Metals and Non-metals · Lesson 8 of 13
Properties Of Ionic Compounds
“Ionic compounds stay rigid as solids but become electrically useful once their ions can move.”
• Explain the physical nature of ionic compounds. • Explain why ionic compounds have high melting and boiling points. • Describe their typical solubility in water, petrol and kerosene. • Explain conductivity in solid, molten and aqueous states using ion mobility. • Connect strong inter-ionic attraction with hardness, brittleness and thermal behaviour.
The behaviour of ionic compounds follows directly from the strong attraction between positive and negative ions. The same forces explain hardness, brittleness, high melting points and the conditions under which electrical conduction becomes possible.
Properties Of Ionic Compounds
Physical Nature
Ionic compounds are generally solids and somewhat hard because positive and negative ions are strongly attracted. They are also brittle and can break into pieces when pressure is applied.
Melting And Boiling Points
A considerable amount of energy is needed to overcome strong inter-ionic attraction. Ionic compounds therefore have high melting and boiling points.
| Ionic Compound | Melting Point In K | Boiling Point In K |
|---|---|---|
| NaCl | 1074 | 1686 |
| LiCl | 887 | 1600 |
| CaCl₂ | 1045 | 1900 |
| CaO | 2850 | 3120 |
| MgCl₂ | 981 | 1685 |
Solubility
Ionic compounds are generally soluble in water and insoluble in solvents such as kerosene and petrol.
Conduction Of Electricity
Electric current requires charged particles that can move. Ionic compounds contain ions, but the ability of those ions to move depends on the physical state.
Solid Ionic Compounds
In a solid, ions are held in fixed positions in a rigid structure. They cannot move through the material, so the solid does not conduct electricity.
Molten Ionic Compounds
On melting, ions become mobile. Positive and negative ions can move and carry electric charge, so the molten compound conducts.
Ionic Compounds In Water
When a soluble ionic compound dissolves, ions can move through the solution and travel toward oppositely charged electrodes. The solution therefore conducts.
Problem
Why does solid salt fail to conduct while salt solution conducts?
- 1.Both contain ions.
- 2.In the solid, ions are locked in fixed positions.
- 3.Fixed ions cannot carry charge through the material.
- 4.In water, ions become mobile.
- 5.Mobile ions carry charge, so the solution conducts.
The presence of ions is not enough. Electrical conduction depends on whether the charged particles are free to move.
Quiz
Why are ionic compounds generally hard?
Why do ionic compounds have high melting points?
In which state does a typical ionic compound not conduct?
Why can molten ionic compounds conduct?
Which solvent generally dissolves ionic compounds better?
Practice Problems
- Explain why ionic compounds are hard yet brittle.
- Use inter-ionic attraction to explain high melting and boiling points.
- Compare solubility in water, petrol and kerosene.
- Explain why solid sodium chloride does not conduct.
- Explain why molten and aqueous sodium chloride conduct.
- Write a cause-and-effect chain linking strong attraction, rigid structure, melting point and conductivity.
Key Takeaways
• Ionic compounds are generally hard, brittle solids because of strong attraction between opposite ions. • Their high melting and boiling points reflect the large energy needed to overcome inter-ionic forces. • Ionic compounds are generally soluble in water and insoluble in petrol and kerosene. • Solid ionic compounds do not conduct because their ions cannot move. • Molten and aqueous ionic compounds conduct because their ions are mobile.