Q.Which test is used to detect the presence of proteins ?

Sometimes a chemist does not need to know exactly how much of a substance is present — only whether it is present at all. A test that answers a simple yes-or-no question of this kind, usually by producing an obvious colour change, is called a qualitative test. One of the most familiar examples in the study of biomolecules is the test that reveals starch.

The reason the test works comes from the shape starch takes. Starch does not stay as a loose thread; parts of it coil up into a helical arrangement, rather like a spiral staircase. This coil has just enough room inside it to trap small molecules. When you add iodine, the iodine molecules slip into the coils of the helix, and the resulting starch–iodine combination takes on a striking blue colour. That sudden appearance of blue is the visible signal that starch is there. …