What does the phrase "Those two mix like oil and water" really mean?

We use the term “Those two mix like oil and water,” when describing two people who don’t like each other. Maybe you’ve also noticed shimmering oil floating on the surface of water on the road after it rains. In both cases you understand that water and oil don’t mix well —but have you ever wondered why? So many other things can dissolve in water—why not oil? In this activity we’ll explore what makes oil so special.

Background

BackgroundMany substances such as fruit juice, coffee or even salt and sugar mix well with water, what makes oil different? The answer can be found in the properties of water and oil. Water molecules (H2O) are a combination of 2 hydrogen and 1 oxygen atom. In addition to a very simpler H2O structure, water molecules are polar, meaning that there is a plus and minus charge across the molecule. The Hydrogen atoms have a practical positive charge and oxygen atom has a practical positive charge. This charges or polarity allow the atoms to look together and from the liquid water we see made out of strong hydrogen bonds. The positive oxygen atom locks onto the negative charge of the Hydrogen.

Oils are very different; they are nonpolar meaning they don’t have positive and negative charges and as a consequence are not attracted to the polarity of water molecules. Quite on the contrary oils are “water fearing” or hydrophobic. This means oil molecules are repelled when getting in contact with water. The result is oil forming a thin layer, and due to it lesser density it is floating on top of water.

This is a short try-out on how surfactant changes the properties of oil and water. We will use washing up liquid. The detergent will break up the surface tension due to is amphiphilic composition. Amphiphilic means it is partly polar and nonpolar and therefore is able to bind to both nonpolar oil molecules and polar water molecules. Below are the materials for a short experiment to see this in action.

Equipment

2 clear plastic or glass bottles with lidsEnough water to fill the bottles halve waySome oil (any cooking oil will work, Sunflower, Olive, Rapeseed etc.)Washing up liquidSharpy or markerMeasuring jar

Preparation

Remove any labels from your bottles. Using the marker, label the bottles: “Oil+Water” and “Oil+Water+Soap.” Fill the bottle halve way with water.

Procedure

Stand the bottles next to each other on a flat surface. Carefully measure a small amount of oil (e.g. 50ml) and add it carefully to each of the bottles. (don’t add any detergent yet)

Does the oil sink to the bottom of the bottle, sit on top of the water or mix with it?

Carefully add squeeze some washing up detergent to the bottle labelled Oil+Water+Soap. Try not to shake the bottle as you add the dish soap.

Does the detergent sink to the bottom of the bottle, sit on top of the water or mix with the water and oil?

Now screw on the bottle caps tightly to each bottle and powerfully shake the bottles for 20 seconds holding a bottle in each hand.

After 20 seconds set the bottles on a flat surface. – Set a timer for 10 minutes or note the time on your clock. Observe the contents, hold each bottle up to a light so you can clearly see what is happening inside the bottle.

Did anything change when you shook the bottles? Do the mixtures look the same in both bottles? If not, what is different between them? How would you explain the differences that you observe?

After 10 minutes have passed look at the contents of the bottles and note the changes. What does the oil and water look like in each bottle? Has the oil mixed with the water, sink to the bottom or rise to the top?

Extra: Test other types of soap, such as toothpaste, hand soap and shampoo by mixing them with oil and water.

Please share your observations and results!

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