Answer: They are hard and brittle, have high melting and boiling points, and conduct electricity when molten or dissolved in water.
- Ionic compounds are made of oppositely charged ions held by strong electrostatic forces, so they are hard, crystalline and brittle — they break when struck.
- Large amounts of energy are needed to break the ionic lattice, so their melting and boiling points are high. They are usually solids at room temperature.
- In the solid state the ions cannot move, so they do not conduct electricity; when melted or dissolved in water the ions become free to move and the solution conducts electricity. They are generally soluble in water and insoluble in organic solvents.