How does detergent repel oil

Two test tubes, pipette, test tube holder or container to place the test tubes.

Water, oil, detergent.

Two test tubes are filled with water and about 4-5 drops of oil. One test tube is used for comparison at the end of the experiment, so that it can be set aside for the time being.
The other test tube is shaken and then left to stand for a moment. You can see a temporary fragmentation of the oil layer. However, the oil soon settles on the water again.
A few drops of washing-up liquid are then added and the test tube is shaken again.
Mixing of the two substances can now be observed. The oil no longer settles on the water, but is distributed in the finest droplets in it. An emulsion was created.
The other test tube is used for comparison.

Factual information:
Oil and water are two immiscible liquids. This is due to the hydrophobic properties of fats and the lipophobic properties of water. The two repel each other.
One liter of fat has a density of around 0.9-0.97 g / cm and thus weighs 900-970 g. One liter of water weighs exactly one kilogram. Thus, oil is lighter and floats on water.
An emulsion is formed when one of the two liquids in two or more immiscible liquids is distributed in the other in the form of small droplets. This condition is stabilized by adding emulsifiers, as these reduce the surface tension. ("Emulsifiers are surface-active substances that have a polar, water-soluble [hydrophilic] and a non-polar, fat-soluble [lipophilic] side").
In this experiment, the detergent acts as an emulsifier. When this is added, the oil is distributed in the form of small droplets in the water. It emulsifies.
A naturally occurring emulsion known to everyone is milk.
When it comes to washing or rinsing, emulsions have the advantage that they are easy to rinse off. The dirt does not settle again, but remains dissolved in the washing water.

(Photo: Katharina)