Carbon and Its Compounds
Properties of Covalent Compounds | कार्बन एवं उसके यौगिक
CBSE + Foundation + Competitive Level
Properties of Covalent Compounds | सहसंयोजक यौगिकों के गुण
1. Introduction | परिचय
English:
Covalent compounds are compounds formed by the sharing of electrons between atoms. Carbon forms a large number of covalent compounds because it has tetravalency and a strong tendency to form bonds with carbon and other elements.
हिन्दी:
सहसंयोजक यौगिक वे यौगिक हैं जिनमें परमाणु इलेक्ट्रॉनों की साझेदारी करके बंध बनाते हैं। कार्बन अपनी चतुसंयोजकता तथा कार्बन और अन्य तत्वों के साथ मजबूत बंध बनाने की क्षमता के कारण बड़ी संख्या में सहसंयोजक यौगिक बनाता है।
Covalent compounds are compounds formed by the sharing of electrons between atoms. Carbon forms a large number of covalent compounds because it has tetravalency and a strong tendency to form bonds with carbon and other elements.
हिन्दी:
सहसंयोजक यौगिक वे यौगिक हैं जिनमें परमाणु इलेक्ट्रॉनों की साझेदारी करके बंध बनाते हैं। कार्बन अपनी चतुसंयोजकता तथा कार्बन और अन्य तत्वों के साथ मजबूत बंध बनाने की क्षमता के कारण बड़ी संख्या में सहसंयोजक यौगिक बनाता है।
2. Main Properties of Covalent Compounds | सहसंयोजक यौगिकों के प्रमुख गुण
Covalent compounds generally show the following properties:
मुख्य गुण:
मुख्य गुण:
- Generally have low melting and boiling points.
- Usually exist as gases, liquids or soft solids.
- Generally do not conduct electricity.
- Usually have weak intermolecular forces.
- Many are insoluble in water but soluble in organic solvents.
- They are generally molecular in nature.
- They may have characteristic odours.
3. Low Melting and Boiling Points | निम्न गलनांक एवं क्वथनांक
English:
Most covalent compounds have low melting and boiling points because the forces of attraction between their molecules are generally weak. For example, methane (CH₄), carbon dioxide (CO₂) and ammonia (NH₃) are molecular covalent substances.
हिन्दी:
अधिकांश सहसंयोजक यौगिकों के गलनांक और क्वथनांक कम होते हैं क्योंकि उनके अणुओं के बीच आकर्षण बल सामान्यतः कमजोर होते हैं।
Most covalent compounds have low melting and boiling points because the forces of attraction between their molecules are generally weak. For example, methane (CH₄), carbon dioxide (CO₂) and ammonia (NH₃) are molecular covalent substances.
हिन्दी:
अधिकांश सहसंयोजक यौगिकों के गलनांक और क्वथनांक कम होते हैं क्योंकि उनके अणुओं के बीच आकर्षण बल सामान्यतः कमजोर होते हैं।
Important: Covalent bonds inside a molecule are strong, but the forces between separate molecules are often weak. Therefore, many molecular covalent substances have low melting and boiling points.
महत्वपूर्ण: अणु के अंदर सहसंयोजक बंध मजबूत होते हैं, लेकिन अलग-अलग अणुओं के बीच बल अक्सर कमजोर होते हैं।
महत्वपूर्ण: अणु के अंदर सहसंयोजक बंध मजबूत होते हैं, लेकिन अलग-अलग अणुओं के बीच बल अक्सर कमजोर होते हैं।
4. Important Exceptions | महत्वपूर्ण अपवाद
Not every covalent substance has a low melting point.
Diamond and silicon dioxide (SiO₂) have very high melting points because they form giant covalent/network structures.
हीरा और सिलिकॉन डाइऑक्साइड (SiO₂) जैसे पदार्थों में विशाल सहसंयोजक संरचना होती है, इसलिए इनके गलनांक बहुत अधिक होते हैं।
हीरा और सिलिकॉन डाइऑक्साइड (SiO₂) जैसे पदार्थों में विशाल सहसंयोजक संरचना होती है, इसलिए इनके गलनांक बहुत अधिक होते हैं।
5. Physical State | भौतिक अवस्था
| Substance | पदार्थ | State | अवस्था | Example | उदाहरण |
|---|---|---|
| Gas | गैस | Many covalent compounds are gases. | CH₄, CO₂, NH₃ |
| Liquid | द्रव | Some covalent compounds are liquids. | Ethanol, benzene |
| Solid | ठोस | Some are solids. | Sugar, iodine, naphthalene |
| Network solid | जालक ठोस | Very hard and high melting. | Diamond, SiO₂ |
6. Electrical Conductivity | विद्युत चालकता
English:
Covalent compounds generally do not conduct electricity because they do not contain free ions or freely moving charged particles.
हिन्दी:
सहसंयोजक यौगिक सामान्यतः विद्युत का चालन नहीं करते क्योंकि इनमें स्वतंत्र आयन या स्वतंत्र रूप से गतिशील आवेशित कण नहीं होते।
Covalent compounds generally do not conduct electricity because they do not contain free ions or freely moving charged particles.
हिन्दी:
सहसंयोजक यौगिक सामान्यतः विद्युत का चालन नहीं करते क्योंकि इनमें स्वतंत्र आयन या स्वतंत्र रूप से गतिशील आवेशित कण नहीं होते।
Covalent compound → No free ions → Generally poor conductor
Exception: Graphite conducts electricity because it contains delocalised electrons.
अपवाद: ग्रेफाइट विद्युत का चालन करता है क्योंकि इसमें विस्थानीकृत (delocalised) इलेक्ट्रॉन होते हैं।
अपवाद: ग्रेफाइट विद्युत का चालन करता है क्योंकि इसमें विस्थानीकृत (delocalised) इलेक्ट्रॉन होते हैं।
7. Solubility | विलेयता
Covalent compounds are generally insoluble or only slightly soluble in water.
Many covalent compounds are soluble in organic solvents such as benzene, ether and carbon tetrachloride.
हिन्दी:
सहसंयोजक यौगिक सामान्यतः जल में अघुलनशील या बहुत कम घुलनशील होते हैं। कई सहसंयोजक यौगिक कार्बनिक विलायकों में घुलनशील होते हैं।
हिन्दी:
सहसंयोजक यौगिक सामान्यतः जल में अघुलनशील या बहुत कम घुलनशील होते हैं। कई सहसंयोजक यौगिक कार्बनिक विलायकों में घुलनशील होते हैं।
General rule: "Like dissolves like" — समान प्रकृति वाले पदार्थ एक-दूसरे में अधिक घुलते हैं।
8. Intermolecular Forces | अंतराअणुक बल
Molecular covalent compounds are held together in the solid or liquid state by intermolecular forces.
These forces are generally weaker than ionic or network covalent bonds.
हिन्दी:
अणुक सहसंयोजक यौगिकों के अणुओं के बीच अंतराअणुक बल कार्य करते हैं। ये बल सामान्यतः आयनिक बंध या विशाल सहसंयोजक जालक के बंधों की तुलना में कमजोर होते हैं।
हिन्दी:
अणुक सहसंयोजक यौगिकों के अणुओं के बीच अंतराअणुक बल कार्य करते हैं। ये बल सामान्यतः आयनिक बंध या विशाल सहसंयोजक जालक के बंधों की तुलना में कमजोर होते हैं।
9. Hardness | कठोरता
Most molecular covalent compounds are soft because their molecules are held together by weak intermolecular forces.
However, giant covalent structures such as diamond are extremely hard.
Example: Diamond is one of the hardest naturally occurring substances.
Example: Diamond is one of the hardest naturally occurring substances.
10. Covalent Compounds vs Ionic Compounds
सहसंयोजक एवं आयनिक यौगिकों में अंतर
सहसंयोजक एवं आयनिक यौगिकों में अंतर
| Property | गुण | Covalent | सहसंयोजक | Ionic | आयनिक |
|---|---|---|
| Formation | Sharing of electrons | Transfer of electrons |
| Particles | Usually molecules | Ions |
| Melting point | Generally low | Generally high |
| Boiling point | Generally low | Generally high |
| Electrical conduction | Generally poor | Conduct when molten or dissolved in water |
| Solubility | Often soluble in organic solvents | Often soluble in water |
| Physical nature | Many gases/liquids/soft solids | Generally hard crystalline solids |
11. Why Do Covalent Compounds Generally Not Conduct Electricity?
सहसंयोजक यौगिक सामान्यतः विद्युत का चालन क्यों नहीं करते?
सहसंयोजक यौगिक सामान्यतः विद्युत का चालन क्यों नहीं करते?
English:
- Electric current requires moving charged particles.
- Covalent compounds generally consist of neutral molecules.
- They do not normally produce free ions.
- Therefore, they generally do not conduct electricity.
- विद्युत धारा के लिए गतिशील आवेशित कण आवश्यक होते हैं।
- सहसंयोजक यौगिक सामान्यतः उदासीन अणुओं से बने होते हैं।
- इनमें स्वतंत्र आयन सामान्यतः नहीं होते।
- इसलिए ये सामान्यतः विद्युत का चालन नहीं करते।
12. Diamond and Graphite | हीरा एवं ग्रेफाइट
| Property | Diamond | Graphite |
|---|---|---|
| Structure | Each carbon bonded strongly in 3D network. | Carbon atoms arranged in layers. |
| Hardness | Extremely hard | Soft |
| Electrical conductivity | Does not conduct electricity | Conducts electricity |
| Reason | No freely moving electrons | Delocalised electrons are present |
13. Giant Covalent Structures | विशाल सहसंयोजक संरचनाएँ
A giant covalent structure contains a very large number of atoms connected by strong covalent bonds.
Examples:
- Diamond
- Graphite
- Silicon dioxide (SiO₂)
- Very high melting points
- High hardness in many cases
- Strong covalent bonds throughout the structure
14. 30 MCQs | बहुविकल्पीय प्रश्न
1. Covalent compounds are formed by:
सहसंयोजक यौगिक बनते हैं:
सहसंयोजक यौगिक बनते हैं:
A. Transfer of electrons
B. Sharing of electrons
C. Loss of protons
D. Gain of neutrons
Answer: B — Electrons are shared between atoms.
2. Covalent compounds generally have:
सहसंयोजक यौगिकों का सामान्यतः होता है:
सहसंयोजक यौगिकों का सामान्यतः होता है:
A. Low melting point
B. Very high melting point
C. No melting point
D. Infinite melting point
Answer: A
3. Most covalent compounds are poor conductors because they lack:
A. Atoms
B. Molecules
C. Free charged particles
D. Covalent bonds
Answer: C
4. Which allotrope of carbon conducts electricity?
A. Diamond
B. Graphite
C. Fullerene only
D. Coal
Answer: B
5. Diamond has a high melting point mainly because:
A. It contains ions
B. It has weak forces
C. It contains a giant covalent network
D. It contains water
Answer: C
6. Which is generally a covalent compound?
A. NaCl
B. MgO
C. CH₄
D. KBr
Answer: C
7. Covalent compounds generally consist of:
A. Neutral molecules
B. Only positive ions
C. Only negative ions
D. Free protons
Answer: A
8. Which substance is extremely hard?
A. Graphite
B. Diamond
C. Wax
D. Methane
Answer: B
9. Which compound is a gas at room temperature?
A. Methane
B. Sodium chloride
C. Calcium carbonate
D. Copper
Answer: A
10. Which statement is correct?
A. All covalent compounds are gases.
B. All covalent compounds conduct electricity.
C. Molecular covalent compounds often have low boiling points.
D. All covalent compounds are ionic.
Answer: C
11. Which has delocalised electrons?
A. Diamond
B. Graphite
C. Methane
D. Ethane
Answer: B
12. Silicon dioxide is an example of:
A. Ionic liquid
B. Giant covalent structure
C. Metallic compound
D. Simple ion
Answer: B
13. Which force is generally weak between molecular covalent molecules?
A. Intermolecular force
B. Nuclear force
C. Strong covalent bond
D. Proton force
Answer: A
14. Covalent compounds are often soluble in:
A. Organic solvents
B. Molten metals
C. Iron
D. Copper
Answer: A
15. Which is NOT generally a property of molecular covalent compounds?
A. Low boiling point
B. Poor electrical conductivity
C. Presence of free ions
D. Weak intermolecular forces
Answer: C
16. Graphite conducts electricity due to:
A. Free protons
B. Delocalised electrons
C. Sodium ions
D. Water molecules
Answer: B
17. Diamond is:
A. Soft and a good conductor
B. Hard and generally an electrical insulator
C. Ionic
D. Metallic
Answer: B
18. Which property is due mainly to weak intermolecular forces?
A. Low boiling point
B. High nuclear charge
C. Atomic number
D. Proton number
Answer: A
19. Which contains a giant covalent structure?
A. CH₄
B. CO₂
C. Diamond
D. H₂
Answer: C
20. Ionic compounds conduct electricity in molten state because:
A. Molecules disappear
B. Ions become free to move
C. Covalent bonds form
D. Atoms become neutral
Answer: B
21. A molecular covalent substance usually has:
A. Weak intermolecular forces
B. Only ionic bonds
C. Free electrons everywhere
D. Metallic bonding
Answer: A
22. Which is a network covalent solid?
A. Diamond
B. Methane
C. Ethane
D. Ammonia
Answer: A
23. Covalent compounds generally do not have:
A. Molecules
B. Shared electrons
C. Free ions
D. Covalent bonds
Answer: C
24. Which statement about diamond is correct?
A. It has weak intermolecular forces only.
B. It has a strong three-dimensional covalent network.
C. It is a metal.
D. It contains Na⁺ ions.
Answer: B
25. Which one generally has a higher melting point?
A. Molecular covalent compound
B. Giant covalent solid
C. Methane
D. Ethane
Answer: B
26. Which is an example of a molecular covalent compound?
A. CH₄
B. NaCl
C. MgCl₂
D. CaO
Answer: A
27. Most covalent compounds are poor conductors because:
A. They contain no atoms.
B. Their electrons are not available as free charge carriers.
C. They contain only metals.
D. They contain protons only.
Answer: B
28. Which substance is soft because of its layered structure?
A. Diamond
B. Graphite
C. SiO₂
D. Silicon carbide
Answer: B
29. Low boiling points of many covalent compounds are associated with:
A. Strong ionic lattice
B. Weak intermolecular forces
C. Free ions
D. Metallic bonds
Answer: B
30. Which pair contains covalent substances?
A. CH₄ and CO₂
B. NaCl and KCl
C. MgO and CaO
D. NaOH and KOH
Answer: A
15. 30 Subjective Questions with Answers
वर्णनात्मक प्रश्न एवं उत्तर
वर्णनात्मक प्रश्न एवं उत्तर
1 Mark
Q1. What is a covalent compound?
सहसंयोजक यौगिक क्या है?
Q1. What is a covalent compound?
सहसंयोजक यौगिक क्या है?
A compound formed by sharing of electrons between atoms is called a covalent compound.
इलेक्ट्रॉनों की साझेदारी से बनने वाले यौगिक को सहसंयोजक यौगिक कहते हैं।
इलेक्ट्रॉनों की साझेदारी से बनने वाले यौगिक को सहसंयोजक यौगिक कहते हैं।
1 Mark
Q2. Why do covalent compounds generally have low melting points?
Q2. Why do covalent compounds generally have low melting points?
Because the intermolecular forces between their molecules are generally weak.
क्योंकि इनके अणुओं के बीच अंतराअणुक बल सामान्यतः कमजोर होते हैं।
क्योंकि इनके अणुओं के बीच अंतराअणुक बल सामान्यतः कमजोर होते हैं।
1 Mark
Q3. Name one covalent compound that conducts electricity.
Q3. Name one covalent compound that conducts electricity.
Graphite is a covalent form of carbon that conducts electricity.
1 Mark
Q4. Give one example of a giant covalent substance.
Q4. Give one example of a giant covalent substance.
Diamond or silicon dioxide (SiO₂).
2 Marks
Q5. Why are covalent compounds generally poor conductors of electricity?
Q5. Why are covalent compounds generally poor conductors of electricity?
They generally do not contain free ions or freely moving charged particles.
इनमें सामान्यतः स्वतंत्र आयन या स्वतंत्र रूप से गतिशील आवेशित कण नहीं होते।
इनमें सामान्यतः स्वतंत्र आयन या स्वतंत्र रूप से गतिशील आवेशित कण नहीं होते।
2 Marks
Q6. State two properties of covalent compounds.
Q6. State two properties of covalent compounds.
1. Generally low melting and boiling points.
2. Generally poor electrical conductivity.
2. Generally poor electrical conductivity.
2 Marks
Q7. Why are many covalent compounds soluble in organic solvents?
Q7. Why are many covalent compounds soluble in organic solvents?
Many covalent compounds and organic solvents have similar molecular/non-polar characteristics, so they dissolve more readily in each other.
2 Marks
Q8. Why does diamond have a high melting point?
Q8. Why does diamond have a high melting point?
Diamond has a giant three-dimensional network of strong covalent bonds. A large amount of energy is required to break these bonds.
3 Marks
Q9. Explain three properties of molecular covalent compounds.
Q9. Explain three properties of molecular covalent compounds.
1. Low melting and boiling points.
2. Poor electrical conductivity.
3. Many are soluble in organic solvents and have limited solubility in water.
2. Poor electrical conductivity.
3. Many are soluble in organic solvents and have limited solubility in water.
3 Marks
Q10. Differentiate between molecular covalent and giant covalent substances.
Q10. Differentiate between molecular covalent and giant covalent substances.
Molecular covalent: Consist of separate molecules; generally low melting points; weak intermolecular forces.
Giant covalent: Atoms are connected throughout a large network; generally very high melting points; strong covalent bonds throughout.
Giant covalent: Atoms are connected throughout a large network; generally very high melting points; strong covalent bonds throughout.
3 Marks
Q11. Explain why graphite conducts electricity but diamond does not.
Q11. Explain why graphite conducts electricity but diamond does not.
In graphite, some electrons are delocalised and can move through the layers, allowing electrical conduction. In diamond, all valence electrons are involved in strong covalent bonds, so there are no freely moving electrons.
3 Marks
Q12. Write three differences between covalent and ionic compounds.
Q12. Write three differences between covalent and ionic compounds.
- Covalent compounds involve electron sharing; ionic compounds involve electron transfer.
- Covalent compounds generally have lower melting points; ionic compounds generally have higher melting points.
- Covalent compounds generally do not conduct electricity; ionic compounds conduct when molten or dissolved.
4 Marks
Q13. Explain the electrical conductivity of covalent compounds.
Q13. Explain the electrical conductivity of covalent compounds.
Covalent compounds generally consist of neutral molecules. Their electrons are involved in covalent bonds and they generally do not produce free ions. Since electrical conduction requires mobile charged particles, most covalent compounds are poor conductors. Graphite is an exception because it contains delocalised electrons.
4 Marks
Q14. Explain the melting and boiling points of covalent compounds.
Q14. Explain the melting and boiling points of covalent compounds.
Most molecular covalent compounds have weak intermolecular forces. Therefore, only a small amount of energy is needed to separate their molecules, resulting in low melting and boiling points. Giant covalent substances such as diamond are exceptions because strong covalent bonds extend throughout their structures.
4 Marks
Q15. Explain the solubility behaviour of covalent compounds.
Q15. Explain the solubility behaviour of covalent compounds.
Most covalent compounds are generally insoluble or slightly soluble in water. Many are soluble in organic solvents because substances with similar molecular characteristics tend to dissolve in each other. The exact solubility depends on polarity and intermolecular interactions.
4 Marks
Q16. Compare diamond and graphite.
Q16. Compare diamond and graphite.
| Diamond | Graphite |
|---|---|
| Three-dimensional covalent network | Layered structure |
| Very hard | Soft |
| Does not conduct electricity | Conducts electricity |
| All valence electrons participate in bonding | Delocalised electrons are present |
4 Marks
Q17. Why are giant covalent substances generally hard and have high melting points?
Q17. Why are giant covalent substances generally hard and have high melting points?
They contain a huge network of atoms joined by strong covalent bonds. Breaking or disrupting this network requires a very large amount of energy. Hence they are generally hard and have high melting points.
5 Marks
Q18. Describe the important physical properties of covalent compounds.
Q18. Describe the important physical properties of covalent compounds.
Important properties include:
- Generally low melting and boiling points for molecular substances.
- Usually poor electrical conductivity.
- Many are gases, liquids or soft solids.
- Many are soluble in organic solvents.
- Giant covalent substances are exceptions and may be extremely hard with high melting points.
5 Marks
Q19. Explain why molecular covalent compounds have low melting points while diamond has a high melting point.
Q19. Explain why molecular covalent compounds have low melting points while diamond has a high melting point.
Molecular covalent compounds consist of separate molecules held together by relatively weak intermolecular forces. Therefore, little energy is required to separate molecules. Diamond, however, has a giant three-dimensional network in which carbon atoms are connected by strong covalent bonds. Breaking this network requires enormous energy, giving diamond a very high melting point.
5 Marks
Q20. Explain the electrical behaviour of diamond and graphite.
Q20. Explain the electrical behaviour of diamond and graphite.
Diamond does not conduct electricity because each carbon atom forms four covalent bonds and there are no freely moving electrons. Graphite conducts electricity because each carbon atom forms three bonds and one electron remains available for delocalisation. These delocalised electrons can move through the layers and carry electric charge.
5 Marks
Q21. Explain five general properties of covalent compounds.
Q21. Explain five general properties of covalent compounds.
- Generally low melting points.
- Generally low boiling points.
- Poor electrical conductivity.
- Often soluble in organic solvents.
- Usually exist as gases, liquids or soft solids when molecular.
6 Marks
Q22. Discuss the properties of covalent compounds in detail.
Q22. Discuss the properties of covalent compounds in detail.
Covalent compounds are formed by sharing electrons. Their major properties are:
- Most molecular covalent compounds have low melting and boiling points.
- They generally do not conduct electricity.
- They usually do not contain free ions.
- Many have limited solubility in water.
- Many dissolve in organic solvents.
- Giant covalent substances such as diamond and SiO₂ have high melting points because of strong covalent networks.
6 Marks
Q23. Compare covalent compounds with ionic compounds on the basis of six properties.
Q23. Compare covalent compounds with ionic compounds on the basis of six properties.
| Property | Covalent | Ionic |
|---|---|---|
| Bond formation | Sharing of electrons | Transfer of electrons |
| Particles | Molecules | Ions |
| Melting point | Generally low | Generally high |
| Boiling point | Generally low | Generally high |
| Electrical conduction | Generally poor | Molten/aqueous state conducts |
| Physical state | Gas/liquid/soft solid often | Generally crystalline solid |
6 Marks
Q24. Explain the structure-property relationship in diamond and graphite.
Q24. Explain the structure-property relationship in diamond and graphite.
Diamond has a three-dimensional network in which each carbon atom is strongly bonded to four other carbon atoms. This makes diamond extremely hard and gives it a high melting point. It does not conduct electricity because there are no free electrons.
Graphite has carbon atoms arranged in layers. Each carbon atom is bonded to three neighbouring carbon atoms, leaving one electron delocalised. The layers can slide over one another, making graphite soft, and the delocalised electrons allow it to conduct electricity.
6 Marks
Q25. Explain why most covalent compounds are poor conductors but graphite is a good conductor.
Q25. Explain why most covalent compounds are poor conductors but graphite is a good conductor.
Most covalent compounds contain neutral molecules and their electrons are strongly associated with individual covalent bonds. There are no freely moving ions or charge carriers. Hence they generally do not conduct electricity.
Graphite has a special layered structure. Each carbon atom forms three covalent bonds, while one electron per carbon atom becomes delocalised. These electrons can move through the structure and carry electric charge. Therefore graphite conducts electricity.
6 Marks
Q26. Discuss molecular covalent compounds and giant covalent compounds with examples.
Q26. Discuss molecular covalent compounds and giant covalent compounds with examples.
Molecular covalent compounds: They consist of separate molecules. The molecules are held together by relatively weak intermolecular forces. They generally have low melting and boiling points. Examples include methane, carbon dioxide and ammonia.
Giant covalent compounds: They contain a continuous network of atoms joined by strong covalent bonds. They generally have high melting points. Examples include diamond and silicon dioxide.
Giant covalent compounds: They contain a continuous network of atoms joined by strong covalent bonds. They generally have high melting points. Examples include diamond and silicon dioxide.
6 Marks
Q27. Why are covalent compounds generally insoluble in water?
Q27. Why are covalent compounds generally insoluble in water?
Many covalent compounds are non-polar or have weak interactions with water. Water is a polar solvent and therefore tends to dissolve polar or ionic substances more effectively. Many non-polar covalent substances do not interact strongly enough with water molecules to dissolve. However, polar covalent substances can be soluble in water, so this is a general trend rather than an absolute rule.
6 Marks
Q28. Explain the importance of intermolecular forces in covalent compounds.
Q28. Explain the importance of intermolecular forces in covalent compounds.
Intermolecular forces act between separate molecules. In many molecular covalent compounds these forces are weaker than the covalent bonds within the molecules. Their strength affects physical properties such as melting point, boiling point, viscosity and physical state. Weak intermolecular forces often lead to low melting and boiling points.
6 Marks
Q29. A substance has a very high melting point and is extremely hard. It does not conduct electricity. Is it likely to be a molecular covalent compound? Explain.
Q29. A substance has a very high melting point and is extremely hard. It does not conduct electricity. Is it likely to be a molecular covalent compound? Explain.
No. These properties suggest a giant covalent structure such as diamond rather than a simple molecular covalent substance. Molecular covalent compounds generally have low melting points and are soft or volatile. Diamond has a strong three-dimensional covalent network, giving it high hardness and high melting point, while the absence of free electrons makes it a poor electrical conductor.
6 Marks
Q30. Give a comprehensive account of the properties of covalent compounds with suitable examples.
Q30. Give a comprehensive account of the properties of covalent compounds with suitable examples.
Covalent compounds are formed through sharing of electrons. Molecular covalent compounds generally have low melting and boiling points due to weak intermolecular forces. They are usually poor electrical conductors because they lack free ions or freely moving charge carriers. Many are insoluble or slightly soluble in water but dissolve in organic solvents. They can occur as gases, liquids or soft solids.
However, giant covalent substances such as diamond and silicon dioxide are exceptions. They contain strong covalent bonds throughout a large network, so they have high melting points. Graphite is another special case because it contains delocalised electrons and therefore conducts electricity.
16. Quick Revision | त्वरित पुनरावृत्ति
Remember these key points:
- Covalent bond: Sharing of electrons.
- Molecular covalent compounds: Usually low melting and boiling points.
- Electrical conductivity: Generally poor.
- Reason: Absence of free ions/mobile charge carriers.
- Solubility: Many are soluble in organic solvents.
- Diamond: Giant covalent structure, very hard, high melting point.
- Graphite: Layered structure, soft and electrically conducting.
- SiO₂: Giant covalent structure.
- Important exception: Not every covalent substance has low melting point.
17. Foundation + Competitive Key Concept
Concept:
The statement "covalent compounds have low melting points" should not be treated as an absolute rule.
Correct scientific understanding:
Correct scientific understanding:
- Simple molecular covalent substances → generally low melting/boiling points.
- Giant covalent substances → generally very high melting points.
- Graphite → covalent network with electrical conductivity due to delocalised electrons.
- Diamond → giant covalent network and electrical insulator.
CBSE Exam Tip: Always write "generally" when describing the properties of covalent compounds because diamond, graphite and SiO₂ show important exceptions.
