A student carries out an emulsion test on two food samples. Sample A gives a cloudy white emulsion. Sample B remains clear. Explain what the emulsion test involves, what the results show about the contents of each sample, and state the elements found in a lipid molecule.

The emulsion test is carried out by adding a small amount of the food sample to a test tube, then adding a few drops of ethanol and shaking well. After this, an aqueous solution (usually water or a dilute acid) is added and the tube is shaken again. If lipids are present, the ethanol extracts them and the subsequent addition of water causes the lipids to form a milky‑white, cloudy emulsion.

In Sample A the mixture becomes cloudy and milky, indicating that lipids have been extracted and have formed an emulsion – therefore Sample A contains lipids. In Sample B the mixture remains clear, showing that no emulsion is formed and that lipids are absent.

A typical lipid molecule is composed of carbon, hydrogen and oxygen atoms.