Experiment 8 : Grease Spot Test

Description of the test
In the test, some oil and some water are smeared onto a piece of paper. Some time later, the water smear would become not translucent. But the smear of oil would keep translucent for a long time. This is known as the grease spot test.

Working principle
The working principle is that most grease or fat have a high boiling point. So, they are non-volatile. In room temperature, the spot of water can absorb enough heat from the air and evaporized. But the spot of grease can never absorb enough heat to evaporized. When the liquid is inside the sheet of paper, it diffracts light. So, light can pass from one side of the paper to another side. This gives the phenomenon of "translucent". When there is no liquid in the paper, there is no diffraction. So, light cannot pass it through.