Re: Oil spot test for fat in food.

I think we can say you are both correct. Your teacher has a good point 
though. There may be a few things in certain foods that will leave the 
translucent spot on the bag but they are not lipid. So to be more certain 
that the spot was made by lipid you need to see if the spot disappears when 
you use a solvent that dissolves the lipid. You must use a mixture of 
orgainc solvents to do this. 
When scientists do  experiments, they try to confirm that what they found. 
they do this several ways. First, they always repeat the experiment several 
times to be sure they get the same answer. They also try to do something 
else to confirm what they found. Your teacher has given you a way of 
confirming that what you found was actually lipid. If it leaves a mark on 
the bag it might be lipid. If t the mark dissolves in a solvent that 
dissolves lipid, you can be even more confident that what it was lipid that 
made the mark in the first place.

By the way, I have used the term lipid and not fat. Lipid is composed mainly 
of two types of compounds, fat and oil. Fat is solid lipid and oil is liquid 
lipid. Butter is a fat and corn oil is an oil. Both are lipids. I can say 
very truthfully that corn oil is 100% fat free. Using the word fat when you 
mean lipid is like saying apple when you mean fruit. 

Hope this helps

Good Luck