The Iodine Test: What Is a Common Test to Detect the Presence of Starch?

October 30, 2025 •

4 min read

The well-known starch-iodine reaction has been used for centuries to detect carbohydrates. The most common test to detect the presence of starch relies on a simple chemical reaction using iodine solution, which produces a dramatic color change to indicate a positive result. This technique is used widely in biology and food science experiments.

Quick Summary

The iodine test is a standard method for detecting starch. A blue-black color change occurs when a sample containing starch reacts with iodine solution, which is typically orange-brown.

Key Points

Iodine Test is Common: The most frequently used method to detect starch is the iodine test, which is a simple and reliable chemical test.
Blue-Black Color is Positive: When iodine solution is added to a sample containing starch, it changes color from its original yellowish-brown to a deep blue-black.
Reaction with Amylose: The color change occurs because iodine molecules become trapped within the helical structure of amylose, a component of starch.
No Starch = No Color Change: If a substance does not contain starch, the iodine solution will remain its original orange-brown color.
Test Requires Iodine-KI Solution: The iodine reagent is prepared by dissolving iodine crystals in a potassium iodide solution to increase its solubility and effectiveness.
Different for Sugars: This test specifically targets starch and does not react with simple sugars like glucose, which are detected using different chemical tests such as Benedict's test.

How the Iodine Test Works

The iodine test for starch is a qualitative chemical test that relies on a specific interaction between iodine and a component of starch called amylose. Starch is a polysaccharide composed of glucose units linked together, forming long, coiled chains. There are two main types of starch molecules: the linear amylose and the branched amylopectin. The coiled structure of amylose is key to the test's success.

When an iodine solution is added to a substance containing starch, the triiodide anion (${I_3^-}$) from the iodine solution becomes trapped within the helical structure of the amylose molecules. This causes a charge-transfer complex to form, resulting in a change in the light absorption properties of the complex. The result is the appearance of a distinct, deep blue-black color, which indicates a positive result for starch. If no starch is present, the iodine solution retains its original yellowish-brown color, indicating a negative result.

Preparing the Iodine Reagent

Pure molecular iodine ($I_2$) is not very soluble in water. To create an effective and stable testing solution, iodine is dissolved in a solution of potassium iodide (KI). This creates polyiodide ions, such as the triiodide ion (${I_3^-}$), which are water-soluble and can readily interact with the amylose helix. This is why the reagent is often referred to as iodine-potassium iodide (I2-KI) solution or simply Lugol's iodine.

Practical Procedures for Testing for Starch

The iodine test is a versatile and straightforward procedure that can be adapted for different types of samples. The following steps outline how to perform the test on solid and liquid food items.

Testing Solid Food Samples

Preparation: Take a small piece of the food sample to be tested, such as a slice of potato, a piece of bread, or a cracker. Place it on a clean, white surface like a porcelain tile or a paper plate for clear observation of the color change.
Application: Using a dropper, add a few drops of iodine solution directly onto the food sample.
Observation: Wait for a few moments and observe any color change. If the sample turns a dark blue, purple, or black color, starch is present. If there is no color change, or the color remains the yellowish-brown of the iodine, then starch is not present.

Testing Liquid Food Samples

Preparation: Add a few milliliters of the liquid food sample, such as rice water or milk, into a clean test tube.
Application: Add several drops of the iodine solution into the test tube.
Observation: Mix gently and observe for any color change. A dark blue-black color indicates a positive result, while no color change confirms a negative result.

Comparison of Starch and Sugar Tests

It is important to distinguish the iodine test from other carbohydrate tests, such as the Benedict's test for reducing sugars. The following table highlights the key differences.


Feature	Iodine Test (for Starch)	Benedict's Test (for Reducing Sugars)
Reagent Used	Iodine-potassium iodide solution (Lugol's iodine)	Benedict's solution
Initial Reagent Color	Yellowish-brown or orange-brown	Blue
Positive Result Indicated by...	Color change to dark blue or black	Color change to green, yellow, orange, or brick-red precipitate upon heating
Tested Carbohydrate	Starch (polysaccharide)	Reducing sugars (monosaccharides and some disaccharides, e.g., glucose)
Heat Requirement	Not required	Required to induce the color change

Potential Complications and Considerations

While the iodine test is generally reliable, certain factors can affect the results. For example, if a sample has a naturally dark color, a positive result may be difficult to observe clearly. Additionally, if starch has been fully hydrolyzed (broken down) into smaller sugar units, the iodine test will not yield a positive result because the amylose structure required for the reaction is no longer present. This is why the test can also be used to indicate the completion of starch hydrolysis, a process that occurs during digestion or manufacturing.

Another important consideration is safety. Iodine solution is a chemical that can stain skin and clothes and is poisonous if ingested. Proper handling, including wearing gloves and eye protection, is crucial during experimentation.

Conclusion

What is a common test to detect the presence of starch? The iodine test is the standard answer. This simple yet effective chemical test uses an iodine-potassium iodide solution, which reacts with the amylose component of starch to produce a striking blue-black color. Its straightforward procedure makes it an invaluable tool for students and scientists to identify starch in various food and plant samples. By understanding the principles behind this reaction, we can confidently interpret the presence or absence of starch in a given substance. The test's reliability and simplicity continue to make it a cornerstone of introductory biology and chemistry experiments worldwide.

A Note on Starch Hydrolysis

For more information on how starch is broken down and the role of the iodine test in monitoring this process, visit the detailed entry on the Chemistry LibreTexts website.

Frequently Asked Questions

The principle behind the iodine test is that iodine molecules, in the presence of potassium iodide, form a triiodide complex that fits into the helical structure of amylose, a component of starch. This interaction results in the formation of a dark blue-black complex.

A positive result for the starch test is a distinct color change from the iodine solution's original yellowish-brown to a deep blue-black or purple-black color.

A negative result occurs when the iodine solution remains its original yellowish-brown or orange-brown color, indicating the absence of starch in the sample.

No, the iodine test is specific to starch and will not produce a positive result for simple sugars like glucose or sucrose. A different reagent, such as Benedict's solution, is used to test for reducing sugars.

Potassium iodide is added because molecular iodine ($I_2$) is not very soluble in water. The KI helps form a water-soluble polyiodide complex, such as triiodide (${I_3^-}$), which is necessary for the reaction with starch.

Both solid and liquid samples can be tested. Common examples include pieces of potato, bread, rice, crackers, and starch solutions.

If a sample has a naturally dark color, it may be difficult to observe the blue-black color change clearly. You might need to use a lighter-colored, aqueous extract of the sample or test a very small amount on a white background to see the result.