How do polar covalent bonds dissolve in water?

1 Answer
Dec 21, 2013


Covalent bonds do not dissolve in water.


Rather, compounds with covalent bonds dissolve in water.

The water surrounds the polar sites of the molecules at the interface with the solute (whether it is a solid, a liquid, or a gas) and strips the molecules away.

When a solute dissolves in a solvent, the individual particles of the solute separate from their neighbours and move between the spaces of the solvent particles.

The solvent particles collide with the solute particles and the intermolecular forces of attraction between solute and solvent particles "hold" the solute particles in the spaces.

There are three steps to the dissolving process:

  1. The solvent particles must move apart to make room for solute particles. This process requires energy to overcome forces of attraction between solvent particles. This first step is endothermic.

  2. The solute particles must separate from their neighbours. This process also requires energy to overcome the forces of attraction between the solute particles. The second step is endothermic.

  3. When the solute particles move between the solvent particles, the intermolecular forces of attraction between solute and solvent take hold and the particles "snap" back and move closer. This process releases energy. The final step in the dissolving process is exothermic.

Consider the process of dissolving a cube of sugar (C₁₂H₂₂O₁₁) in water.

In the space-filling model of sucrose (below), red represents oxygen, light gray represents hydrogen, and dark gray represents carbon.

Like water, sucrose has oxygen atoms bonded to hydrogen atoms (O-H bonds). The areas near the oxygen atoms are slightly negative, and the areas near the hydrogen atoms are slightly positive. That is, the O-H bonds are polar.

Sucrose molecules are attracted to each other because of the dipole-dipole attractions among the O atoms in one molecule and the H atoms in the neighbouring molecules. These particularly strong attractions are called hydrogen bonds.

Sucrose has several polar O-H groups. This is why it dissolves in water

However, the covalent bonds within the molecule aren't broken. Rather, you are breaking the hydrogen bonds that hold that hold the sucrose molecules to each other in the crystal.

If we add water, the O-H groups in the water form hydrogen bonds to the sucrose molecules in the crystal. In turn, the sucrose molecules use their O H groups to form H-bonds with the water molecules.

We see below a picture of water molecules attacking the surface of sucrose.

The intermolecular forces among the sucrose molecules are weaker than those between the sucrose molecules and water.

The water molecules surround the sucrose molecules, replacing the sucrose-sucrose H-bonds with sucrose-water H bonds.

Eventually, the sucrose molecules leave the surface of the crystal and disperse themselves throughout the water as hydrated sucrose molecules.

We end up with a solution of hydrated sucrose molecules in water.

The same processes occur when any polar molecule dissolves in a polar solvent.

If the solute-solvent intermolecular forces of attraction are greater than the solvent-solvent forces, the substance will be soluble.

The solubility depends on the strengths of the intermolecular forces (i.e. the polarity of the solvent).

The relative strengths are: H-bonds > dipole-dipole >dipole-induced dipole > London dispersion.

Thus, a solvent that is capable only of dipole-dipole interactions will not be as good a solvent for sucrose as water (which has H-bonding).