| MadSci Network: Biochemistry |
hi Nabelle,
I was very intrigued by your question, so I did some "Googling" and discovered that this experiment is done in lots of schools in the USA when you are looking at "food testing" - working out what types of substances are in food. So I learned something too!
To answer your question, we have to think about what happens when we put oily or greasy things onto paper, as compared to something else such as water. If you place a water drop onto uncoated paper, it will soak into the paper. If you looked at paper fibres under the microscope, you would be able to see that they contain compartments, surrounded by walls. In fact, these are the remains of the cells from the trees from which paper is made. When you place water on paper, it enters these compartments and they swell up. This means that light can pass through the paper more easily, so you will see a translucent spot where the water is. However, if you wait for a while, the water will evaporate, the spot will dry up, the paper fibres shrink and the spot will disappear.
Now - if you rub something greasy or oily onto paper, you will also see a translucent spot. But if you wait a while, this spot will not disappear. Why? Well, like water, the lipids soak into the paper fibres, but the lipid spot evaporates much more slowly than water (because the forces that hold the lipid molecules together are stronger than those in water molecules). So the spot remains.
By the way, this question has come up before here at MadSci and that answer also explains why this is not a perfect test for a lipid! I was also interested to discover that early settlers in the USA used to use paper coated in oil to cover their windows - it would let some light in but keep their homes private if viewed from outside.
I hope this helps with your question,
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