Physical Properties of Covalent Substances (DP IB Chemistry): Revision Note

Physical properties of covalent substances

• The physical properties of molecular covalent compounds are largely influenced by their intermolecular forces

• Using the type of intermolecular forces, the following physical properties can be predicted:

  • Melting and boiling point (volatility)

  • Solubility

  • Conductivity

Melting and boiling point

• When molecular covalent substances change state, the intermolecular forces between their molecules are overcome

  • These forces are much weaker than covalent bonds

  • This explains why many covalent substances are gases or liquids at room temperature

• The stronger the intermolecular forces, the higher the melting and boiling point

  • Substances with low boiling points are described as volatile

• The strength of the intermolecular forces depends on:

  • The size (molar mass) of the molecule

  • The polarity of the molecule

  • The type of intermolecular force present:

London < dipole–dipole < hydrogen bonding

Solubility

• The general rule for solubility is “like dissolves like”:

  • Nonpolar substances tend to dissolve in nonpolar solvents

    • This is due to the formation of London (dispersion) forces between solute and solvent

  • Polar substances tend to dissolve in polar solvents

    • This occurs through dipole–dipole attractions or hydrogen bonding

• For example, ethanol and water:

• This explains why small alcohols (e.g. ethanol, C₂H₅OH) are highly soluble in water

• As molecules increase in size, the nonpolar hydrocarbon region dominates and solubility in water decreases

  • For example:

    • Ethanol (C₂H₅OH) is soluble in water

    • Hexanol (C₆H₁₃OH) is much less soluble

• Polar substances do not dissolve well in nonpolar solvents

  • Their permanent dipoles cannot interact effectively with nonpolar molecules

• Giant covalent substances are generally insoluble in both polar and nonpolar solvents

  • This is because too much energy is required to break the strong covalent bonds in the lattice

Conductivity

• Most covalent substances do not conduct electricity in the solid or liquid state

  • This is because they lack free-moving charged particles

Molecular covalent substances

• Molecular covalent compounds do not conduct electricity in any state

  • They consist of neutral molecules with no delocalised electrons or mobile ions

• However, some polar covalent substances can conduct electricity in solution:

  • They ionise in water to produce ions that carry charge

  • Example: HCl forms H⁺ and Cl^- in solution and conducts electricity

Giant covalent substances

• Most giant covalent structures do not conduct electricity

• Exceptions include:

  • Graphite and graphene, which have delocalised electrons that can move through the structure

• Diamond and silicon dioxide do not conduct, as all electrons are held in covalent bonds

Comparing the properties of covalent substances