Cornstarch reacts powerfully with iodine, producing one of the most recognized color changes in chemistry. Cornstarch is a common, accessible form of starch, a complex carbohydrate produced by plants for energy storage. The iodine solution, typically an amber or yellowish-brown liquid, is usually prepared using potassium iodide (KI) to ensure the necessary triiodide ions are present. This interaction forms the basis of the classic starch-iodine test, a widely used method for detecting starch.
The Immediate Visible Result
The immediate and observable result of mixing cornstarch and iodine is a rapid change in color. The initial iodine solution, whether a tincture or Lugol’s solution, is a translucent amber or brownish-yellow color. Upon contact with the starch, this color instantly shifts to a deep, intense blue or blue-black, sometimes appearing dark purple.
This color change serves as the positive indicator for the presence of starch. The reaction’s intensity is sensitive to external conditions. If the mixture is heated, the deep blue color temporarily fades as the complex breaks apart. The color usually regenerates as the solution cools, demonstrating the compound’s conditional stability.
The test cannot be performed effectively in highly acidic conditions. Low pH can cause the starch to break down, preventing the reaction.
The Chemistry Behind the Color Change
The color change results from the formation of a coordination complex, not a simple chemical reaction. Starch is a polysaccharide composed of two main types of glucose polymers: amylose and amylopectin. Natural starches are mixtures, typically containing 10–20% amylose and 80–90% amylopectin.
The reaction primarily depends on the amylose component, which has a distinct molecular structure. Amylose consists of long, unbranched chains of glucose units that coil into a left-handed helix, resembling a hollow spring. The iodine solution contains the triiodide ion (I3-), formed when molecular iodine (I2) reacts with iodide ions (I-) present in the solution.
The linear triiodide ions are the right size and shape to slip into the hollow core of the amylose helix. This trapping forms the starch-iodine complex, which is a supramolecular association, not a covalent bond. When the polyiodide chains are encased within the helix, their electronic structure changes, altering how they absorb light. The complex absorbs light in the red-yellow spectrum, causing the solution to appear the complementary deep blue or blue-black color. Amylopectin, the branched component of starch, does not form this stable complex, making the reaction a specific test for the helical structure of amylose.
Practical Uses of the Starch-Iodine Test
The specificity and visibility of this reaction make the starch-iodine test a valuable tool across scientific and industrial fields. In food science, it is routinely used to verify the presence of starch in products or to check for adulteration. For example, the test can be applied to cut apples to estimate maturity by detecting the disappearance of starch as it converts to sugar during ripening.
In biological laboratories, the test is a standard procedure for assaying the activity of the enzyme amylase. Amylase is the enzyme responsible for breaking down starch into smaller sugar molecules, such as maltose. By adding amylase to a starch solution and periodically testing with iodine, scientists can observe the blue color gradually fading as the starch is hydrolyzed, indicating the enzyme’s effectiveness.
This simple chemical test has applications in forensic science and medicine. The principle also forms the basis for iodometric titrations in analytical chemistry. Here, a starch solution is used as a highly sensitive indicator to signal the endpoint of a reaction by the sudden appearance or disappearance of the dark blue color.