A student investigating the properties of biological molecules tests four different food samples. One sample gives a blue-black colour with iodine solution, one gives a brick-red precipitate when heated with Benedict's solution, one gives a purple colour with biuret reagent, and one turns emulsified (milky-white) when mixed with ethanol and water. Explain what each result tells the student about the biological molecules present in each food sample.

1. The blue‑black colour with iodine shows that the sample contains starch, a polysaccharide that forms a complex with iodine.
2. The brick‑red (or orange‑red) precipitate after heating with Benedict’s solution indicates a reducing sugar, such as glucose, present in the sample.
3. The purple colour produced by biuret reagent demonstrates that the sample contains protein, as the reagent reacts with peptide bonds.
4. The milky‑white emulsion that forms when ethanol is added to water reveals the presence of a lipid (fat or oil), which is insoluble in water but soluble in ethanol.
5. Each of these positive results occurs because the reagent reacts specifically with that type of biological molecule, making the tests selective for starch, reducing sugars, proteins and lipids respectively.