Like Dissolves Like

When hexane is added to a solution of aqueous potassium permanganate, it remains as a clear layer on top of the water solution. When water is added to a solution of iodine in hexane, it remains as a clear layer below the hexane solution. 

Curriculum Notes 

This demo is usually performed when intermolecular forces and solubility are being discussed. Allow about 10 minutes for this demo. One day of lead time is required for this project. 

Lead Time 
One day of lead time is required for this project.
Discussion 

Non-polar solutes do not dissolve in polar solvents like water because they are unable to compete with the strong attraction that the polar solvent molecules have for each other. Thus neither hexane nor iodine dissolves in water. Polar and ionic solutes do not dissolve in non-polar solvents because they have a stronger attraction for each other than for the non-polar solvent molecules. Thus neither water nor potassium permanganate dissolves in hexane. Iodine is a non-polar molecule because the iodine-iodine bond is pure covalent. Hexane is non-polar because the electronegativity difference between the hydrogen and carbon atoms is very small. Water is polar because the electronegativity difference between the hydrogen and oxygen atoms is large. Water is capable of hydrogen bonding with itself. Potassium permanganate is an ionic compound. It separates into ions when mixed with a polar solvent.

Materials 
  • 4 - 500 mL Erlenmeyer flasks, containing pure hexane, iodine dissolved in hexane, pure water, and potassium permanganate dissolved in water
  • 2 1L separatory funnels mounted in ring stands
  • 2 large funnels
Procedure 
  • Be sure that the stopcocks on the separatory funnels are closed.
  • Using a funnel, pour the purple aqueous solution of potassium permanganate into one of the separatory funnels.
  • Pour the purple iodine/hexane solution into the other separatory funnel.
  • Pour the pure hexane into the funnel containing the aqueous solution of potassium permanganate.
  • Pour the pure water into the funnel containing the iodine/hexane solution.
  • Place the glass stoppers in the separatory funnels and shake them. Vent them to prevent the buildup of hexane vapor pressure. Be sure that the funnel is not pointing at any one when you vent it. The layers will separate back out..

 

Safety Precautions 
  • Hexane is very volatile and very flammable. Do not perform this demo near a possible source of ignition. Keep a fire extinguisher handy.
  • Iodine is an oxidizer and corrosive. Potassium permanganate is a strong oxidizer. Avoid contact with skin. Wear safety goggles.
  • Be sure that the separatory funnels are not pointing at any one when you vent them.
Prep. Notes 

Do not try to transport these chemicals in the separatory funnels when you retrieve this demo.They are top-heavy and likely to tip over. Take a few moments after class to separate the fractions back into their respective flasks.

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group