MadSci Network: Chemistry

Re: Why won't oil and water mix?

Date: Wed Jan 26 13:38:13 2000
Posted By: Sarah Earley, Grad student, CU Boulder
Area of science: Chemistry
ID: 948653204.Ch

Hi Raymond,

Enemies, huh? The actual reason for why water (H2O) and oil don't mix involves the charges or polarity of the two molecules. Remember the phrase, "like dissolves like." Think of oil as an uncharged molecule, and water as a slightly charged molecule. You can figure out why oil and water don't mix well by recognizing that the molecules are unlike, and following the "like dissolves like" idea.

Here are some details behind the idea:

(An ion is a positively or negatively charged particle. Examples of ions are Na+ (sodium) and Cl- (chloride) which combine to form table salt, NaCl. Salt is an ionic compound because there is such a large charge difference between the two atoms that make it up.)

The atoms that make up water do not have these large charge differences, so water is non-ionic. However, the oxygen atom of H2O is slightly negatively charged, whereas the two hydrogen atoms are slightly positively charged. So there is a non-uniform charge distribution throughout a water molecule, which makes it a polar molecule. (Water is called the "universal solvent" due to these properties. That is, water can interact with both ionic AND polar non-ionic molecules, which is more than what most other solvents can do.)

Oil, on the other hand, is primarily made of carbon (C) and hydrogen (H) atoms. Oil is non-ionic because there isn't a large charge difference between the two atoms. Moreover, the charge distribution between the C and H atoms is uniform (unlike in water), and neither C nor H has even a slight charge on it. Therefore, oil is non-polar.

When you put oil into water, there will always be a very clear boundary between the two chemicals. This is because the molecules will not mix with one another. They minimize the amount of interaction between them, which means staying completely away from one another and forming a minimum number of boundaries.

This explanation is more complicated than the enemy explanation, but I hope it clears something up for you.

Sarah Earley
CU Boulder

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