What is the difference between hard and soft water?

Hard water contains dissolved Ca²⁺ and Mg²⁺ salts (bicarbonates, chlorides, sulfates) which prevent lather formation with soap. Soft water is free of these ions and lathers easily with soap. Temporary hardness (due to bicarbonates) is removed by boiling; permanent hardness (due to chlorides and sulfates) requires ion exchange or treatment with washing soda.

Hydrogen — NCERT Class 11 Chemistry

Q: What are the different types of hydrides?

Hydrides are classified into three types: (1) Ionic or saline hydrides — formed by Group 1 and 2 metals (e.g., NaH, CaH₂), crystalline solids, good conductors when molten. (2) Covalent or molecular hydrides — formed by p-block elements (e.g., CH₄, NH₃, H₂O), exist as discrete molecules, volatile. (3) Metallic or interstitial hydrides — formed by d- and f-block elements (e.g., TiH₂, VH), hydrogen occupies interstitial sites in the metal lattice.

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This page provides complete, step-by-step NCERT solutions for CBSE Class 11 Chemistry Chapter 9 — Hydrogen. Every intext question (grouped by page number) and every chapter-end exercise question is answered with clear explanations, chemical equations, and conceptual reasoning. Use the section headings to jump to the question you need help with.

Key Concepts (Quick Recap)

Position of Hydrogen: Resembles alkali metals (Group 1: one valence electron, forms H⁺) and halogens (Group 17: needs one electron, forms H⁻, diatomic H₂). Placed separately.
Isotopes of H: ¹H (protium), ²H or D (deuterium), ³H or T (tritium). Protium is most abundant.
Preparation of H₂: Laboratory — Zn + dil. H₂SO₄; Industrial — steam reforming of methane (CH₄ + H₂O → CO + 3H₂); electrolysis of water.
Properties of H₂: Colourless, odourless, lighter than air, insoluble in water, neutral gas. Burns with pale blue flame.
Ionic (Saline) Hydrides: Formed by Group 1 and 2 metals. NaH, CaH₂. Crystalline, conduct electricity when molten, react vigorously with water to give H₂.
Covalent (Molecular) Hydrides: Formed by p-block elements. CH₄, NH₃, H₂O, HF. Volatile molecular compounds. Further classified as electron-deficient (BH₃), electron-precise (CH₄), and electron-rich (NH₃, H₂O).
Metallic (Interstitial) Hydrides: Formed by transition metals. H₂ occupies interstitial sites. Non-stoichiometric (e.g., TiH₁.₇).
Water Structure: V-shaped, H–O–H angle ≈ 104.5°, extensive H-bonding, high boiling point, high specific heat.
Hard Water: Contains Ca²⁺/Mg²⁺. Temporary hardness (bicarbonates) — removed by boiling. Permanent hardness (chlorides, sulfates) — removed by washing soda (Na₂CO₃), ion exchange, or Calgon's method.
Hydrogen Peroxide (H₂O₂): Pale blue liquid, strong oxidizing agent, also reducing agent. Decomposes to H₂O and O₂. Uses: bleaching, antiseptic, rocket fuel.
Heavy Water (D₂O): Used as moderator in nuclear reactors, tracer in metabolic studies.
Dihydrogen as Fuel: Clean fuel (product is H₂O), high calorific value, but storage and transport challenges.

Intext Questions

Page 222–223

Q1. Justify the position of hydrogen in the periodic table.

AnswerHydrogen is placed separately in the periodic table because it shows dual behaviour:

Resemblance with alkali metals (Group 1):

One electron in valence shell (1s¹)
Forms H⁺ ion (like Na⁺, K⁺)
Reacts with halogens to form HX (like NaX)
Reducing agent like alkali metals

Resemblance with halogens (Group 17):

Needs one electron to complete its shell (like F, Cl)
Forms H⁻ ion (hydride), like Cl⁻
Exists as a diatomic molecule (H₂), like F₂, Cl₂
High ionization enthalpy, high electronegativity

Due to this dual character, hydrogen is not placed in Group 1 or Group 17 but separately at the top.

Q2. Write the names of isotopes of hydrogen. What is the difference between them?

AnswerThe three isotopes of hydrogen are: 1. Protium (¹H): 1 proton, 0 neutrons, mass number = 1 2. Deuterium (²H or D): 1 proton, 1 neutron, mass number = 2 3. Tritium (³H or T): 1 proton, 2 neutrons, mass number = 3

They differ in the number of neutrons and hence in mass. They have the same electronic configuration (1s¹) and similar chemical properties, but show differences in reaction rates (kinetic isotope effect). Physical properties like density, boiling point, and freezing point differ due to mass differences.

Q3. Why is dihydrogen not preferred as a fuel despite its high calorific value?

AnswerDespite its high calorific value, dihydrogen is not preferred as a common fuel because:

It is difficult to store and transport (being a gas, it requires high pressure or cryogenic temperatures)
Handling is hazardous (explosive when mixed with air)
Lack of infrastructure for production, storage, and distribution
Currently expensive to produce from water using renewable energy
Low boiling point (−253 °C) makes liquefaction energy-intensive

Page 228–230

Q4. How is dihydrogen prepared in the laboratory?

AnswerLaboratory preparation:

From metals and acids: Zn + dil. H₂SO₄ → ZnSO₄ + H₂↑
From metals and water: 2Na + 2H₂O → 2NaOH + H₂↑ (cold water, violent)
From metals and alkali: Zn + 2NaOH → Na₂ZnO₂ + H₂↑ (sodium zincate)

Industrial methods:

Steam reforming: CH₄ + H₂O → CO + 3H₂ (at 1270 K, Ni catalyst)
Coal gasification: C + H₂O → CO + H₂ (water-gas reaction)
Electrolysis of water: 2H₂O → 2H₂ + O₂

Q5. What are ionic hydrides? Give two examples.

AnswerIonic (saline) hydrides are formed when hydrogen combines with highly electropositive metals (Group 1 and Group 2 elements). In these hydrides, hydrogen exists as H⁻ ion.

Properties: Crystalline solids, high melting points, conduct electricity when molten, react vigorously with water to give H₂.

Examples: NaH (sodium hydride), CaH₂ (calcium hydride)

Reaction with water: CaH₂ + 2H₂O → Ca(OH)₂ + 2H₂↑

Q6. Classify the following as electron-deficient, electron-precise, or electron-rich hydrides: (a) BH₃ (b) CH₄ (c) NH₃ (d) H₂O

Answer

(a) BH₃: Electron-deficient — B has only 6 valence electrons (3 bonds), needs 2 more for octet.

(b) CH₄: Electron-precise — C has exactly 8 electrons (4 bonds), complete octet, no lone pairs.

(d) H₂O: Electron-rich — O has 8 electrons (2 bonds + 2 lone pairs), has lone pairs available for donation.

Q7. Explain the structure of water.

AnswerWater (H₂O) has a V-shaped (bent) structure with:

O–H bond length: 95.7 pm
H–O–H bond angle: 104.5° (less than ideal tetrahedral 109.5° due to lone pair–bond pair repulsion)
Oxygen is sp³ hybridized with two bond pairs and two lone pairs
Extensive hydrogen bonding between water molecules accounts for:
High boiling point (100 °C vs predicted ~−100 °C)
High specific heat capacity
High surface tension
Anomalous density behaviour (ice floats on water — maximum density at 4 °C)

Page 236–237

Q8. What is hydrogen peroxide? How is it prepared?

AnswerHydrogen peroxide (H₂O₂) is a pale blue liquid with the structure H–O–O–H (dihydrogen dioxide). The O–O bond is called the peroxide linkage.

Preparation methods:

Laboratory method: Na₂O₂ + H₂SO₄ → Na₂SO₄ + H₂O₂ (cold, dilute acid)

Industrial method (anthraquinone process): Anthraquinone is hydrogenated then oxidized by air to give H₂O₂ and regenerate anthraquinone.

From barium peroxide: BaO₂ + H₂SO₄ → BaSO₄↓ + H₂O₂

Q9. What are the uses of hydrogen peroxide?

Answer1. Bleaching agent — for textiles, paper, hair 2. Antiseptic and disinfectant — for cleaning wounds (3% solution) 3. Oxidizing agent — in chemical reactions 4. Reducing agent — with strong oxidizers like KMnO₄ 5. Rocket propellant — as oxidizer with hydrazine 6. Environmental applications — treating industrial effluents 7. Manufacturing — sodium perborate (detergent bleach)

Q10. What is heavy water? How is it used?

AnswerHeavy water (D₂O) is water in which both hydrogen atoms are replaced by deuterium (²H).

Uses: 1. Moderator in nuclear reactors — slows down fast-moving neutrons for sustained fission 2. Tracer compound — in metabolic and mechanistic studies (deuterium labeling) 3. Exchange reactions — to study reaction mechanisms 4. Production of deuterium — by electrolysis of D₂O

Q11. Discuss the consequences of using hard water.

Answer1. Does not lather with soap — wastes soap (forms scum with Ca²⁺/Mg²⁺) 2. Scale formation — in boilers, pipes, and kettles, reducing efficiency and causing blockages 3. Corrosion — metal pipes may corrode 4. Cooking — hard water may toughen vegetables and affect taste 5. Industrial use — unsuitable for dyeing, laundering, and paper manufacturing

Chapter-End Exercise

Q1. Hydrogen resembles halogens in which of the following properties? (a) Electronegativity (b) Ionization enthalpy (c) Diatomic nature (d) Both (a) and (c)

Answer(d) Both (a) and (c). Hydrogen has high electronegativity (2.1, close to halogens) and exists as a diatomic molecule (H₂, like F₂, Cl₂).

Q2. Why does hydrogen occupy a unique position in the periodic table?

AnswerHydrogen shows properties of both alkali metals (Group 1) and halogens (Group 17). Like alkali metals, it has one valence electron, forms H⁺, and is a reducing agent. Like halogens, it needs one electron to complete its shell, forms H⁻, exists as H₂, and has high ionization enthalpy. Since it fits neither group completely, it occupies a unique position.

Q3. Write the electronic configuration of hydrogen and explain its resemblance to alkali metals and halogens.

AnswerElectronic configuration of H: 1s¹

Resemblance to alkali metals:

1s¹ configuration similar to ns¹ of Group 1
Loses one electron to form H⁺ (like Na⁺, K⁺)
Acts as a reducing agent
Forms oxides (H₂O like Na₂O), halides (HCl like NaCl)

Resemblance to halogens:

Needs one electron for noble gas configuration (like F, Cl)
Forms H⁻ (hydride), like Cl⁻
Exists as diatomic molecule H₂
High electronegativity and ionization enthalpy
Forms covalent compounds with non-metals

Q4. Differentiate between the three isotopes of hydrogen.

Answer

Property	Protium (¹H)	Deuterium (²H/D)	Tritium (³H/T)
No. of protons	1	1	1
No. of neutrons	0	1	2
Mass number	1	2	3
Abundance	99.985%	0.015%	Trace (radioactive)
Nature	Stable	Stable	Radioactive (β-emitter)

Q5. How is dihydrogen prepared from: (a) metals and acids, (b) metals and water, (c) metal hydrides?

Answer

(a) Metals + acids: Zn + dil. H₂SO₄ → ZnSO₄ + H₂↑

(b) Metals + water:

3Fe + 4H₂O (steam) → Fe₃O₄ + 4H₂↑

Cold water (active metals): 2Na + 2H₂O → 2NaOH + H₂↑
Steam (less active metals): Mg + 2H₂O (steam) → Mg(OH)₂ + H₂↑

Q6. Discuss the consequences of high bond dissociation enthalpy of the H–H bond.

AnswerThe H–H bond dissociation enthalpy is very high (435.8 kJ mol⁻¹) due to the small size of hydrogen and effective orbital overlap. This has the following consequences:

1. Low reactivity at room temperature — H₂ is relatively inert under ordinary conditions 2. High activation energy needed for reactions involving H₂ 3. Inertness as a gas — H₂ does not react easily with most substances at room temperature 4. Need for catalysts or high temperatures — many reactions of H₂ require catalysts (e.g., hydrogenation needs Ni/Pd) or elevated temperatures

Q7. What are the different types of hydrides? Give one example of each.

Answer

1. Ionic (Saline) Hydrides: Formed by Group 1 and 2 metals. H exists as H⁻. Example: NaH (sodium hydride). React vigorously with water.

2. Covalent (Molecular) Hydrides: Formed by p-block elements. Discrete molecules. Example: H₂O (water). Can be electron-deficient (BH₃), electron-precise (CH₄), or electron-rich (NH₃).

3. Metallic (Interstitial) Hydrides: Formed by d- and f-block elements. H₂ occupies interstitial spaces. Example: TiH₂ (titanium hydride). Often non-stoichiometric.

Q8. Explain why water has a higher boiling point than H₂S, despite H₂S having a higher molecular mass.

AnswerWater has a much higher boiling point (100 °C) compared to H₂S (−60 °C) because of extensive hydrogen bonding in water. Each water molecule can form up to 4 hydrogen bonds (2 through H atoms and 2 through lone pairs on O). These strong intermolecular H-bonds require significant energy to break, resulting in a high boiling point. In H₂S, hydrogen bonding is negligible (S is less electronegative than O), so only weak van der Waals forces operate, giving it a much lower boiling point despite higher molecular mass.

Q9. Why does ice float on water?

AnswerIce floats because it is less dense than liquid water. In ice, water molecules form an extensive hydrogen-bonded network with a hexagonal structure that creates empty spaces (channels). This makes ice about 9% less dense than liquid water (density of ice = 0.917 g/cm³, water = 1.0 g/cm³ at 0 °C). This unusual property (most solids are denser than their liquids) is crucial for aquatic life — ice insulates the water below, preventing it from freezing solid.

Q10. Explain the term 'heavy water'.

AnswerHeavy water (D₂O) is water in which both hydrogen atoms are replaced by deuterium (²H or D). Its molecular mass is 20 (vs. 18 for H₂O). It is prepared by prolonged electrolysis of ordinary water (deuterium accumulates because H₂O decomposes 6 times faster than D₂O). Properties: higher boiling point (101.4 °C), higher density (1.11 g/cm³), and lower dielectric constant than H₂O.

Q11. Write the chemical reactions to show the amphoteric nature of water.

AnswerWater acts as both an acid and a base (amphoteric):

As a base (with strong acid): H₂O + HCl → H₃O⁺ + Cl⁻ (water accepts H⁺)

As an acid (with strong base): H₂O + NH₃ → NH₄⁺ + OH⁻ (water donates H⁺)

This amphoteric nature is explained by the Bronsted-Lowry theory — water can donate or accept a proton depending on the species it reacts with.

Q12. What is the difference between temporary and permanent hardness of water?

Answer

Feature	Temporary Hardness	Permanent Hardness
Cause	Bicarbonates of Ca²⁺/Mg²⁺	Chlorides and sulfates of Ca²⁺/Mg²⁺
Removal	By boiling (bicarbonates decompose to insoluble carbonates)	Cannot be removed by boiling
Chemical change on boiling	Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂	No change on boiling
Removal methods	Boiling, Clark's method	Ion exchange, washing soda, Calgon's method

Q13. Describe the peroxide linkage in H₂O₂.

AnswerIn H₂O₂, the two oxygen atoms are connected by a single bond called the peroxide linkage (–O–O–). The structure is H–O–O–H. The O–O bond length is 147.5 pm (longer than O₂ at 121 pm), and each O–H bond length is 95 pm. The molecule is non-planar — it has a skewed (open-book) structure with a dihedral angle of 111.5° in the solid state and about 90° in the gas phase. The O–O bond is weak (bond energy ≈ 142 kJ/mol), which is why H₂O₂ easily decomposes.

Q14. Why is H₂O₂ stored in dark, plastic bottles?

AnswerH₂O₂ decomposes on exposure to light and in the presence of metal ions (catalysts): 2H₂O₂ → 2H₂O + O₂ + heat

This decomposition is accelerated by:

Light (photolytic decomposition)
Metal ions like Fe²⁺, Cu²⁺, Mn²⁺ (catalytic decomposition)
Rough surfaces (provide nucleation sites)
Heat (thermal decomposition)

Therefore, H₂O₂ is stored in dark, plastic bottles (or wax-lined glass) to minimize decomposition, and a stabilizer (like acetanilide) is often added.

Q15. H₂O₂ acts as both oxidizing and reducing agent. Give one reaction each.

Answer

As oxidizing agent (H₂O₂ gets reduced to H₂O): H₂O₂ + 2Fe²⁺ + 2H⁺ → 2Fe³⁺ + 2H₂O (in acidic medium)

PbS + 4H₂O₂ → PbSO₄ + 4H₂O (black PbS → white PbSO₄, used in restoring old paintings)

As reducing agent (H₂O₂ gets oxidized to O₂): 2KMnO₄ + 5H₂O₂ + 3H₂SO₄ → K₂SO₄ + 2MnSO₄ + 8H₂O + 5O₂

H₂O₂ + Cl₂ → 2HCl + O₂ (bleaching action)

Q16. Why are ionic hydrides also called saline hydrides?

AnswerIonic hydrides are called saline hydrides because they are salt-like in nature. They are crystalline solids with high melting points, conduct electricity when molten, and are formed by highly electropositive metals (like Na, Ca) with hydrogen — similar to the way salts are formed from metals and non-metals. Like NaCl, NaH is a crystalline ionic solid (Na⁺H⁻).

Q17. What is the use of hydrogen as a fuel? What are its advantages and limitations?

Answer

Advantages: 1. High calorific value (≈ 150 kJ/g, about 3 times that of gasoline) 2. Clean fuel — only product is H₂O (no CO₂, CO, or particulates) 3. Abundant raw material (water) 4. Renewable if produced from water using solar energy

Limitations: 1. Storage difficulty — requires high pressure or cryogenic temperatures 2. Safety concerns — highly flammable and explosive when mixed with air 3. High cost of production (electrolysis is expensive) 4. Lack of distribution infrastructure 5. Low energy density per unit volume (light gas)

Q18. Write balanced equations for the reaction of water with: (a) Sodium (b) Calcium (c) Iron (steam)

Answer

(a) 2Na + 2H₂O → 2NaOH + H₂↑ (vigorous, explosive)

(b) Ca + 2H₂O → Ca(OH)₂ + H₂↑ (less vigorous)

Q19. Explain the structure of H₂O₂.

AnswerH₂O₂ has a non-planar, open-book (skewed) structure:

The two O atoms are connected by a peroxide bond (O–O, 147.5 pm)
Each O is bonded to one H atom (O–H, 95 pm)
The H–O–O angle is approximately 94.8°
The dihedral angle between the two O–H planes is 111.5° (solid) and ~90° (gas)
It is not a linear molecule — the repulsion between lone pairs and bond pairs creates the bent structure

Q20. Differentiate between H₂O and D₂O in terms of physical properties.

Answer

Property	H₂O	D₂O
Molecular mass	18	20
Density (at 20 °C)	0.997 g/cm³	1.105 g/cm³
Boiling point	100 °C	101.4 °C
Freezing point	0 °C	3.8 °C
Viscosity	Lower	Higher
Dielectric constant	78.5	78.06

Q21. What happens when H₂O₂ is added to acidified KMnO₄ solution?

AnswerH₂O₂ acts as a reducing agent and decolorizes the purple KMnO₄ solution:

2KMnO₄ + 5H₂O₂ + 3H₂SO₄ → K₂SO₄ + 2MnSO₄ + 8H₂O + 5O₂↑

The purple colour of KMnO₄ disappears as MnO₄⁻ (purple) is reduced to Mn²⁺ (colourless). This reaction is used to estimate H₂O₂ volumetrically.

Q22. What is the structure of the water molecule?

AnswerThe water molecule has a bent (V-shaped) structure:

Central atom: Oxygen (sp³ hybridized)
Two O–H bonds and two lone pairs (tetrahedral electron geometry)
H–O–H bond angle: 104.5° (less than 109.5° due to greater lone pair repulsion)
O–H bond length: 95.7 pm
Dipole moment: 1.84 D (polar molecule)
Extensive hydrogen bonding between molecules

Q23. Why does H₂O₂ act as both oxidizing and reducing agent?

AnswerIn H₂O₂, oxygen has an oxidation number of −1, which is intermediate between 0 (in O₂) and −2 (in H₂O). This allows O to be both oxidized (−1 → 0, releasing O₂) and reduced (−1 → −2, forming H₂O). Therefore, H₂O₂ can act as both an oxidizing agent (getting reduced to H₂O) and a reducing agent (getting oxidized to O₂), depending on the reaction partner.

Q24. What is water gas? How is it produced?

AnswerWater gas is a mixture of CO and H₂, also called synthesis gas or syngas.

Production: C(s) + H₂O(g) → CO(g) + H₂(g) (at 1270 K)

It is called water gas because it is produced by passing steam over red-hot coke. Water gas is used as an industrial fuel and as a source of H₂ for the synthesis of ammonia (Haber process).

Q25. Discuss the biological importance of hydrogen.

Answer1. Water (essential for all life processes, major component of cells) 2. H⁺ ions maintain pH in biological systems (stomach acid, blood pH) 3. Hydrogen bonding stabilizes protein structure (alpha helix, beta sheet) and DNA double helix 4. Hydrogen in organic molecules — carbohydrates, proteins, lipids all contain hydrogen 5. Energy metabolism — NADH and FADH₂ carry hydrogen (and electrons) in cellular respiration 6. Heavy water (D₂O) — used as tracer in biochemical studies

Internal Links

[Chapter 9 Guide — Hydrogen](/school/cbse/class/11/chemistry/cbse-hydrogen)
[All Class 11 Chemistry Solutions](/school/cbse/class/11/chemistry/solutions)
[Class 11 Chemistry Subject Guide](/school/cbse/class/11/chemistry)

Frequently Asked Questions

Hydrogen resembles alkali metals (Group 1) in having one electron in its valence shell, forming H⁺ ion, and reacting with halogens. It also resembles halogens (Group 17) in needing one electron to complete its shell, forming H⁻ ion (hydride), and existing as a diatomic molecule (H₂). Because it shares properties with both groups but fits neatly in neither, it is placed separately at the top of the periodic table.