Water and Oil

Water is the ‘universal solvent’, almost all substances dissolve in it. Oil is the one substance that does not dissolve in water. However hard you try you will always see that if you try to mix the two, you will notice that the oil layer lies about the water.

Water is a polar molecule as compared to oil

The answer behind this puzzle is that oil does not dissolve in water because of the way molecules of each of these substances interact with each other. A basic principle of chemistry is one molecule can easily dissolve in another having similar characteristics.

The polarity of water

Water is a polar molecule. This means this is a molecule that likes to take sides. One side of the molecule is positively charged, while the other is negatively charged. The two atoms of oxygen cling to each other with the single hydrogen atom on the other side.

Oil on the other hand is made of carbon atoms bonded to hydrogen atoms forming hydrocarbon chains, this makes them non-polar. What this means is that oil molecules are attracted to each other more than water molecules.

Oil molecules are attracted to each other more than water molecules.

There is a particular saying in chemistry called ‘Like dissolves like’ which basically means polar substances will only dissolve polar substances and so is the same for non-polar substances.

Size does matter

Did you know that water molecules are much smaller than other molecules? So a number of water molecules have to break their hydrogen bonds to accommodate oil molecules, which is another reason why oil doesn’t easily mix with water.

Oil and water don’t mix – they are described as ‘immiscible’. Crude oil floats on the sea after a spill from a tanker. Motor oil shows up as a sheen on puddles in the road. Olive oil separates out in salad dressing. But why don’t they mix?

What is Oil?

Oil is a slippery liquid that burns (is combustible) and is not soluble in water. Oil is used as a fuel (petrol is made from oil, and oil is used in some houses for heating) and to make things move easily (it is a lubricant.

Oil can come from deep in the earth, or from plant or animal sources. Oil can also be made from chemicals (synthesised).

Why Don’t Oil and Water Mix?

Mix some food colouring into some water in a jar and pour a bit of vegetable oil in. Give it a good shake and leave it for a moment. At first it might look like it has mixed, and then small droplets of oil form, and join with other droplets to make larger and larger drops, until the oil settles on the top of the water.

Water molecules are polar – they have a small positive charge at one end and a small negative charge at the other end, and they stick to each other. Oil molecules are non-polar – they have no charge. Because of this, oil molecules are more attracted to each other than to water molecules, and water molecules are more attracted to each other than to oil molecules.

Oil and water can be forced to mix together by adding an emulsifier (see ‘Making an Emulsion’). This creates a stable mixture of water with droplets of oil spread through it, or oil with droplets of water spread through it, that does not settle out.

Why Does Oil Float?

Make a home-made lava lamp by filling a jar two-thirds full of water and put in some food colouring. Add some cooking oil. The oil will float on top of the water. Shake some salt onto the oil – it will form a blob and sink to the bottom. Once the salt dissolves, the oil will float back up to the top. If the oil doesn’t sink, sprinkle on a bit more salt.

The oil floats on top of the water because it is less dense (a spoonful of oil weighs less than a spoonful of water). The salt is denser than the water – it weighs the oil down and makes it sink. Once the salt dissolves in the water, the oil floats back up to the top of the water.

Make a glitter globe by filling a jar one-quarter full of surgical spirit, and then almost filling the jar with cooking oil. Drop in a few sequins and some glitter, or other small shiny things. Fill the jar right up to the brim with oil, and screw the lid on tightly. Shake the jar. The oil and alcohol will mix together and then separate, and the glitter and sequins will sparkle as they move around in the mixture.
The alcohol floats on top of the oil because it is less dense (a spoonful of alcohol weighs less than a spoonful of oil). Like water, alcohol is a polar molecule so does not mix with oil.

The Practical Side

Having an oily coat or oily feathers helps animals that live in rivers or the sea keep warm in cold water, because the oil keeps the water away from their skin.

Oil and water not mixing also means that crude oil spills from tankers stay on the surface of the sea. This makes the oil accessible to remove, but it also means that sea animals and birds get caught up in the oil, which weighs down their feathers and fur and is poisonous if they swallow it when they are trying to clean themselves Oil is composed, in part, of long chains of carbon atoms with hydrogens attached. These chains aren’t very polar. It shouldn’t be too hard to pull them apart, because they are held together only by London interactions. The chains really aren’t long enough to create the strong London interactions that would prevent oil from mixing with water.

On the other hand, it is pretty difficult to pull water molecules away from each other, and the oil does not have the means to do so; it just isn’t polar enough. If the water molecules don’t move away from each other, there will be no room between them for the individual oil molecules to become dissolved. These two substances will not mix together very well.

Consequently, if placed in the same vessel, they will remain separate and form two different layers. The more dense layer (the water) will sink to the bottom while the lighter, less dense one (the oil) will float to the top.

The same situation is true for a number of other, non-polar organic compounds, such as benzene and toluene. These liquids are too non-polar to dissolve very well in water. Consequently if you mix benzene and water, the two liquids will form two separate layers. Benzene has a specific gravity or density of 0.874 g/mL, whereas the density of water is 1.0 g/mL. As a result, benzene would float on the top, while water would sink to the bttom. Mixing the two layers up as hard as you can may produce temporary mixing (the mixture would form a sort of cloudy, sparkly mess called schlieren), but once left alone the benzene and water would separate out again.

There’s one more useful example we should look at. Suppose we have a molecule that is very polar on one end, but non-polar at the other. Soap, for instance, is an ionic compound, but while the cation is usually just a sodium ion, the anion is more complicated. This molecular anion most often contains a very polar “carboxylate group”, composed of a carbon with two attached oxygens. It also contains a very long carbon chain, just like in the oil. So one part of the molecule should dissolve well in water, while the other does not. There is a trade-off, and a balance will be struck that determines exactly how soluble the soap is in the water. Interestingly, when placed in water, these soap molecules will arrange themselves in groups so that the polar ends face outward, towards the water, while the nonpolar ends are tucked on the inside. Think of the “circle the wagons” scene in a classic western movie.

There are two reasons why this phenomenon is useful. Micelle formation, as the behaviour is called, allows nonpolar substances, such as dirt and oils, to be dissolved in the polar water. The dirt can interact perfectly well with the nonpolar soap tails, and so it will end up in the middle of the micelles, and something that could not be dissolved in plain water turns out to be perfectly soluble in soapy water. But at another level, micelle formation is a very good model for some of the phenomena of cell and molecular biology. For instance, cell membranes are composed of molecules that are somewhat similar to soap molecules. These molecules form groupings similar to large micelles, but with an additional layer of molecules on the inside of the circle, with their polar ends pointing inward. That leaves the nonpolar ends of both layers sandwiched together, out of the water. Proteins also have polar and non-polar regions, and getting the non-polar regions away from the water leads the protein to adopt a specific shape that, in turn, determines the behaviour of the protein.

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