Ask A Scientist© Chemistry Archive

 >
 >    name         Emily Hartman
 >    age          10

 >    Question -   Why don't oil and water mix?

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    Well, at your school, do the boys and girls mix?  Probably not.
And that is not because the girls don't like the boys (usually), or
because the boys don't like the girls.  It's just that young girls
like spending time with other young girls more than with young boys.
Boys and girls tend to like to do different things and talk about
different things when they are young.
    Imagine you are a girl going outside for recess.  You look around.
You see a boy or two, and a girl or two.  Naturally you go over to the
girl to talk.  Later, another girl comes out.  She, too, goes over to
talk to the girls.  After a while, after every child is outside, you
have patches of boys and patches of girls all over the place, but not
many areas where boys and girls are together.
    The same thing happens with oil and water.  Oil is made of
molecules (which are like very small, sticky, rubber balls).  So is
water.  But water molecules are not the same as oil molecules.  Most
importantly, while all molecules like to stick to each other, oil
molecules like to stick to other oil molecules more than they like to
stick to water molecules.  Same with the water: water molecules like
to stick to oil molecules, but they like to stick to other water
molecules even more.
    So, if you pour some water into some oil, the water molecules
coming in see oil molecules and (at first only a few) water molecules.
Naturally they prefer to join the other water molecules.  After all
the water is in, you have patches of water, and patches of oil, but
nowhere the two kinds of molecules mixed up together.
    It turns out that oil is lighter than water, so that the patches of
oil tend to float up on top of the water, and join together into a big
oil slick.  If you get enough oil, you get a thick layer of it on top
of the water.  You can sort of mix up the oil and water by shaking the
jar, but you will only break up the layer into smaller patches, and
they will come back together again quickly because of the lightness
business.  Now, if you were on the Space Shuttle, the patches wouldn't
form a thick layer, they'd just stay all jumbled up.  But the oil and
water wouldn't mix!
    You CAN force oil and water to mix.  What you need is another type
of molecules which both the water and the oil like to stick to more
than they like to stick to each other.  Such a molecule is called an
emulsifier, or, more simply, a soap.  When you get a soap molecule in
there, the water and the oil stick to it, and then the oil and water
mix.
    A lot of things can be emulsifiers.  Soap is one of them.  If you
get grease on your hands, plain water won't wash it off -- the grease
won't mix with the water and come off.  But if you put soap on your
hands, the grease will mix with the water, and come off.
    Egg yolks are another thing that works.  So if you mix salad oil
and vinegar (which is mostly water), and then put some eggs in, the
oil and vinegar will mix -- you get mayonnaise.  You can make some
with oil, vinegar, and eggs in your blender at home. 
Grayce
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Water is a polar molecule which means that it has a negatively charged end
and a positively charged end.  So water molecules attract each other.  They
also attract other polar molecules.  Oil is NOT a polar molecule-it doesn't
have a separation of charge.  So water and oil aren't attracted to each
other.  Just remember-like dissolves like.

Van Hoeck
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