Soap

First, you put some oil and some water in a test-tube. You will find that they don't mix but instead form layers. The oil is less dense than the water so it floats on top:

If you try to mix oil and water you will find that they don't stay together. Imagine that you shake the test-tube vigorously...

By shaking you manage to mix everything up initially (although close inspection reveals that there are actually small patches of oil and water, not really a mixture). However, the oil and water gradually separate out again until the are two large areas again. When two liquids have this property of not mixing, we say they are immiscible.

But, if we add some dishwashing soap (*) to the test-tube and then repeat the shaking we find that the oil and water stay mixed. We say that they form an emulsion (*). This is made possible by the soap molecules.

What does the soap do to achieve this?