Here’s why oil and water don’t mix… in plain english

Oil and water
A while ago, I was in the process of dressing a gorgeous looking salad with some olive oil, until I accidentally spilled quite a bit of the olive oil into my glass of water that happened to be sitting right by my salad bowl. As usual, the oil floated to the top of the glass of water, forming a distinct layer at the top. I shrugged and dumped my ruined glass of water into the sink and got another glass of water to enjoy with my meal. While eating, I wondered how many of us actually took time to ponder the following interesting questions about the relationship between these two fluids:

  1. Why is it pretty much impossible to mix oil and water?
  2. Why does oil always float to the top when poured into a glass of water

As you might have already guessed, there are scientific reasons behind the answers to both of the questions above. Let’s look a little deeper to see if we can illuminate them both.
 

Why is it pretty much impossible to mix oil and water?

Good question. This is probably a weird one for most of us because we are accustomed to easily mixing fluids together. Bleach mixes well with water, Kool aid mixes well with water, champagne mixes well with orange juice, and good ol’ Jack Daniels mixes quite well with coke. So why then is it that no matter how hard you try, you cannot get oil to mix with water assuming they are the only two things in your attempted mixture? The reason for this strangeness is largely due to the molecular structure(s) and chemical properties of oil and water.

Oil and water are naturally occurring chemical compounds. In plain english, a chemical compound is basically a group of atoms bonded together in a structured way. As an analogy, you can think of a chemical compound as one of those LEGO structures you built as a child, with each atom of the compound being like the individual LEGO blocks you used to build the entire structure. Depending on the nature and physical arrangement of the atoms present in a compound, it takes on specific qualities that cause it to be polar or non polar.

You might be wondering what on earth makes a molecule polar or non polar… fair question, let’s have a look. A compound is usually rendered polar if there is an imbalance in the degree of electronegativity of its constituent atoms. Electronegativity can be loosely defined as the degree of “love” that say a hydrogen atom, an oxygen atom, or any other atom for that matter has for electrons. You see, when atoms get together to form a molecule, they often do so by sharing their electrons. However, if one atom is more electronegative than the atom to which it is bonded to, it will tend to attract some of the electrons away from the other atom. Oxygen for instance is much more electronegative than hydrogen and thus when the two are bonded together in a water molecule for instance, oxygen tends to attract the electrons from both of the hydrogen atoms to which it is bonded. This results in a partial negative charge on the oxygen atom (because electrons are negatively charged), and a partial positive charge on the hydrogen atoms because the partial loss of their negatively charged electrons results in both of them being more positively charged. If it helps you, feel free to think of it as the partial loss of a negative – hydrogen’s electron – resulting in a partial positive charge on each of the hydrogen atoms in a water molecule.

Although water is a net neutral substance, it is also polar because of the slightly negative and positive charges that its constituent parts have. Notice the slight positive charge on the hydrogen atom and a slight negative charge on the oxygen atom on the water molecule above .
Although water is a net neutral substance, it is also polar because of the slightly negative and positive charges that its constituent parts have. Notice the slight positive charge on the hydrogen atom and a slight negative charge on the oxygen atom on the water molecule above .

According to dictionary.com, a polar molecule is an asymmetric molecule with a non-uniform positive and negative charge distribution. In plain english, this just means that a polar molecule generally has a side(s) that is more positively charged, as well as another side(s) that is more negatively charged, giving the molecule electrical poles… sort of like a AA battery. A good example of a polar molecule is the compound we just discussed above which happens to be the most essential fluid on the planet – water. On the other hand, non-polar molecules are symmetric and do not have electrical poles. In general, cooking oils (olive oil, vegetable oil, peanut oil) are good examples of non polar compounds.

So… why is all this important for helping anyone understand why oil and water don’t mix? Well as nature would have it, polar fluids tend to mix well with other polar fluids, and not so well with non polar fluids. The reason why water and alcohol mix so well is because they are both polar. The regions of partial positive charge on the water molecule are attracted to the regions of partial negative charge on the alcohol molecules and vice versa. The weak electrically based attraction between the water and alcohol molecules is what keeps both fluids mixed when you put them in the same container. As a random aside, these interactions between water and alcohol molecules that keep them mixed are called hydrogen bonds, and are the same exact forces that keep both strands of your DNA bound to each other. The illustration below should elucidate further.

Since both water and ethanol are polar molecules, they mix well with each other. The Hydroxyl (O-H) group on the ethanol molecule forms hydrogen bonds with the oxygen and hydrogen atoms that form each water molecule.
Since both water and ethanol are polar molecules, they mix well with each other. The Hydroxyl (O-H) group on the ethanol molecule forms hydrogen bonds with the oxygen and hydrogen atoms that form each water molecule.

When oil and water are mixed together on the other hand, the water molecules are much more electrically attracted to each other than to the oil molecules because of the presence of the partial negative and positive charges that water molecules carry. As a result, the water molecules tend to stay attracted and weakly bonded to one another, leaving all the oil molecules in a separate layer. This is a big part of the reason why oil and water are pretty much impossible to mix together when they are the only two components of a mixture.

Water molecules forming weak hydrogen bonds with each other. Because water molecules are polar, they can form weak hydrogen bonds with each other... a much more favorable interaction than one in which a water molecule interacts with an oil molecule.
Water molecules forming weak hydrogen bonds with each other. Because water molecules are polar, they can form weak hydrogen bonds with each other… a much more favorable interaction than one in which a water molecule interacts with an oil molecule.

Why does oil always float to the top when poured into a glass of water

The simple answer here is that oil floats on top of water because it is less dense than water. As you may already know, the density of a substance is defined as the amount of mass of that substance per unit volume. In plain english, the higher the mass of a given substance that you can stuff into a cubic centimeter space, the more dense it is. The reason why water is more dense than oil is because its individual molecules are much smaller and thus more of them can be stuffed into a cubic centimeter space relative to oil molecules.

OK so assuming all of this is clear, the next question that might have popped into your head is the following. “Are there any conditions under which oil and water will mix”? The answer to that question is yes, but you have to add a third thing to the mixture. Can you guess what that is? Till next time friends, please take care of yourselves and each other.

Without Wax
Oyolu B.C. Ph.D.
chubaoyolu.org

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