Hydrogen and Its Compounds: notes and MCQs for State PSC prelims

Hydrogen and Its Compounds is part of Chemistry. Below are notes on 9 sub-topics, followed by practice MCQs from State PSC and UPSC papers.

Hard and Soft Water: Causes of hardness and methods for removal

Hard Water: Water that does not lather easily with soap. Due to dissolved salts of calcium (Ca²⁺) and magnesium (Mg²⁺).
Soft Water: Water that lathers easily with soap.
Causes of Hardness:

Temporary Hardness:

Caused by dissolved bicarbonates (hydrogen carbonates) of calcium (Ca(HCO₃)₂) and magnesium (Mg(HCO₃)₂).

Permanent Hardness:

Caused by dissolved chlorides (e.g., CaCl₂, MgCl₂) and sulphates (e.g., CaSO₄, MgSO₄) of calcium and magnesium.

Methods for Removal of Hardness:

Removal of Temporary Hardness:

Boiling: Decomposes bicarbonates into insoluble carbonates (CaCO₃) and hydroxides (Mg(OH)₂), which precipitate out.
Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)
Mg(HCO₃)₂(aq) → Mg(OH)₂(s) + 2CO₂(g)
Clark's Method: Adding a calculated amount of lime (calcium hydroxide, Ca(OH)₂), which precipitates Ca²⁺/Mg²⁺ as insoluble carbonates/hydroxides.

Removal of Permanent Hardness (Boiling does NOT remove permanent hardness):

Washing Soda Method (Sodium Carbonate, Na₂CO₃): Washing soda reacts with soluble calcium and magnesium salts to form insoluble carbonates, which then precipitate.
CaSO₄(aq) + Na₂CO₃(aq) → CaCO₃(s) + Na₂SO₄(aq)
Calgon's Method (Sodium Hexametaphosphate, (NaPO₃)₆): Calgon sequesters Ca²⁺ and Mg²⁺ ions by forming soluble complex anions, preventing them from reacting with soap.
Ion-Exchange Method (Zeolite/ Permutit Method or Synthetic Resins):
Zeolite Method: Hard water passes through a bed of sodium zeolite (NaZ), which exchanges Na⁺ ions for Ca²⁺/Mg²⁺ ions.
Synthetic Resins Method (Demineralization/Deionization):
Water first passed through a cationexchange resin (RH⁺), which exchanges H⁺ ions for all cations (Ca²⁺, Mg²⁺, Na⁺).
Then passed through an anionexchange resin (ROH⁻), which exchanges OH⁻ ions for all anions (Cl⁻, SO₄²⁻, HCO₃⁻).
The H⁺ and OH⁻ ions combine to form water (H₂O), yielding pure, demineralized (deionized) water, free from all soluble mineral salts.

Heavy Water (D2O): Preparation, properties, and uses

Heavy water (D2O) is also known as Deuterium oxide.
It is an oxide of the heavier isotope of hydrogen (deuterium), containing deuterium in place of ordinary hydrogen.
Heavy water occurs naturally and can be produced by the process of electrolysis.
Pure heavy water is not radioactive.
It is extensively used as a moderator in nuclear reactors.
It is used in nuclear power plants as a coolant.
The dielectric constant of heavy water is smaller than that of ordinary water.
It can be used in exchange reactions to study reaction mechanisms.
Though algae and bacteria can live using 100% heavy water, its consumption by humans may cause sterilisation.

Hydrides: Classification (Ionic, Covalent, Metallic/Interstitial)

Hydrides: Compounds formed by hydrogen with other elements.
Classified into three main categories based on chemical bonding and properties: Ionic, Covalent, and Metallic.
1. Ionic Hydrides (Saline/Salt-like Hydrides):

Formed by highly electropositive sblock elements (Group 1 and Group 2, except Be, Mg).
Examples: LiH, NaH, CaH₂, SrH₂, BaH₂.
Crystalline, non-volatile, non-conducting in solid state.
Conduct electricity in molten state or in solution, liberating H₂ gas at anode (H⁻ ion).
React violently with water producing H₂ gas.

2. Covalent Hydrides (Molecular Hydrides):

Formed by hydrogen with p-block elements (e.g., C, N, O, F, Si, P, S, Cl).
Volatile compounds.
Further classified based on electron count:

Electron-deficient: Group 13 elements (e.g., B₂H₆).
Electron-precise: Group 14 elements (e.g., CH₄, SiH₄).
Electron-rich: Group 15-17 elements (e.g., NH₃, H₂O, HF) possessing lone pairs.

3. Metallic Hydrides (Interstitial/Nonstoichiometric Hydrides):

Formed by hydrogen with d-block and f-block elements. (Note: Group 7, 8, 9 metals generally do not form hydrides, known as the 'hydride gap').
Often non-stoichiometric compounds (e.g., LaH₂.₈₇, TiH₁.₇₃), meaning the ratio of hydrogen to metal is not a simple whole number.
Hydrogen atoms occupy interstitial sites within the metal lattice.
Retain metallic luster and electrical conductivity.
Can accommodate a very large volume of hydrogen, making them potential hydrogen storage media.
This property of non-stoichiometric hydrides allows for high hydrogen storage density.

Isotopes of hydrogen: Protium, Deuterium, Tritium and their properties

Hydrogen (H) is the lightest and most abundant element in the universe.
It exists as three isotopes: Protium ($^1$H), Deuterium ($^2$H or D), and Tritium ($^3$H or T).
Isotopes: Atoms of the same element having the same atomic number (number of protons) but different mass numbers (due to different number of neutrons).
Protium ($^1$H):

Most common isotope (~99.98% abundance).
Contains 1 proton and 0 neutrons.
Stable isotope.

Deuterium ($^2$H or D):

Also known as 'heavy hydrogen'.
Contains 1 proton and 1 neutron.
Abundance ~0.0156%.
Stable isotope.
Deuterium oxide (D$2$O) is 'heavy water', used as a moderator in nuclear reactors.

Tritium ($^3$H or T):

Contains 1 proton and 2 neutrons.
Naturally rare, produced in upper atmosphere by cosmic rays.
Radioactive isotope of hydrogen.
Undergoes beta decay (half-life ~12.33 years).

Chemical Properties:

Due to the same number of protons and electrons (i.e., same electronic configuration), Protium, Deuterium, and Tritium exhibit nearly identical chemical properties.

Physical Properties:

Significant differences in physical properties (e.g., density, boiling point, melting point) exist due to their large relative mass differences (1:2:3 ratio). This leads to the kinetic isotope effect.

Hydrogen gas (H$2$) forms an explosive mixture with oxygen (O$2$).

Physical and chemical properties of Dihydrogen

Dihydrogen (H₂) is a colourless, odourless, and tasteless gas.
It is the lightest gas known.
Dihydrogen is lighter than air.
It is insoluble in water.
Dihydrogen is inert at room temperature due to its high H–H bond dissociation enthalpy (~436 kJ/mol).
It is combustible.
Hydrogen typically exhibits an oxidation state of +1 (most common), but can also be -1 (in metal hydrides like NaH), and 0 (as elemental H₂).
Dihydrogen reacts with alkali metals at high temperatures to form metal hydrides (e.g., 2Na + H₂ → 2NaH).
Syngas (synthesis gas) is a mixture of carbon monoxide (CO) and dihydrogen (H₂).

Position of hydrogen in the periodic table

Hydrogen has unique position due to resemblance with both Group 1 (Alkali Metals) and Group 17 (Halogens).
Resemblance with Alkali Metals (Group 1):

Electronic configuration: 1s¹ (like ns¹ for alkali metals).
Forms unipositive ion (H⁺) by losing one electron.
Combines with electronegative elements.

Resemblance with Halogens (Group 17):

Needs one electron to complete its duplet (1s²) (like halogens needing one electron for octet).
Forms uninegative ion (H⁻, hydride) by gaining one electron.
Exists as diatomic molecule (H₂).
High ionization energy and nonmetallic nature.

Anomaly: Due to these conflicting properties, hydrogen is often placed separately or considered as having an anomalous position in the periodic table.
Non-metal forming positive ion: Hydrogen is the only non-metal known to form a simple monoatomic positive ion (H⁺).

Preparation of Dihydrogen (H2): Laboratory and Industrial methods

Laboratory Preparation of Dihydrogen (H₂):

Action of Active Metals on Dilute Acids:

Active metals (e.g., Zn, Mg, Fe) react with dilute non-oxidizing acids (e.g., HCl, H₂SO₄).
Example: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g).
Example: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
Note: Nitric acid (HNO₃) is an oxidizing agent and generally produces oxides of nitrogen (e.g., NO, NO₂, N₂O), not H₂ (unless very dilute).
Note: Less reactive metals (e.g., Cu, Ag) do not displace H₂ from acids.

Action of Highly Electropositive Metals on Cold Water:

Highly reactive metals (e.g., Na, K, Ca) react with cold water.
Example: 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g) (Vigorous reaction).

Action of Amphoteric Metals on Strong Alkalis:

Amphoteric metals (e.g., Zn, Al) react with hot concentrated strong alkalis (e.g., NaOH, KOH).
Example: Zn(s) + 2NaOH(aq) → Na₂ZnO₂(aq) + H₂(g) (Sodium zincate).

Reactions NOT Producing H₂:

Mixing water to Plaster of Paris (hydration reaction).
Carbonates (e.g., CaCO₃) with acids (produce CO₂).

Industrial Preparation of Dihydrogen (H₂):

Electrolysis of Acidified Water:

Electrolysis of acidified water (e.g., with trace H₂SO₄) using platinum electrodes.
2H₂O(l) --(electrolysis)--> 2H₂(g) + O₂(g).
Yields very pure H₂.

Electrolysis of Brine (Aqueous NaCl Solution):

H₂ is obtained as a byproduct during the electrolytic production of NaOH and Cl₂.
2NaCl(aq) + 2H₂O(l) --(electrolysis)--> 2NaOH(aq) + Cl₂(g) + H₂(g).

Steam Reforming of Hydrocarbons or Coke:

From Steam on Hot Coke: C(s) + H₂O(g) --(1270 K)--> CO(g) + H₂(g) (Water gas).
From Hydrocarbons (e.g., Methane) and Steam: CH₄(g) + H₂O(g) --(Ni catalyst, 1270 K)-> CO(g) + 3H₂(g) (Syngas).
The mixture of CO and H₂ is called syngas or water gas.

Water-Gas Shift Reaction:

To increase the yield of H₂ from syngas (CO + H₂), carbon monoxide (CO) is reacted with steam.
CO(g) + H₂O(g) --(FeCrO₄ catalyst, 673 K)--> CO₂(g) + H₂(g).
CO₂ is then removed by scrubbing (e.g., with sodium arsenite solution).

Uses of Dihydrogen

Used in the hydrogenation of vegetable oils to produce Vanaspati Ghee/Vanaspati.
Raw material for the industrial synthesis of ammonia (Haber process).
Used in the synthesis of methanol (CH₃OH).
Used in the manufacture of hydrogen chloride (HCl).
Employed as a reducing agent in metallurgical processes to convert metal oxides to metals.
Used as a rocket fuel.
Used in fuel cells for electricity generation.
Utilized in atomic hydrogen and oxy-hydrogen torches for cutting and welding metals.
Highly combustible, making it less safe for filling balloons compared to non-flammable gases like Helium.

Water (H2O): Structure, physical and chemical properties

Structure: Bent (V-shaped) geometry with a bond angle of approximately 104.5°. Oxygen is sp3 hybridized and has two lone pairs.
Polarity: A highly polar molecule due to its bent shape and the significant electronegativity difference between oxygen and hydrogen. Possesses a high dipole moment.
Hydrogen Bonding: Forms strong intermolecular hydrogen bonds (H-bonds). Each H₂O molecule can form up to 4 H-bonds with other water molecules.
Physical Properties (Influenced by Hydrogen Bonding & Polarity):

High Boiling Point & Melting Point: Significantly higher than hydrides of similar molecular weight (e.g., H₂S) due to strong intermolecular H-bonds requiring more energy to break.
High Heat of Fusion: Large amount of energy is required to convert ice to liquid water.
High Heat of Vaporization: Large amount of energy is required to convert liquid water to steam.
High Specific Heat Capacity: Water resists temperature changes effectively due to H-bonds absorbing/releasing large amounts of energy.
Excellent Solvent (Universal Solvent): Its high polarity and dipole moment enable it to dissolve a wide range of ionic compounds and many polar covalent substances.
Density Anomaly:

Maximum density at 4°C.
Ice (solid water) is less dense than liquid water and thus floats.
Reason: Hydrogen bonding in ice leads to an open, cage-like three-dimensional structure with interstitial holes, resulting in a larger volume and lower density compared to liquid water where H-bonds are constantly breaking and reforming.

Chemical Properties: Water is amphoteric, meaning it can act as both an acid and a base.
Purest Form of Water: Rainwater (naturally occurring); Distilled water (laboratory purified) is considered the purest.

9. Sulphur, Nitrogen, Halogen, Inert

Gases

Practice MCQs: Hydrogen and Its Compounds

Hard and Soft Water: Causes of hardness and methods for removal

Which of the following substances cause temporary hardness in water? 1. Mg(HCO3)2 2. Ca(HCO3)2 3. CaCl2 4. MgSO4 Select the correct answer using the code given below.
Asked in: UPSC CDS 1 2018
1. 3 and 4
2. 2 and 3
3. 1 and 4
4. 1 and 2
Show answer
Answer: D. 1 and 2
Temporary hardness is primarily caused by dissolved calcium and magnesium bicarbonates.
Permanent hardness of water cannot be removed by which one of the following methods?
Asked in: UPSC NDA 2 2018
1. Treatment with washing soda
2. Calgon's method
3. Boiling
4. Ion exchange method
Show answer
Answer: C. Boiling
Pure, demineralized water, free from all soluble mineral salts is obtained by which of the following method?
Asked in: UPSC CAPF 2023
1. Passing water through microfiltration mem‐ brane
2. Calgon’s method
3. Passing water through a cation exchange and an anion exchange resin bed
4. By boiling
Show answer
Answer: C. Passing water through a cation exchange and an anion exchange resin bed
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).

Heavy Water (D2O): Preparation, properties, and uses

Which of the following statements are correct regarding Heavy Water? 1. Heavy water occurs naturally or it is produced by the process of electrolysis. 2. Pure heavy water is radioactive. 3. Heavy water is used in nuclear power plants as a coolant. 4. Though algae and bacteria can live using 100% heavy water, but consumption of heavy water by humans may cause sterilisation.
Asked in: GPSC Prelims CSP 2 2022
1. 1, 2, 3 and 4
2. 2 and 3 only
3. 1, 2 and 4 only
4. 1, 3 and 4 only
Show answer
Answer: D. 1, 3 and 4 only
Heavy water (D2O) is non-radioactive, occurs naturally, is used in certain nuclear reactors, and can have biological effects if consumed in large quantities by complex organisms.
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:
Asked in: UPSC NDA 2 2015
1. 1 and 2
2. 2 and 3
3. 3 and 4
4. 1 and 4
Show answer
Answer: D. 1 and 4
This question tests specific properties of heavy water compared to ordinary water.
Heavy water is so called because it
Asked in: UPSC CAPF 2011
1. contains a number of salts as in sea water
2. is the mercury contaminated water
3. is saline water used in hospitals
4. contains the heavier isotope of hydrogen in water
Show answer
Answer: D. contains the heavier isotope of hydrogen in water
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 of an atomic reactor is :
Asked in: UPSC CAPF 2015
1. deionised water
2. an oxide of heavier isotope of oxygen
3. a mixture of ice and water
4. an oxide of heavier isotope of hydrogen
Show answer
Answer: D. an oxide of 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.
Which one of the following is the chemical name of heavy water ?
Asked in: UPSC CAPF 2017
1. Hydrogen oxide
2. Deuterium dioxide
3. Deuterium oxide
4. Heavy hydrogen oxide
Show answer
Answer: C. Deuterium oxide
Heavy water is water composed of deuterium and oxygen. Deuterium is an isotope of hydrogen.
Heavy water (Nuclear science)
Asked in: UPPSC Upper Mains 2007
1. contains more dissolved air
2. contains deuterium in place of hydrogen
3. contains more dissolved minerals and salts
4. contains organic impurities
Show answer
Answer: B. contains deuterium in place of hydrogen
Heavy water is a form of water in which the hydrogen atoms are replaced by deuterium, an isotope of hydrogen that has one neutron in addition to the proton (unlike standard hydrogen, protium, which has only a proton). Its chemical formula is D₂O or ²H₂O.
Heavy water is
Asked in: BJS 2023
1. pH7
2. rainwater
3. deuterium oxide
4. tritium oxide
Show answer
Answer: C. deuterium oxide
This question tests knowledge of basic chemistry, specifically isotopes and their compounds.
The water in which hydrogen atoms have been replaced by deuterium atoms is called
Asked in: Group C Exam 2011
1. Warm water
2. Cold water
3. Heavy water
4. Volatile water
Show answer
Answer: C. Heavy water
Heavy water (D₂O) is a form of water with a different isotopic composition.

Hydrides: Classification (Ionic, Covalent, Metallic/Interstitial)

A very large volume of hydrogen can be accommodated by making
Asked in: UPSC NDA 2 2019
1. non-metallic hydrides.
2. hydrogen peroxide.
3. non-stoichiometric hydrides.
4. alkali metal hydrides.
Show answer
Answer: 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.
The compounds formed when hydrogen combines with p-block elements are called
Asked in: TNPSC CS Prelims 2017
1. Ionic hydrides
2. Molecular hydrides
3. Metallic hydrides
4. Polynuclear hydrides
Show answer
Answer: B. Molecular hydrides
Hydrides are binary compounds formed between hydrogen and other elements. They are broadly classified into three main types based on their chemical bonding and properties: 1. Ionic (or Saline) hydrides: Formed by alkali metals (Group 1) and larger alkaline earth metals (Ca, Sr, Ba - Group 2). These are ionic compounds like NaH, CaH₂. 2. Metallic (or Interstitial) hydrides: Formed by most d-block and f-block elements. Hydrogen atoms occupy interstitial sites in the metal lattice. Examples include TiH₂, PdHₓ. 3. Covalent (or Molecular) hydrides: Formed by most p-block elements, as well as some s-block elements like Be and Mg. These involve covalent bonds between hydrogen and the other element. Examples include CH₄, NH₃, H₂O, HCl, PH₃, H₂S.

Isotopes of hydrogen: Protium, Deuterium, Tritium and their properties

Which one of the following statements is not correct?
Asked in: UPSC CDS 1 2016
1. Hydrogen is an element
2. Hydrogen is the lightest element
3. Hydrogen has no isotopes
4. Hydrogen and oxygen form an explosive mix‐ ture
Show answer
Answer: C. Hydrogen has no isotopes
Hydrogen is indeed an element and the lightest element. A mixture of hydrogen and oxygen is highly explosive. However, hydrogen has three naturally occurring isotopes: Protium (¹H), Deuterium (²H or D), and Tritium (³H or T). They differ in the number of neutrons (0, 1, and 2 respectively) while all having one proton.
The radioactive isotope of hydrogen is
Asked in: UPSC CDS 2 2020
1. protium
2. deuterium
3. tritium
4. hydronium
Show answer
Answer: C. tritium
Hydrogen has three naturally occurring isotopes: Protium (¹H, 1 proton, 0 neutrons), Deuterium (²H or D, 1 proton, 1 neutron), and Tritium (³H or T, 1 proton, 2 neutrons). Protium and Deuterium are stable isotopes.
Assertion - Chemical properties of Protium ($^1$H), Deuterium ($^2$D) and Tritium ($^3$T) are same. Reason - $^1$H, $^2$D and $^3$T are isobars. Choose the appropriate option. - Question not attempted
Asked in: Department Recruitment 2024 (Chemist) 2024
1. Assertion is correct but Reason is incorrect.
2. Assertion and Reason both are correct but Reason does not explain the Assertion.
3. Assertion and Reason both are correct, also Reason explains the Assertion.
4. Assertion is incorrect but Reason is correct.
Show answer
Answer: A. Assertion is correct but Reason is incorrect.
The assertion relates to the chemical properties of hydrogen isotopes, while the reason describes their relationship in terms of nuclear mass.

Physical and chemical properties of Dihydrogen

Which one of the following statements is correct?
Asked in: UPSC CDS 2 2016
1. The oxidation number for hydrogen is always zero.
2. The oxidation number for hydrogen is always +1.
3. The oxidation number for hydrogen is always -1.
4. Hydrogen can have more than one oxidation number.
Show answer
Answer: D. Hydrogen can have more than one oxidation number.
The oxidation number for hydrogen is typically +1 when bonded to nonmetals (e.g., H₂O, HCl, NH₃). However, when bonded to more active metals (forming metal hydrides like NaH, CaH₂), hydrogen is more electronegative than the metal and has an oxidation number of -1. In its elemental form (H₂), the oxidation number is 0.
Which one of the following gases is the lightest?
Asked in: UPSC Geoscientist 2022
1. Hydrogen
2. Helium
3. Oxygen
4. Nitrogen
Show answer
Answer: A. Hydrogen
- Gas density is directly proportional to its molar mass at constant temperature and pressure (Ideal Gas Law). - Comparing molar masses allows identification of the lightest gas.
Which one of the following statements about dihydrogen (H₂) is not correct ?
Asked in: UPSC NDA 1 2021
1. H₂ is lighter than air and insoluble in water.
2. H₂ is inert at room temperature due to high H – H bond dissociation enthalpy.
3. H₂ reacts with alkali metals at high temperat‐ ure to yield metal hydrides.
4. A mixture of NO₂ and H₂ is known as Syngas.
Show answer
Answer: D. A mixture of NO₂ and H₂ is known as Syngas.
Syngas is a fuel gas mixture that can be produced from various carbon-containing materials like natural gas, coal, biomass, or plastic waste. It is used as a fuel or as an intermediate for the production of other chemicals like methanol and ammonia. Nitrogen dioxide (NO₂) is a toxic gas primarily associated with air pollution.
Which one of the following is the colour of hydrogen gas?
Asked in: UPSC NDA 1 2022
1. Light yellow
2. Orange
3. Black
4. Colourless
Show answer
Answer: D. Colourless
- Hydrogen is the lightest and most abundant element in the universe. - At standard temperature and pressure, hydrogen exists as a diatomic molecule (H₂) and is a gas. - Pure hydrogen gas is colourless, odourless, and tasteless. - It is also highly flammable.
Which of the following gasses is/are lighter than air? 1. Hydrogen chloride 2. Sulfur dioxide 3. Helium . Select the correct answer from the code given below : Code :
Asked in: uppsc 2025 2025
1. 1 and 2
2. 2 and 3
3. Only 3
4. Only 1
Show answer
Answer: C. Only 3
To determine if a gas is lighter than air, we need to compare its molecular weight with the average molecular weight of air. The average molecular weight of air is approximately 29 g/mol (about 80% N2, molecular weight 28 g/mol, and 20% O2, molecular weight 32 g/mol). 1. Hydrogen chloride g/mol. This is heavier than air. 2. Sulfur dioxide (SO2): Molecular weight = 32 (S) + 2 16 (O) = 64 g/mol. This is much heavier than air. 3. Helium (He): Atomic weight = approximately 4 g/mol. This is significantly lighter than air.

Position of hydrogen in the periodic table

The only non-metal which forms positive ion is
Asked in: ae 2020
1. hydrogen
2. helium
3. argon
4. oxygen
Show answer
Answer: A. hydrogen
Non-metals generally gain electrons to form negative ions (anions) because they have high electronegativity. However, hydrogen is unique because it can also lose its single electron to form a positive ion (cation), H+. This is especially evident in its compounds with highly electronegative elements like oxygen and halogens (e.g., HCl, H2O).

Preparation of Dihydrogen (H2): Laboratory and Industrial methods

Reaction between which of the following two reactants will produce hydrogen gas?
Asked in: UPSC CDS 2 2018
1. Magnesium and hydrochloric acid
2. Copper and dilute nitric acid
3. Calcium carbonate and hydrochloric acid
4. Zinc and nitric acid
Show answer
Answer: A. Magnesium and hydrochloric acid
Reactive metals, generally those above hydrogen in the reactivity series (like Magnesium, Zinc, etc.), react with dilute acids to displace hydrogen and produce hydrogen gas and a salt.
Which of the following reactants can be used to prepare ammonia ? a. NH$_{4}$Cl + Ca(OH)$_{2}$ b. N$_{2}$ + H$_{2}$ c. CaCN$_{2}$ + H$_{2}$O
Asked in: MPSC CS Prelims 2019
1. a and b
2. b and c
3. a and c
4. a, b and c
Show answer
Answer: D. a, b and c
Ammonia can be prepared through various chemical reactions, including the reaction of ammonium salts with strong bases, direct synthesis from nitrogen and hydrogen, and hydrolysis of calcium cyanamide.
Which one of the following reactions does not result in the evolution of hydrogen gas ?
Asked in: UPSC NDA 1 2021
1. Reaction of zinc metal with dilute sulphuric acid solution
2. Mixing water to Plaster of Paris
3. Heating zinc metal with sodium hydroxide solution
4. Reaction of potassium metal with water
Show answer
Answer: B. Mixing water to Plaster of Paris
The reactions in options A, C, and D all involve the liberation of hydrogen gas: A) Active metals like zinc react with dilute acids (Zn + H₂SO₄ → ZnSO₄ + H₂). C) Amphoteric metals like zinc react with strong bases upon heating (Zn + 2NaOH → Na₂ZnO₂ + H₂). D) Highly reactive metals like potassium react vigorously with water (2K + 2H₂O → 2KOH + H₂).
The gas generated on reacting zinc with dilute sulphuric acid is :
Asked in: UPSC CAPF 2023
1. Argon
2. Helium
3. Hydrogen
4. Nitrogen
Show answer
Answer: C. Hydrogen
Active metals react with dilute acids to produce hydrogen gas and a salt.
Which of the following gases combines together to produce ammonia gas?
Asked in: Assistant Engineer (Civil) 2018 19 2018
1. Nitrogen and Oxygen
2. Nitrogen and Helium
3. Nitrogen and Hydrogen
4. Nitrogen and Carbon dioxide
Show answer
Answer: C. Nitrogen and Hydrogen
This question tests basic knowledge of chemical reactions and the synthesis of common gases.

Uses of Dihydrogen

The gas used in the manufacture of Vanaspati Ghee from Vanaspati Oil is
Asked in: BPSC Prelims Exam 60 62rd UPPSC Upper Mains 2013
1. Hydrogen
2. Oxygen
3. Nitrogen
4. Carbon dioxide
Show answer
Answer: A. Hydrogen
Hydrogenation is a chemical process used to saturate organic compounds with hydrogen. It is commonly applied in the food industry to convert liquid oils into solid or semi-solid fats.
The gas used in the manufacture of Vanaspati from vegetable oil is
Asked in: UPPSC Upper Mains 2007
1. Oxygen
2. Nitrogen
3. Hydrogen
4. Carbon dioxide
Show answer
Answer: C. Hydrogen
Hydrogenation is a chemical reaction in which hydrogen is added to unsaturated organic compounds, such as vegetable oils, in the presence of a catalyst (commonly nickel). This process increases the saturation of the fatty acids in the oil, changing its physical properties from liquid to solid or semi- solid at room temperature.
It would be less safe to fill a gas balloon with the following gas :
Asked in: APPSC Panchayat secretary Prelims 2016
1. Nitrogen
2. Air
3. Hydrogen
4. Helium
Show answer
Answer: C. Hydrogen
Helium (He) is also lighter than air and makes balloons float, but it is an inert gas and non- flammable, making it a much safer choice than Hydrogen for balloons.

Water (H2O): Structure, physical and chemical properties

Which one of the following statements about water is not true ?
Asked in: UPSC NDA 2 2019
1. Hydrogen bonds are present in liquid water.
2. Water has a high boiling point.
3. Water has a high heat of fusion.
4. Water is a non-polar molecule.
Show answer
Answer: D. Water is a non-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.
Which property of water makes it a good solvent?
Asked in: PPSC CS Prelims 2015
1. High boiling point
2. High dipole moment
3. High specific heat
4. High specific gravity
Show answer
Answer: B. High dipole moment
Water is a polar molecule because of the uneven distribution of charge due to the higher electronegativity of oxygen compared to hydrogen. This creates a significant dipole moment. The polarity of water molecules allows them to form strong interactions (hydrogen bonds) with other polar molecules and ions, effectively surrounding and separating solute particles, which is the process of dissolution.
Ice is lighter than pure water and floats on the surface. Which one of the following statements is correct to explain this phenomenon ?
Asked in: UPSC CAPF 2010
1. The structure of ice is similar to that of water
2. Ice has a disordered 3-dimensional structure
3. Hydrogen-bonding in ice gives an open type structure with interstitial holes
4. There is no hydrogen bonding in water
Show answer
Answer: C. 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.
Water is essential to life because it has many special properties. Which one of the following is a special property of water?
Asked in: WBCS Main Paper 4 2022
1. It is able to covalently bond to other water mo‐ lecules
2. It is good at dissolving other substances
3. It easily heats up
4. It easily cools
Show answer
Answer: B. It is good at dissolving other substances
Water is known as the "universal solvent" because of its polar nature, which allows it to dissolve a wide range of polar and ionic substances. This property is crucial for biological processes, as it facilitates the transport of nutrients and removal of waste.
पानी का शुद्धम रूप है :
Asked in: Assistant Prosecuting Officer (APO) Exam 2021
1. कु एँ का जल
2. भूlमगत जल
3. सतह जल
4. बरसात का जल
Show answer
Answer: D. बरसात का जल
Rainwater is the purest form of natural water among the given options as it is least contaminated with dissolved minerals and salts.
Intramolecular Hydrogen bond is present in which of the following?
Asked in: Department Recruitment 2024 (Chemist) 2024
1. (1) NH₃
2. (2) H₂O
3. (3) o-nitrophenol
4. (4) p-nitrophenol
Show answer
Answer: C. (3) o-nitrophenol
Intramolecular hydrogen bonds are formed within a single molecule. This requires a specific spatial arrangement where a hydrogen atom bonded to an electronegative atom is close enough to another electronegative atom within the same molecule. Ortho-substituted phenols are common examples where intramolecular hydrogen bonding occurs.
Dry ice is - solid $\text{NH}_3$ - solid $ \text{CO}_2$ - solid $\text{H}_2\text{O}$ - solid $\text{SO}_2$
Asked in: Lab Assistant Higher Education (Group C) Exam 2023
1. solid $\text{NH}_3$
2. solid $\text{CO}_2$
3. solid $\text{H}_2\text{O}$
4. solid $\text{SO}_2$
Show answer
Answer: B. solid $\text{CO}_2$
Dry ice is the solid state of carbon dioxide ($ \text{CO}_2$), characterized by its sublimation at room temperature and pressure.
Dry ice is
1. solid NH₃
2. solid CO₂
3. solid H₂O
4. solid SO₂
Show answer
Answer: B. solid CO₂

Hard and Soft Water: Causes of hardness and methods for removal

Heavy Water (D2O): Preparation, properties, and uses

Hydrides: Classification (Ionic, Covalent, Metallic/Interstitial)

Isotopes of hydrogen: Protium, Deuterium, Tritium and their properties

Physical and chemical properties of Dihydrogen

Position of hydrogen in the periodic table

Preparation of Dihydrogen (H2): Laboratory and Industrial methods

Uses of Dihydrogen

Water (H2O): Structure, physical and chemical properties

9. Sulphur, Nitrogen, Halogen, Inert

Gases

Practice MCQs: Hydrogen and Its Compounds

Hard and Soft Water: Causes of hardness and methods for removal

Heavy Water (D2O): Preparation, properties, and uses

Hydrides: Classification (Ionic, Covalent, Metallic/Interstitial)

Isotopes of hydrogen: Protium, Deuterium, Tritium and their properties

Physical and chemical properties of Dihydrogen

Position of hydrogen in the periodic table

Preparation of Dihydrogen (H2): Laboratory and Industrial methods

Uses of Dihydrogen

Water (H2O): Structure, physical and chemical properties

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