Chapter 3 · Question 9

List the general properties of ionic compounds. Explain why ionic compounds have high melting and boiling points, why they conduct electricity in the molten state but not in the solid state, and why they are generally soluble in water but not in organic solvents.

Answer Revealed

Direct Answer:

Properties of ionic compounds: (i) They are solids at room temperature and are hard but brittle — they break on applying pressure. (ii) They have high melting and boiling points because strong electrostatic forces of attraction between oppositely charged ions require a large amount of energy to overcome. (iii) In the solid state, ions are held in fixed positions in a rigid lattice and cannot move — hence solids do not conduct electricity. In the molten (liquid) state, heat energy overcomes the electrostatic forces, freeing the ions to move towards opposite electrodes — hence they conduct electricity. In aqueous solution too, ions dissociate and conduct electricity. (iv) They are soluble in polar solvents like water (which separates the ions) but insoluble in non-polar organic solvents like petrol and kerosene.

Simple Explanation

Ionic compounds are solid crystals at room temperature. You have to heat them very strongly to melt them — NaCl melts at 1074 K — because the positive and negative ions grip each other tightly. They do not conduct electricity as solids because the ions are locked in place, but melt them or dissolve them in water, and the ions break free to swim around and carry electric current. They dissolve in water (water molecules pull the ions apart) but not in petrol or kerosene (those molecules cannot separate charged ions).

Exam-Ready Structure

The properties of ionic compounds are a direct consequence of the strong electrostatic forces between cations and anions, which NCERT explores through Activity 3.13 and Table 3.4. 1. Physical nature: Ionic compounds are solids and somewhat hard. They are generally brittle — when pressure is applied, layers of ions shift so that like charges face each other, causing repulsion and fracture. 2. High melting and boiling points (Table 3.4): NaCl melts at 1074 K and boils at 1686 K. CaO melts at 2850 K and boils at 3120 K. The high values exist because a considerable amount of energy is required to break the strong inter-ionic electrostatic attractions in the crystal lattice. The greater the charge on the ions, the stronger the attraction and the higher the melting/boiling point (CaO, with Ca²⁺ and O²⁻, has higher values than NaCl with Na⁺ and Cl⁻). 3. Electrical conductivity: (a) Solid state: Does not conduct. The ions are fixed in their lattice positions by strong electrostatic forces and cannot move. Electrical conduction requires mobile charged particles. (b) Molten state: Conducts electricity. On heating, the thermal energy overcomes the electrostatic forces of attraction. The ions become free and mobile, moving towards oppositely charged electrodes. (c) Aqueous solution: Conducts electricity because water molecules surround and separate the ions (hydration), allowing them to move freely in solution. 4. Solubility: Electrovalent compounds are generally soluble in water but insoluble in organic solvents such as petrol and kerosene. Water molecules, being polar, can pull the charged ions apart from the lattice (solvation/hydration), whereas non-polar organic solvent molecules cannot effectively separate the ions. Activity 3.13 tests solubility of NaCl, KI, and BaCl₂ in water, petrol, and kerosene to verify this property.

Key Points

Ionic compounds are solid, hard, brittle; break when pressure is applied
High mp/bp due to strong electrostatic forces (NaCl mp: 1074 K; CaO mp: 2850 K)
Solid state: no conductivity — ions locked in rigid lattice
Molten/aqueous state: conducts electricity — free mobile ions move to opposite electrodes
Soluble in water (polar solvent separates ions); insoluble in petrol/kerosene (non-polar)

List the general properties of ionic compounds. Explain why ionic compounds have high melting and boiling points, why they conduct electricity in the molten state but not in the solid state, and why they are generally soluble in water but not in organic solvents.