“Like dissolves like” is a more-than-decent guideline, but it’s not gospel truth. If you’ve taught solubility equilibria and Ksp, you know that even the most insoluble compound has some teeny, tiny, yet finite solubility.
You can show this to your students. Get a couple of separatory funnels, some kerosene1 and two beakers. A molecular model of water and one of a long-chain hydrocarbon — to represent kerosene — will help illustrate the discussion.
Start by showing a molecular model of a C-14 alkane, typical of kerosene. It won’t take long for students to agree that kerosene is non-polar and will not be water soluble. This follows our common-sense guideline: “like dissolves like”.
Now, pour — carefully — some kerosene into a separatory funnel that already contains some water. The kerosene will float. Shake the funnel to mix the liquids and allow the layers to separate.2 This observation is not surprising and is in keeping with your previous prediction based on molecular polarity.
Now send a student out of the room. When he or she is out of sight (and earshot) drain some water layer from the separatory funnel into a beaker. In a second beaker, add about the same volume of tap water. Both samples will look the same.
Call the student in.
Ask him or her to smell the (odourless) tap water. “If you were very thirsty and I said that this was pure water, would you drink it?” “Probably, Sir.”
Now have him or her smell the water that was previously in contact with kerosene. “Smells bad — I wouldn’t drink it.”
I rest my case: kerosene, in spite of its non-polar-ness, is still (minimally) soluble in water.
And there you have it: “Like dissolves like” is not a rule, it’s a guideline. Put another way, there is no such thing as insoluble. Now you can teach the “dipole-induced dipole” intermolecular force based on empirical evidence.
Notes
I like kerosene. It smells bad and you can buy it at the hardware store. Otherwise, use any smelly non-polar liquid. Remember: kerosene is flammable.
This may take a while, so you may want to have a second separatory funnel with the already-separated layers. Think cooking show.
