Why Don’t Water and Oil Mix? Part 1

I remember the first time I saw kerosene and water mix together.  We used to have an old kerosene heater and I was curious to see what would happen if you mix the kerosene and water together.  Since the kerosene was a nice sky blue colour, I wondered whether it would make the water blue.

Much to my surprise, the kerosene and water separated into two discrete layers, with the kerosene on top.  I was fascinated by this – I’d never seen this before.  Two liquids were mixed together, but not mixing at all.

This phenomenon explains an awful lot of what we see in the world around us, particularly in the kitchen, so let’s look at – why don’t oil and water mix?

In a later post, I’ll look at oil – and other organic compounds , but for now let’s just look at water.

You see, strange as this may seem, water is a very unusual molecule.  It is only because it is so abundant that it doesn’t seem unusual, but if it was a lot less abundant than it was, it would be considered quite unusual.  The reason is, that it is the second most polar molecule in existence.

By the use of the term polar I simply mean that the charges on the molecule are unevenly distributed.  That is, in just the same way that the Earth has the North Pole and the South Pole, and magnets have a north and south ends, the water molecule has a positive end and a negative end,.

The oxygen (the red atom) sucks all the electrons up its end, thereby causing a negative charge.  At the other end of the molecule, is the hydrogen is (the blue atoms).  There is a net positive charge at this end as the oxygen has pinched all its electrons.

Another simplified way to look at it might be like this, with a charge separation:

Now we all know that opposites attract.  A negative change it attracted towards a positive charge and a positive charge is attracted towards a negative charge.

Some of the molecules in water will align themselves to look like this:

In other words, the negative end of one molecule will be attracted towards the positive end of another molecule, and this provides a very energetically stable structure for the water as it stacks into a regular lattice.

Oil, on the other hand, is very different, and we will look at it tomorrow

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