Oil and Water
The misconception that I will be talking about today is that oil and water repel each other.
Students and teachers, we’ve all seen that when you mix oil and water together they
separate into layers with the less dense oil on top and the water on the bottom. There are
several explanations for why this happens. One explanation is that oil is hydrophobic.
That doesn’t explain much, that oil is afraid of water. Another explanation is that water
and oil repel each other, This is not actually true. You can do some small experiments by
putting droplets of liquids on different surfaces and there are several forces that you have
to consider. If I were to put a drop of water onto wax paper you would have water-water
forces (the forces of attraction between 2 water molecules), water-wax forces (the force
of attraction between a water molecule and a wax molecule) and wax-wax attraction (the
force of attraction between 2 wax molecules) If you were to put a drop of liquid onto a
surface like wax paper it would bead up, and what that tells you is that the water-water
forces are stronger than the water-wax forces. If you put a drop of oil on the surface of
water it spreads out. That means that there is a stronger attraction between the oil and the
water than there is between the oil and the oil. Therefore oil and water don’t repel each
other. They actually attract each other. So if they attract each other then why do oil and
water not mix? Well it’s because there are more forces that you have to consider. In this
example of oil and water there are oil-oil forces, oil-water forces which we just said are
pretty strong, and there are water-water forces. Well it’s the water-water forces that cause
this phenomenon to happen. Water molecules exhibit a force called hydrogen bonding
because of the bent shape of the water molecule. The water molecule is shaped like a v
with the oxygen at the angle of the v and the hydrogens at the ends of the tips of the v.
This gives it a partial positive charge on the hydrogen end and a partial negative charge
on the oxygen end. So it’s slightly polar and that means that the hydrogens on one water
molecule are attracted to the oxygens on the other molecule. This explains why such a
light molecule has a high boiling point. Other molecules that are in the same range of
weights as water that don’t exhibit hydrogen bonding boil at a much lower boiling point.
For example carbon dioxide is heavier than a water molecule yet it’s a gas at room
temperature, whereas water is a liquid. Hydrogen bonding explains that. So the reason
that oil and water don’t mix is the water-water intermolecular forces are much stronger
than the water-oil intermolecular forces and the water-oil intermolecular forces are
stronger than the oil-oil intermolecular forces. This is demonstrated by the fact that oil
doesn’t bead up when you put it on the surface of water; it spreads out. So the
misconception that oil and water repel each other is one that comes from a lack of a good
explanation for a common observation that students make that oil and water don’t mix.
So you can have your students do an experiment to investigate this further and to
remember it longer by putting different liquids on different surfaces and seeing if they
bead up or if they wet the surface. If you put a drop of water on a sheet of paper it will
wet the paper, and that means that the water-paper forces are stronger than the waterwater forces. It would bead up if the water-water forces were stronger but if you put
water on wax paper again it beads up because the water-water forces are stronger than the
water-wax forces.