Describe the properties of ionic compounds: (a) high melting points and boiling points (b) good electrical conductivity when aqueous or molten and poor when solid

Atoms, Elements and Compounds – Ions and Ionic Bonds

Properties of Ionic Compounds

⚡️ High Melting & Boiling Points – Because ions are tightly packed in a crystal lattice, you need a lot of energy to break the strong electrostatic forces. Think of it like a giant magnet holding a whole city together – you need a lot of heat to pull it apart.

🧊 Good Conductivity in Aqueous or Molten State – When ionic compounds dissolve in water or melt, the ions become free to move, allowing electric current to flow. It’s like giving the ions a “free‑ride” on a conveyor belt.

❄️ Poor Conductivity in Solid State – In the solid crystal, ions are locked in place, so they can’t carry charge. Imagine a traffic jam where cars (ions) are stuck in a gridlock.

PropertyWhy It HappensExample
High \$Tm\$ & \$Tb\$Strong electrostatic attraction in crystal lattice.NaCl, MgO
Conductive when molten/aqueousIons are mobile.Molten NaCl, 1 M NaCl solution.
Non‑conductive when solidIons fixed in lattice.Solid NaCl block.

Analogy Corner

Imagine ions as Lego blocks. In a solid, the blocks are glued together tightly – you can’t move them. When you melt the Lego set, the glue melts and the blocks can slide freely, making it easy to build new shapes (conduct electricity).

Key Terms

  • Ionic bond – electrostatic attraction between cation and anion.

  • Crystal lattice – 3D arrangement of ions in a solid.

  • Melting point (\$T_m\$) – temperature at which solid becomes liquid.

  • Boiling point (\$T_b\$) – temperature at which liquid becomes gas.

📚 Exam Tips

  1. Remember: High \$Tm\$ & \$Tb\$ = strong lattice → need lots of energy.

  2. Use the “free‑ride” analogy to explain conductivity in molten/aqueous state.

  3. Draw a simple crystal lattice diagram to show ions locked in place for the solid state.

  4. When answering, state the property, give the reason, and give an example.