What change did you observe in the boiled and chewed rice after adding iodine solution?

December 28, 2025 •

4 min read

The human mouth contains the enzyme salivary amylase, which starts breaking down starches into simpler sugars the moment you begin to chew. This surprising fact is clearly demonstrated when testing boiled versus chewed rice with iodine solution, revealing the very first step of carbohydrate digestion in action.

Quick Summary

Adding iodine solution to boiled rice turns it blue-black, indicating the presence of starch. The same solution added to chewed rice shows no color change, as saliva's amylase breaks down starch into sugars, which do not react with iodine.

Key Points

Positive Test: Boiled rice turns deep blue-black when iodine is added, indicating the presence of starch.
Negative Test: Chewed rice shows no significant color change with iodine, as its starch has been broken down by saliva.
Enzymatic Action: The enzyme salivary amylase, found in saliva, is responsible for digesting the starch in chewed rice.
End Product: Amylase breaks down the complex starch molecules into simpler sugars, like maltose.
Digestive Significance: The experiment proves that carbohydrate digestion begins in the mouth, not just in the stomach.
Complex Required: For the blue-black color to appear, iodine must interact with the helical structure of the starch molecule, which is absent after digestion.

The Boiled Rice Result: A Positive Test for Starch

When an iodine solution is added to boiled rice, a clear and dramatic color change occurs. The solution, which is typically a yellow-brown color, immediately turns to a deep blue-black. This striking color change is a classic positive result for the presence of starch and confirms that boiled rice is rich in this complex carbohydrate. The boiling process gelatinizes the starch, making it more accessible for chemical reactions and further digestion. This reaction is a fundamental principle of the iodine test, a standard method used in laboratories to detect starch.

The Science Behind the Color Change

The mechanism behind the iodine-starch reaction is a fascinating example of chemistry. Starch is a polysaccharide composed of long, coiled chains of glucose molecules. Specifically, the linear component of starch known as amylose is responsible for this reaction. The iodine solution, which contains triiodide ions ($I_3^-$), interacts with the helical structure of the amylose chain. The triiodide ions become trapped within the helix, forming a charge-transfer complex that absorbs light differently, resulting in the deep blue-black color we observe.

The Chewed Rice Result: A Negative Test due to Digestion

The observation for chewed rice is fundamentally different from that of boiled rice. After chewing a piece of rice for a few moments, the subsequent addition of iodine solution does not produce a significant color change; the solution remains its original yellow-brown or shows only a very faint discoloration. This is because the chewing process, or mastication, has mixed the rice with saliva, which contains a powerful digestive enzyme.

The Role of Salivary Amylase

The lack of a positive starch test in the chewed rice is a direct result of the action of salivary amylase, also known as ptyalin. This enzyme, secreted by the salivary glands, begins the chemical digestion of carbohydrates in the mouth. Salivary amylase breaks down the long polysaccharide chains of starch into smaller, simpler sugars, primarily maltose and some dextrins. Unlike starch, these simpler sugars do not have the complex helical structure required to form the colored complex with iodine. Therefore, with the starch largely converted, the iodine has nothing to react with, and no color change occurs.

The Bigger Picture: Digestion Starts in the Mouth

This simple experiment powerfully illustrates that the digestive process for carbohydrates begins long before food reaches the stomach. The presence of salivary amylase in the mouth demonstrates the importance of chewing, as it not only physically breaks down food but also initiates chemical breakdown. This early enzymatic action is a crucial part of the overall digestive efficiency of the human body. For instance, chewing starchy foods for an extended period can lead to a slightly sweet taste, a direct result of the amylase converting starch into sugars. The significance of salivary amylase has been recognized in evolutionary biology, particularly in populations whose diets became more reliant on high-starch foods after the agricultural revolution. National Institutes of Health (NIH) - Salivary Amylase: Digestion and Metabolic Syndrome.

A Step-by-Step Laboratory Experiment

Materials Required

Two test tubes or small, clear cups
Boiled rice
Iodine solution (Lugol's iodine or tincture of iodine diluted with water)
Dropper
Water (for mixing with chewed rice)

Procedure

Preparation: Label one test tube 'Boiled Rice' and the other 'Chewed Rice'.
Sample 1: Place a small amount of boiled rice into the 'Boiled Rice' tube.
Sample 2: Chew a similar amount of boiled rice thoroughly for about 30 seconds. Then, spit the chewed rice into the 'Chewed Rice' tube and add a few milliliters of water to make a paste.
Add Iodine: Using the dropper, add 2-3 drops of iodine solution to each tube.
Observe: Gently mix the contents and observe the color change, or lack thereof, in each tube.

Boiled vs. Chewed Rice: A Comparison


Feature	Boiled Rice	Chewed Rice
Initial State	Contains starch (complex carbohydrate).	Contains starch and saliva.
Iodine Test Observation	Turns a deep blue-black color.	No significant color change (remains yellowish-brown).
Scientific Reason	Starch is present and reacts with iodine to form a blue-black complex.	Salivary amylase breaks down starch into simple sugars, which do not react with iodine.
Enzymatic Activity	No enzymatic activity.	Amylase from saliva is actively breaking down starch.

Conclusion

The difference in observation between the boiled and chewed rice after adding iodine solution is a clear demonstration of carbohydrate digestion beginning in the oral cavity. The positive blue-black result for boiled rice confirms its starch content, while the negative result for chewed rice reveals the effective enzymatic action of salivary amylase. This simple experiment provides a powerful visual aid for understanding the initial stages of human digestion and the vital role of enzymes in breaking down complex food molecules for energy.

Frequently Asked Questions

Boiled rice contains starch. The iodine solution reacts with the long, coiled chains of amylose within the starch, forming a specific complex that appears as a deep blue-black color.

Saliva contains the enzyme salivary amylase, which breaks down the starch in the rice into simpler sugars, such as maltose. Since these sugars do not react with iodine, no blue-black color change is observed.

No. Starch is readily detected in boiled rice. However, it is no longer present in its original form in chewed rice, so the iodine test does not detect it.

The iodine test is used as an indicator to visually confirm the presence or absence of starch. It allows for a clear comparison between the state of carbohydrates in boiled rice versus chewed rice.

The saliva introduced during chewing breaks down the starch molecules. The resulting simpler sugar molecules do not possess the helical structure that iodine needs to produce the blue-black coloration.

No. While salivary amylase initiates starch digestion in the mouth, pancreatic amylase and other enzymes continue the process in the small intestine.

This experiment provides clear evidence that the chemical digestion of carbohydrates begins in the mouth, demonstrating the function of salivary amylase as the first step in breaking down starches.

Yes, increasing the temperature can cause the blue-black color of the iodine-starch complex to fade. The color typically reappears upon cooling, as the helical structure of the amylose is reformed and can again trap the iodine ions.