The Science of Oil and Water's Feud

Why are oil and water enemies?

Any vinegar and oil based dressing needs to be shaken before pouring.  If you are not fast enough you have to shake it again.

This is the classic emulsion.  Two things have been placed together that do not mix.  One liquid is present in droplets and spread throughout the other liquid.  In the case of salad dressing, it is oil spread throughout water and vinegar.  It is called an oil-in-water emulsion.

Water is Special

You could say oil and water are enemies.  The water molecules are bonded in a special way.  Water is a polar molecule.  A polar molecule is when one side of a compound is more negatively charged.  Hydrogen atoms are at the top with a positive charge.  The oxygen is at the bottom with a negative charge.  The positive end of one water molecule attracts to the negative end of another water molecule.  This forms a net-like pattern.  Water will be attracted to other polar molecules like vinegar.   Like mixes with like.  

Why doesn't oil like water?

Oil is non-polar.  Neither side is more negative or positive.  There are no ends on oil that will attract water.  Substances that do not mix are called immiscible.  Oil and water are the ultimate enemies.  Put them together and they make no attempt to mix.  The water settles at the bottom.  When you shake it you will see both substances floating in each other, but they quickly separate again.

Experiment

This makes for great science with kids.  The classic homemade lava lamp.  So much science in two substances.  The goal is to have the glass be mostly oil with just a little bit of water.  The bubbles float romantically in the oil.  Add an effervescent tablet for more surprises. 

Expand Further

Why teacher about oil and water?  Students need to know about science in nature.  Oil spills happen.  They are terrible for wildlife.  Why?  Because oil and water do not get along.  It is so difficult to get the oil layer off of the water.  The classic lava lamp lesson is a way we can introduce students to the complications of oil spills.  

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