Properties of Ionic Compounds (SQA National 5 Chemistry): Revision Note

Melting & boiling points of ionic compounds

• Ionic compounds are typically crystalline solids at room temperature

  • They have very high melting and boiling points

• This property is a direct result of their giant ionic lattice structure

• In the solid state, the positive and negative ions are held in a fixed, rigid lattice by strong electrostatic forces of attraction (the ionic bonds)

• To melt or boil an ionic compound, this strong lattice structure must be broken apart, allowing the ions to become free to move

• Breaking these powerful ionic bonds requires a very large amount of energy

• Since a lot of heat energy is needed, the temperature at which the substance melts or boils is very high

Solubility of ionic compounds

• Many ionic compounds are soluble in water

• When a soluble ionic compound is added to water, a specific process occurs:

  • The water molecules are attracted to the positive and negative ions on the surface of the ionic lattice

• These attractions are strong enough to pull the ions away from their fixed positions, causing the strong ionic lattice to break apart

• The individual ions are now free to move within the water

• Water molecules surround these separated positive and negative ions

Dissolving ionic compounds

• Ionic compounds are generally insoluble in covalent solvents (like oil or hexane)

  • The particles in covalent solvents are not able to overcome the strong electrostatic forces holding the ionic lattice together

Electrical conductivity of ionic compounds

• For any substance to conduct electricity:

  • It must contain charged particles

  • These charged particles must be free to move and carry a current

In the solid state

• Ionic compounds do not conduct electricity when solid

• An ionic lattice does contain charged particles (positive and negative ions)

  • So, it meets the first condition for conducting electricity

• However, in the solid lattice, these ions are held in fixed positions by strong ionic bonds and are not free to move

• Since the charged particles cannot move, solid ionic compounds cannot conduct electricity

When molten or in solution

• Ionic compounds can conduct electricity when molten (melted) or dissolved in water

• When an ionic compound melts or dissolves, the strong lattice structure breaks down

• This releases the ions, so they are now free to move

• Because the substance now has charged particles (ions) that are free to move, it can conduct electricity

How conduction works

• When an electric current is passed through a molten ionic compound or an ionic solution, the ions move in a specific way:

  • The positive ions are attracted to and move towards the negative electrode

  • The negative ions are attracted to and move towards the positive electrode

  • This directed movement of charged ions is the flow of electricity

Conductivity of ionic compounds

ANIMATION