ash. Option D, NaOCl, is the chemical formula for sodium hypochlorite, the main ingredient in many household bleaches.

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6. Which one of the following statements with regard to TNT is not corr

Which one of the following statements with regard to TNT is not correct ?

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TNT stands for trinitrotoluene
It is of creamy yellow colour
It is a solid organic nitrogen compound
It is considered to be a low intensity explosive substance
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UPSC CISF-AC-EXE – 2023
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The correct option is D, as the statement “It is considered to be a low intensity explosive substance” is not correct.
TNT (Trinitrotoluene) is a high explosive. High explosives detonate at high velocity, producing a supersonic shockwave, unlike low explosives which deflagrate (burn rapidly). TNT is one of the most widely used explosive materials for military, industrial, and mining purposes.
Statements A, B, and C are correct. A) TNT stands for trinitrotoluene. B) Pure TNT is indeed a creamy yellow solid. C) It is a solid organic nitrogen compound with the chemical formula C6H2(NO2)3CH3.

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7. Which one among the following is the most reactive metal?

Which one among the following is the most reactive metal?

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Na
K
Ca
Mg
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UPSC CISF-AC-EXE – 2023
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The reactivity of alkali metals increases down the group (K > Na) and alkali earth metals are generally less reactive than alkali metals. Comparing the given options, the standard reactivity series shows Potassium (K) is more reactive than Sodium (Na), which is more reactive than Calcium (Ca), which is more reactive than Magnesium (Mg). Therefore, Potassium (K) is the most reactive metal among the given choices.
– Reactivity of metals is related to their tendency to lose electrons (become oxidized).
– Alkali metals (Group 1) are generally more reactive than alkaline earth metals (Group 2).
– Within alkali metals, reactivity increases down the group.
– Within alkaline earth metals, reactivity generally increases down the group (though sometimes variations occur, but the general trend holds for Ca vs Mg).
– The order of reactivity for the given options is approximately K > Na > Ca > Mg.
Reactivity series can be determined experimentally (e.g., reaction with water, air, acids) or understood based on electronic configuration and ionization energies. Metals higher in the reactivity series can displace metals lower in the series from their compounds.

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8. Which one of the following substances is not used as a lubricant in he

Which one of the following substances is not used as a lubricant in heavy industries ?

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Graphene
Talc
Glass
Sand
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UPSC CISF-AC-EXE – 2023
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The correct answer is D) Sand.
Graphene and Talc are well-known solid lubricants used in various industrial applications to reduce friction and wear. While glass is generally not used as a lubricant, specialized applications exist where molten glass acts as a lubricant in high-temperature processes like hot metal extrusion. Sand, primarily composed of silica, is hard and abrasive, and its properties make it suitable for uses like abrasive blasting or as a proppant in fracking, but it is fundamentally an abrasive material, not a lubricant. Therefore, Sand is the substance among the options that is not used as a lubricant in heavy industries.
Common lubricants in heavy industries include oils, greases, graphite, molybdenum disulfide, PTFE (Teflon), and sometimes talc or mica powder as solid lubricants. Graphene is a relatively new but highly effective lubricant due to its low shear strength between layers. Glass is sometimes used in very high-temperature metalworking processes where conventional lubricants would break down, acting as a ‘molten glass lubricant’, but this is a specific application unlike the general use of lubricants. Sand, being crystalline and hard, increases friction and causes wear, making it unsuitable as a lubricant.

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9. Which one of the following compounds is present in antacid tablets or

Which one of the following compounds is present in antacid tablets or medicines which are used to cure indigestion and acidity in the stomach ?

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Sodium hydroxide
Potassium hydroxide
Lithium hydroxide
Aluminium hydroxide
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UPSC CISF-AC-EXE – 2022
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The correct answer is Aluminium hydroxide.
Antacid tablets or medicines are used to neutralize excess stomach acid (hydrochloric acid, HCl) to relieve indigestion and acidity. Antacids are typically bases or basic salts.
Let’s look at the given options:
A) Sodium hydroxide (NaOH): This is a strong base (lye). It is highly corrosive and would cause severe internal burns if ingested. It is not used as an antacid.
B) Potassium hydroxide (KOH): Similar to sodium hydroxide, this is also a strong base (caustic potash) and is highly corrosive. It is not used as an antacid.
C) Lithium hydroxide (LiOH): A strong base. Not used in antacids.
D) Aluminium hydroxide (Al(OH)₃): This is a weak base commonly used in antacid formulations, often combined with magnesium hydroxide (Mg(OH)₂) or calcium carbonate (CaCO₃). Aluminium hydroxide reacts with stomach acid to form aluminium chloride and water: Al(OH)₃(s) + 3HCl(aq) → AlCl₃(aq) + 3H₂O(l). It helps neutralize acid and can also help protect the stomach lining.
Common active ingredients in antacids include Aluminium hydroxide (Al(OH)₃), Magnesium hydroxide (Mg(OH)₂), Calcium carbonate (CaCO₃), and Sodium bicarbonate (NaHCO₃). Aluminium and magnesium based antacids can have side effects; aluminium salts can cause constipation, while magnesium salts can cause diarrhoea. Combination products are often used to balance these effects. Sodium bicarbonate is fast-acting but can cause belching and flatulence due to CO₂ production, and can be a concern for people on sodium-restricted diets. Calcium carbonate is effective but can also cause belching and constipation.

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10. Which one of the following substances shows sublimation behaviour or p

Which one of the following substances shows sublimation behaviour or property at standard temperature and pressure ?

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Solid carbon dioxide
Water
Liquid nitrogen
Iron
This question was previously asked in
UPSC CISF-AC-EXE – 2022
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The correct answer is Solid carbon dioxide.
Sublimation is a phase transition where a substance changes directly from a solid to a gas without passing through a liquid phase. The question asks which substance exhibits this behaviour at standard temperature and pressure (STP). STP is typically defined as 0°C (273.15 K) and 1 atmosphere (101.325 kPa).
Let’s examine the options:
A) Solid carbon dioxide (Dry ice): At atmospheric pressure (around 1 atm), solid CO₂ sublimes directly into gaseous CO₂ at -78.5°C. While this temperature is below the typical definition of standard temperature (0°C or 25°C), it readily sublimes at standard atmospheric pressure. Compared to other options, it’s the classic example of sublimation under common conditions. Its triple point is -56.6°C at 5.18 atm; below the triple point pressure, it only exists as solid and gas.
B) Water: At 1 atm, ice melts at 0°C and water boils at 100°C. Water exists as a liquid between 0°C and 100°C at 1 atm. Sublimation of ice can occur below 0°C, but water does not typically sublime from solid to gas at standard temperature (0°C or 25°C) and pressure (1 atm) as its primary transition.
C) Liquid nitrogen: Nitrogen boils at -196°C at 1 atm. It is a gas at standard temperature. It doesn’t sublime (solid to gas) at standard temperature and pressure; it transitions from liquid to gas at a much lower temperature.
D) Iron: Iron is a solid at standard temperature and pressure. It melts at 1538°C and boils at 2862°C at 1 atm. It does not sublime under standard conditions.
Solid carbon dioxide is the substance among the options that is well-known for undergoing sublimation at standard atmospheric pressure, even if the temperature is not strictly “standard room temperature”. Given the context of such questions, dry ice is the intended answer for a substance showing sublimation behaviour at standard pressure.
Other substances that sublime at standard atmospheric pressure include iodine, naphthalene (mothballs), camphor, and arsenic. The triple point of a substance is the temperature and pressure at which the three phases (solid, liquid, and gas) of that substance coexist in thermodynamic equilibrium. If the pressure is below the triple point pressure, heating the solid at that pressure will result in sublimation rather than melting. For CO₂, the triple point pressure (5.18 atm) is above standard atmospheric pressure (1 atm), which is why it sublimes instead of melting at 1 atm.

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