Why don't fats and oils dissolve in water?

Answer 1

See:

• http://en.wikipedia.org/wiki/Hydrophobe
• http://en.wikipedia.org/wiki/Water
• http://www.visionlearning.com/library/module_viewer.php?mid=57
  

Water is polar- it is not linear, resembling a boomerang, with a negative oxygen atom in the middle, off to one side, and two positive hydrogen atoms at either end, on the other side. This essentially turns each water molecule into a small magnet.

Oils and fats, on the other hand, tend to be mostly neutral from an electromagnetic standpoint. They happily will sit next anything, including water, but since the polar water actively attracts itself and other polar molecules (each molecule being a tiny magnet), the non-polar oils get squeezed out any time they mix with a polar solvent.

As water heats up, the bend in the boomerang (so-to-speak) straightens, and oils will more readily dissolve. This is the basic premise behind soup and tea- leaves or bones and scraps are boiled, dissolving the oils.

Another way to mix oil and water is through so-called surfactants: these long molecules have a hydrophilic (water-loving) head and a lipohilic (fat-loving) tail. This allows them to "glue" polar and nonpolar molecules together- this is the idea behind soap. It allows the water in your shower or sink to dissolve oils off your body, dishes, car, etc.

Answer 2

Fats and oils are not polar substances, water is.

Generally, polar solutes, like simple alcohols, dissolve in polar solvents, like water. Non-polar solutes, like fats and oils, dissolve in non-polar solvents, like benzene (and many others). Rule of thumb: like dissolves in like.