Why Do Oil and Water Not Mix?
Oil and water do not mix because the molecules (tiny particles) of which they are composed are so different. The molecules in oil are much bigger and contain many more atoms than those of water. When different liquids mix, it is because they have similar types of molecules which readily link up with each other, like milk and water.
In the case of oil and water the groups of molecules prefer to stay apart. When oil and water are mixed and violently shaken, both liquids quickly form a multitude of droplets. These may co-exist momentarily. But the water molecules are drawn to each other, so the little droplets of water soon coalesce, leaving the oil behind.
Oil and water are immiscible, meaning they will not mix together. Liquids tend to be immiscible when the force of attraction between the molecules of the same liquid is greater than the force of attraction between the two different liquids.
The patches of oil floating on top of the water are usually circular because of another characteristic of molecules, which produces what scientist’s term surface tension. This is a cohesive force caused by the attraction of the molecules to each other. They cling so tightly that they produce a surface layer which acts like an elastic skin or the rubber envelope of a balloon. The molecules are trying to pull the liquid into as small a space as possible. As well as producing a circular shape, this tension makes the surface area of each oil patch as small as possible.
Oil and water don’t mix for a basic physical reason curiously easy to explain by comparison with a magnet. Water molecules have a positive side (at the hydrogen end) and a negative side (at the other end). Unevenly distributed electrical charges give rise to polarity in much the same way as a magnet has a north and a south pole. Although there are differing degrees of the characteristic, many molecules are labeled polar or non-polar.
Water is a polar molecule. Oil is a non-polar molecule. Just as the north pole of a magnet attracts the south pole of another magnet, the positive part of one polar molecule attracts the negative part of another polar molecule. Thus water molecules are drawn to each other.