Dissolving

Dissolving is actually a difficult concept to explain. Why do some solutes dissolve in some solvents but not in others? Why are some ionic substances very soluble in water while others appear to be insoluble? Why are some fluids (liquids and gases) miscible in water, while others are not. Why is something like iodine sparingly soluble? Surely it should either be soluble or insoluble!!

Dissolving requires an attraction between the solvent and solute particles to do two things:

1. Break bonds between solute particles
2. Break bonds between solvent particles (in the liquid phase)

Dissolving Molecular Substances

For molecular solutes, we have an easy "rule" to remember: "Like Dissolves Like".

• Polar solvents will dissolve polar solutes.
• Non-polar solvents will dissolve non-polar solutes.

Dissolving Ionic Substances

Ionic solutes can be explained by looking at the strength of the ionic bond. We learned that this is determined by:

• ionic radius
• ionic charge

Water (and other polar solvents) have permanent dipoles, so can form bonds, called ionic-permanent dipole forces. Multiple ionic-permanent dipole forces allow for the ionic bond to be overcome, so for dissolving to occur.

Try to use this understanding to explain things like why iodine (very large molar mass) will dissolve in water (sparingly) despite being non-polar. Why is its solubility so much higher in cyclohexane and cyclohexene?