In this A-Level Biology lesson "The Iodine test for starch" you will learn the procedure for testing foods for the presence of starch.

You must be able to describe the Iodine test and explain the results. For example, a colour change (Purple [blue-black] colour) indicates a positive result, staining with iodine in potassium iodide solution... 

Remember starch is a compact coiled polymer of a-glucose.

When you’re confident you can describe the Iodine test procedure and explain the results it's time for you to complete the accompanying “Iodine test for starch” lesson booklet with knowledge check and exam style questions. As in the previous lesson, you’ll be able to check your answers with mine written in the back of the work book and  see exactly how you should write your answers in a way that gains maximum marks in the exams / assignments.

A-Level Biology "The Iodine Test for Starch"

Here's a summary of the lesson: -

Starch is a complex polysaccharide composed of Amylose and Amylopectin. The combination of these two polysaccharides is the reason starchy foods are a great source of energy.

Remember, Amylopectin is branched (having both a-1,4 glycosidic bonds and a-1,6 glycosidic bonds) making it more readily hydrolysed “releasing those a-glucose molecules”. Whereas Amylose is tightly coiled and compact resulting in a much slower release of the a-glucose molecules.

The Iodine Test:

To test for the presence of Starch in a sample…

Add 10 drops of Iodine in Potassium Iodide Solution to the test sample.

Observe the results: Positive results will show a Purple [blue-black] colour.