Question 19
Give reasons for the following:
Electrovalent compounds are soluble in water, insoluble in organic solvents, good conductors of electricity in molten or aq. solution state, have high melting points and undergo electrolytic dissociation on passage of electric current, while covalent compounds are soluble in organic solvents, insoluble in water, non-conductors of electricity, have low melting points and undergo ionization on passage of electric current.
Answer
Electrovalent compounds
- Solubility — Soluble in water, Insoluble in organic solvents.
Reason — Water [polar solvent] has a high dielectric constant i.e., capacity to weaken the force of attraction, thus resulting in free ions.
Organic solvents [non-polar] have low dielectric constants and do not cause dissolution. - Conduction of Electricity — Good conductors of electricity in molten or aqueous solution state and in solid state, they are non-conductors.
Reason — Strong electrostatic force keeps ions in fixed position in the solid state. The force is weakened in the molten state and disappears in aqueous solution state, hence free ions formed migrate to oppositely charged electrodes. - Melting Point — High melting point.
Reason — Strong electrostatic force of attraction between ions. Large amount of energy required to break the force of attraction. - Dissociation — Undergoes electrolytic dissociation on passage of electric current.
Reason — Process involves separation of ions already present in the ionic compound.
E.g. NaCl ⇌ Na1+ + Cl1- [in molten state]
Covalent compounds
- Solubility — Soluble in organic solvents [non-polar] but insoluble in water [polar].
Reason — Organic solvents [non-polar] e.g., the benzene, alcohol, dissolves non-polar covalent compounds [like dissolves like]. Water [polar solvent] cannot dissolve non-polar compounds, but dissolves polar. - Conduction of Electricity — Non-conductors of electricity.
Reason — Non-polar compounds e.g., CCl4 contain molecules and not free ions. Hence, are non conductors. Polar covalent compounds e.g., HCl, NH3 show charge seperation and dissociate in H2O. - Melting Point —Low melting points.
Reason — Weak Vander Waals force of attraction between molecules. Thus, less amount of energy is required to break these forces of attraction. - Ionization — Undergoes ionization in solution state on passage of electric current. Process involves formation of ions from molecules which are not in ionic state.
E.g. HCl ⟶ H1+ + Cl1- [in solution state]
Answer: Solubility, Conduction of Electricity, Melting Point, Dissociation, Solubility, Conduction of Electricity, Melting Point, Ionization
Key Concepts Covered
This question tests your understanding of the following concepts from the chapter Chemical Bonding: Question, Electrovalent, Compounds, Soluble, Water, Insoluble. These are fundamental topics in Chemistry that students are expected to master as part of the ICSE Class 10 curriculum.
A thorough understanding of these concepts will help you answer similar questions confidently in your ICSE examinations. These topics are frequently tested in both objective and subjective sections of Chemistry papers. We recommend revising the relevant section of your textbook alongside practising these solved examples to build a strong foundation.
How to Approach This Question
Read the question carefully and identify what is being asked. Break down complex questions into smaller parts. Use the terminology and concepts discussed in this chapter. Structure your answer logically — begin with a definition or key statement, then provide supporting details. Review your answer to ensure it addresses all parts of the question completely.
Key Points to Remember
- Balance chemical equations before solving numerical problems.
- Learn the periodic table trends and exceptions.
- Understand reaction mechanisms, not just outcomes.
- Use correct IUPAC nomenclature in your answers.
Practice more questions from Chemical Bonding — Chemistry, Class 10 ICSE
Chapter Overview: Chemical Bonding
Chemical bonding explains how atoms combine to form molecules and compounds. Atoms bond to achieve a stable electronic configuration (octet or duplet). The three primary types of bonds are ionic (electrovalent), covalent, and coordinate (dative) bonds. Ionic bonds form by the transfer of electrons from a metal to a non-metal, creating oppositely charged ions held by electrostatic attraction. Covalent bonds form by the sharing of electron pairs between non-metal atoms. Coordinate bonds are a special type of covalent bond where both shared electrons come from one atom. The type of bond determines the physical properties of a substance: ionic compounds have high melting points and conduct electricity in molten/aqueous state, while covalent compounds generally have low melting points and are poor conductors. Students must draw electron dot (Lewis) structures, understand the octet rule and its exceptions, differentiate between polar and non-polar covalent bonds, and relate bonding type to physical properties. This chapter carries significant weightage and questions frequently involve drawing structures and explaining properties.
Key Definitions
| Term | Definition |
|---|---|
| Ionic Bond | Electrostatic force of attraction between oppositely charged ions formed by electron transfer |
| Covalent Bond | Bond formed by mutual sharing of electron pairs between two atoms |
| Coordinate Bond | Covalent bond where both shared electrons are donated by one atom (donor → acceptor) |
| Electrovalency | Number of electrons lost or gained by an atom to form an ion |
| Covalency | Number of electron pairs shared by an atom in covalent bond formation |
| Polar Covalent Bond | Covalent bond between atoms of different electronegativities; shared pair is displaced towards the more electronegative atom |
| Octet Rule | Atoms tend to gain, lose, or share electrons to achieve 8 electrons in the outermost shell |
| Lone Pair | A pair of valence electrons not involved in bonding |
Must-Know Concepts
- Na (2,8,1) transfers 1 electron to Cl (2,8,7) forming Na+Cl− (ionic bond)
- In H2O, oxygen shares 2 electron pairs with 2 hydrogen atoms (covalent, polar)
- In NH4+, the lone pair on nitrogen forms a coordinate bond with H+
- Single bond (1 shared pair), double bond (2 pairs, e.g. O2), triple bond (3 pairs, e.g. N2)
- Ionic compounds: crystalline solids, high MP/BP, conduct electricity when molten or in solution
- Covalent compounds: low MP/BP, generally insoluble in water, do not conduct electricity
- Electronegativity difference determines bond type: large difference = ionic, small = covalent
Ionic vs Covalent Compounds
| Property | Ionic Compounds | Covalent Compounds |
|---|---|---|
| Bond Formation | Electron transfer | Electron sharing |
| Melting Point | High | Low |
| Solubility | Soluble in water | Soluble in organic solvents |
| Conductivity | Conduct when molten/dissolved | Do not conduct |
| State at Room Temp | Crystalline solids | Gases, liquids, or soft solids |
Important Diagrams to Practice
- Electron dot diagrams for NaCl, MgO, CaCl2 (ionic)
- Electron dot structures for H2O, NH3, CH4, CO2, C2H4, N2 (covalent)
- Coordinate bond formation in NH4+ and H3O+
Common Mistakes
- Drawing electron dot structures without showing lone pairs
- Confusing coordinate bond with ordinary covalent bond (arrow shows direction in coordinate bond)
- Saying ionic compounds conduct electricity in solid state (they do not - ions are fixed)
- Forgetting to show the charge on ions in ionic bond diagrams
- Not recognising that HCl is a polar covalent molecule (not ionic despite containing a metal-like H)
Scoring Tips
- Always show complete electron transfer/sharing with arrows in diagrams
- When comparing ionic and covalent, use a table format for clarity and full marks
- For coordinate bonds, clearly identify the donor (has lone pair) and acceptor (has empty orbital)
- Practice drawing at least 10 electron dot structures till they become automatic
Frequently Asked Questions
Why do ionic compounds have high melting points?
Ionic compounds consist of a lattice of positive and negative ions held by strong electrostatic forces. A large amount of energy is needed to overcome these forces, resulting in high melting points.
Can a molecule have both ionic and covalent bonds?
Yes. For example, NaOH has an ionic bond between Na+ and OH−, while the O-H bond within the hydroxide ion is covalent.
What is the difference between polar and non-polar covalent bonds?
In a non-polar covalent bond, electrons are shared equally (e.g., H2, Cl2). In a polar covalent bond, electrons are shared unequally due to a difference in electronegativity (e.g., HCl, H2O).