Why Does the Iodine Test Result Change from Blue-Black to Brown When Amylase is Added? – Explanation for 15-Year-Old Students

Understanding the Iodine Test and Amylase Effect

The iodine test is a simple experiment used to detect the presence of starch in a sample.

• What happens in the iodine test? When iodine solution is added to starch, it forms a complex with the starch molecules. This complex gives a characteristic blue-black color. So, a blue-black color means starch is present.
• What is amylase? Amylase is an enzyme that breaks down starch into smaller sugar molecules like maltose. It's found in saliva and in some digestive fluids.
• What happens when amylase is added? When you add amylase to starch, the enzyme starts changing starch into simpler sugars by breaking the long starch chains into shorter chains or sugars.
• Why does the color change from blue-black to brown? The blue-black color is due to the large starch molecules. As amylase breaks starch down, there are fewer large starch molecules to react with iodine. Instead, smaller sugar fragments remain, which either do not react strongly or cause a lighter color like brown. Eventually, as starch is mostly broken down, the blue-black color fades completely.

In Simple Terms:

The blue-black color means starch is there. When amylase breaks starch into sugars, there is less starch left. That’s why the blue-black color turns lighter brown or disappears. This color change shows the starch is being digested.

So, the presence of amylase causes the starch to break down, which changes the iodine test color from blue-black (starch present) to brown (less or no starch remaining).