Chemistry 2

Why Substances Dissolve (or Don't)

  1. During previous units we learned that some substances are polar:
    1. Ionic substances
    2. Molecular substances that have molecules with one or more polar covalent bonds joining atoms together and forming a dipole (region of positive and negative charge) on a molecule. See example of ethanol below:

  2. Substance which are polar tend to dissolve in water because water is polar. The positive and negative charges on the polar substances attract to water's own dipoles allowing them to form a solution.
    1. Film: See Another Video Clip of Ionic Substances Dissolving
    2. Hydrated positive ion (cation)
      Hydrated negative ion (anion)

      (This image, used with permission, was created by C.H. Mak at Virginia Tech. University)
    3. Film: See How Polar Covalent Substances Dissolve in Water
  3. Substances which are non-polar are attracted to polar water via London Dispersion forces and induced dipole-dipole attractions. However, the water molecules' attraction for themselves (dipole-dipole attraction) is much stronger than its attraction to the non-polar substace. This prevents the non-polar molecules from getting inbetween the water molecules.

    Why are some ionic compounds (which are very polar) insoluble in water?
  4. When two non-polar substances are mixed together you get roughly equal attractions between the various molecules (weak beacuse only via London Dispersion), so they can easily form a solution.
  5. The general rule for how things dissolve is: Like dissolves like. (polar tends to dissolve polar and non-polar tends to dissolve non-polar)
  6. Click here to see how soaps use this property to clean.