Question

A beaker contains a thick liquid. How do you test whether it is a lipid?

Answer 1

Lipid is the complex compound of the fats and oil present in the food. To detect the presence of lipids three types of tests can be performed which are emulsion test, Sudan III test, and the acrolein test.

Complete answer:
There are three levels of the tests which include a general test, solubility test, and the emulsification test.

General test: Take the given beaker and add some quantity of the solution into a test tube. Add ether or chloroform to the solution drop by drop till the whole sample gets fully dissolved. Now add one drop of the Sudan III reagent into the test tube. The appearance of red colour in the test tube indicates the presence of lipids.
Acrolein test: in this test take a test tube containing the liquid and in the other test tube, take the dry and powdered form of the potassium bisulfate. Now transfer the solution into this test tube and shake well. After mixing heat the test tube. A distinct appearance of the smell of acrolein is felt which indicates the presence of lipids in the solution.
Emulsification test: take a test tube containing 3ml of the sample and add two drops of the oleic acid and shake well. Take another test tube containing the sample solution and add a 10 percent caustic soda to it. If the emulsion is formed then the conclusion can be drawn that the presence of lipids is there.

Note:
Lipids are fatty and oily substances. They do not dissolve in the water and can be dissolved in organic solutions like benzene, ether, and chloroform. If a lipid test is done on the alcohol then the substance containing lipid would sink at the bottom of the alcohol-containing test tube.