Hydrogen and Its Compounds

1. The gas generated on reacting zinc with dilute sulphuric acid is :

The gas generated on reacting zinc with dilute sulphuric acid is :

Argon
Helium
Hydrogen
Nitrogen
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UPSC CAPF – 2023
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When a reactive metal like zinc reacts with a dilute acid like sulphuric acid, it displaces hydrogen from the acid. The reaction is: Zn(s) + H₂SO₄(aq) -> ZnSO₄(aq) + H₂(g). This reaction produces zinc sulphate (a salt) and hydrogen gas.
Active metals react with dilute acids to produce hydrogen gas and a salt.
This is a standard method for preparing hydrogen gas in the laboratory. The hydrogen gas can be tested by bringing a burning splint near it; it will burn with a pop sound.

2. Pure, demineralized water, free from all soluble mineral salts is obta

Pure, demineralized water, free from all soluble mineral salts is obtained by which of the following method?

Passing water through microfiltration membrane
Calgon’s method
Passing water through a cation exchange and an anion exchange resin bed
By boiling
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UPSC CAPF – 2023
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Pure, demineralized water, free from all soluble mineral salts, is obtained by passing water through a cation exchange and an anion exchange resin bed. This process removes dissolved ions (cations and anions) that constitute mineral salts.
Demineralization or deionization is the process of removing mineral salts from water. Ion exchange is a highly effective method for achieving this. Cation exchange resins replace positive ions (like Ca²⁺, Mg²⁺, Na⁺) with H⁺ ions, and anion exchange resins replace negative ions (like Cl⁻, SO₄²⁻, HCO₃⁻) with OH⁻ ions. The H⁺ and OH⁻ ions then combine to form water (H₂O).
Other options are not suitable for obtaining pure, demineralized water. Microfiltration removes suspended solids but not dissolved salts. Calgon’s method softens water by sequestering hardness ions but doesn’t remove all salts. Boiling removes temporary hardness and dissolved gases but leaves behind permanent hardness salts and other dissolved minerals. Distillation is another method to obtain pure water by separating it from dissolved substances through evaporation and condensation. Reverse osmosis can also produce demineralized water.

3. Which one of the following is the chemical name of heavy water ?

Which one of the following is the chemical name of heavy water ?

Hydrogen oxide
Deuterium dioxide
Deuterium oxide
Heavy hydrogen oxide
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UPSC CAPF – 2017
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Heavy water is a form of water in which the hydrogen atoms are replaced by deuterium, an isotope of hydrogen with one proton and one neutron (compared to normal hydrogen, protium, which has one proton and no neutrons). The chemical formula for heavy water is D₂O or ²H₂O. The standard chemical name for D₂O is Deuterium oxide.
Heavy water is water composed of deuterium and oxygen. Deuterium is an isotope of hydrogen.
Heavy water is used primarily as a moderator and coolant in certain types of nuclear reactors (specifically, Pressurized Heavy-Water Reactors like the CANDU reactor) because deuterium absorbs fewer neutrons than normal hydrogen.

4. Heavy water of an atomic reactor is :

Heavy water of an atomic reactor is :

deionised water
an oxide of heavier isotope of oxygen
a mixture of ice and water
an oxide of heavier isotope of hydrogen
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UPSC CAPF – 2015
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Heavy water is an oxide of a heavier isotope of hydrogen.
Heavy water has the chemical formula D₂O, where D represents Deuterium. Deuterium is an isotope of hydrogen that contains one proton and one neutron in its nucleus (whereas common hydrogen, Protium, has only one proton). Thus, deuterium is a heavier isotope of hydrogen. Heavy water (D₂O) is water composed of two deuterium atoms and one oxygen atom.
Heavy water is used in some types of nuclear reactors (like CANDU reactors) as a neutron moderator and coolant. Its function as a moderator is to slow down neutrons, making them more likely to cause nuclear fission in Uranium fuel. D₂O is more effective as a moderator than ordinary water (H₂O) because deuterium absorbs fewer neutrons than protium. Ordinary water also contains a very small natural abundance of D₂O (about 1 part in 6400). Isotopes of oxygen exist (e.g., ¹⁶O, ¹⁷O, ¹⁸O), and water made with ¹⁸O (H₂¹⁸O) is sometimes called “heavy-oxygen water”, but “heavy water” typically refers to D₂O.

5. Heavy water is so called because it

Heavy water is so called because it

contains a number of salts as in sea water
is the mercury contaminated water
is saline water used in hospitals
contains the heavier isotope of hydrogen in water
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UPSC CAPF – 2011
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The correct option is D. Heavy water, chemically represented as D₂O, contains deuterium, which is an isotope of hydrogen. Unlike the common hydrogen atom (protium, ¹H) which has only a proton, deuterium (²H or D) has a proton and a neutron, making it approximately twice as heavy as protium. Therefore, water molecules formed with deuterium (heavy water) are heavier than those formed with protium (ordinary water, H₂O).
The question asks for the reason why heavy water is called “heavy”. It relates to its chemical composition and the presence of a heavier isotope of hydrogen.
Heavy water is primarily used as a neutron moderator in certain types of nuclear reactors (pressurized heavy water reactors – PHWRs) because deuterium has a lower neutron capture cross-section than protium, allowing for the use of unenriched uranium as fuel.

6. Ice is lighter than pure water and floats on the surface. Which one of

Ice is lighter than pure water and floats on the surface. Which one of the following statements is correct to explain this phenomenon ?

The structure of ice is similar to that of water
Ice has a disordered 3-dimensional structure
Hydrogen-bonding in ice gives an open type structure with interstitial holes
There is no hydrogen bonding in water
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UPSC CAPF – 2010
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The correct statement explaining why ice is lighter than pure water is that hydrogen-bonding in ice gives an open type structure with interstitial holes.
In the solid state (ice), water molecules are held in a crystalline structure by hydrogen bonds. This structure is an open lattice containing hexagonal rings, with significant empty space or “interstitial holes” between the molecules. In the liquid state (water), although hydrogen bonds are still present and constantly breaking and reforming, the structure is less ordered and more compact than ice. As a result, a given mass of ice occupies a larger volume than the same mass of liquid water at the same temperature, leading to ice having a lower density and thus floating on water.
Most substances become denser in the solid state than in the liquid state. Water is unusual in this regard, exhibiting anomalous expansion (or contraction) upon melting or freezing. The density of water is maximum at 4°C. This property is crucial for aquatic life in cold climates, as ice forms on the surface, insulating the water below.

7. A very large volume of hydrogen can be accommodated by making

A very large volume of hydrogen can be accommodated by making

non-metallic hydrides.
hydrogen peroxide.
non-stoichiometric hydrides.
alkali metal hydrides.
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UPSC NDA-2 – 2019
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The correct answer is C) non-stoichiometric hydrides.
Certain transition metals (like palladium, titanium, vanadium, etc.) and their alloys can absorb large volumes of hydrogen, forming interstitial (non-stoichiometric) hydrides. In these hydrides, hydrogen atoms occupy interstitial sites within the metal lattice. This property is exploited for hydrogen storage, as these materials can store hydrogen at densities comparable to or even greater than liquid hydrogen, but at less extreme conditions.
Non-metallic hydrides are compounds like water (H2O), ammonia (NH3), etc. Hydrogen peroxide (H2O2) is a chemical compound. Alkali metal hydrides (like NaH, LiH) are ionic hydrides, which react vigorously with water and are not typically used for large-volume, reversible hydrogen storage applications in the same way as interstitial metal hydrides. The ability of metals like palladium to absorb hundreds of times their own volume of hydrogen is a classic example related to non-stoichiometric hydrides.

8. Which one of the following statements about water is not true ?

Which one of the following statements about water is not true ?

Hydrogen bonds are present in liquid water.
Water has a high boiling point.
Water has a high heat of fusion.
Water is a non-polar molecule.
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UPSC NDA-2 – 2019
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Statement D, “Water is a non-polar molecule”, is not true. Water (H2O) has a bent molecular geometry due to the presence of two lone pairs on the oxygen atom. The O-H covalent bonds are polar because oxygen is more electronegative than hydrogen. Because of the bent shape, the bond dipoles do not cancel out, resulting in a net dipole moment for the entire molecule. Therefore, water is a polar molecule.
– Water is a polar molecule due to its bent structure and polar covalent bonds.
– The polarity and the presence of hydrogen atoms bonded to a highly electronegative oxygen atom enable hydrogen bonding between water molecules.
– Hydrogen bonding is responsible for many of water’s unusual properties, such as high boiling point, high heat of fusion, high specific heat capacity, and high surface tension.
– Statement A is true; hydrogen bonds are extensive in liquid water, forming a dynamic network.
– Statement B is true; water has an exceptionally high boiling point (100°C at 1 atm) compared to other molecules of similar size and molar mass without hydrogen bonding (e.g., H2S, which boils at -60°C).
– Statement C is true; water requires a significant amount of energy to melt (heat of fusion) due to the need to break hydrogen bonds holding the ice lattice together.

9. Permanent hardness of water cannot be removed by which one of the fo

Permanent hardness of water cannot be removed by which one of the following methods?

Treatment with washing soda
Calgon's method
Boiling
Ion exchange method
This question was previously asked in
UPSC NDA-2 – 2018
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Permanent hardness of water is caused by the presence of dissolved sulfates and chlorides of calcium and magnesium. Boiling only removes temporary hardness, which is caused by dissolved bicarbonates of calcium and magnesium (these decompose upon heating to form insoluble carbonates). Methods like treatment with washing soda (sodium carbonate), Calgon’s method (using sodium hexametaphosphate), and the ion exchange method (using resins or zeolites) are effective in removing permanent hardness by precipitating or exchanging the dissolved calcium and magnesium ions. Therefore, boiling cannot remove permanent hardness.
This question distinguishes between temporary and permanent hardness of water and tests the knowledge of common methods used to remove water hardness.
Temporary hardness is removed by boiling because soluble bicarbonates (Ca(HCO₃)₂, Mg(HCO₃)₂) decompose into insoluble carbonates (CaCO₃, MgCO₃) which precipitate out. Permanent hardness requires chemical treatment or ion exchange to remove the Ca²⁺ and Mg²⁺ ions from the solution.

10. Which of the following statements regarding heavy water are correct?

Which of the following statements regarding heavy water are correct?

  • 1. It is extensively used as a moderator in nuclear reactors
  • 2. It cannot be used in exchange reaction to study reaction mechanism
  • 3. Viscosity of heavy water is relatively smaller than that of ordinary water
  • 4. The dielectric constant of heavy water is smaller than that of ordinary water

Select the correct answer using the code given below:

1 and 2
2 and 3
3 and 4
1 and 4
This question was previously asked in
UPSC NDA-2 – 2015
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Statement 1 is correct: Heavy water (D₂O) is an excellent moderator in nuclear reactors because deuterium has a low neutron absorption cross-section and can effectively slow down fast neutrons to thermal energies. Statement 2 is incorrect: Heavy water and deuterated compounds are commonly used in isotopic exchange reactions to study reaction mechanisms and kinetics. Statement 3 is incorrect: The viscosity of heavy water is slightly higher than that of ordinary water at typical temperatures. Statement 4 is correct: The dielectric constant of heavy water is slightly smaller than that of ordinary water.
This question tests specific properties of heavy water compared to ordinary water.
Heavy water is chemically similar to ordinary water but has different physical properties due to the heavier deuterium isotope replacing hydrogen. It is essential for some types of nuclear reactors (like CANDU reactors) that use unenriched uranium fuel.
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