Have you ever noticed if the salt you're using says it's "iodized"? Iodine is a micronutrient, which means we need it in small quantities to be healthy. Because iodine is relatively rare in many people's normal diets, it's added to table salt. Then when people salt their food, such as tasty turkey, stuffing and mashed potatoes, they're also getting iodine. In this science activity you'll use some kitchen-friendly chemistry to investigate which types of salt have iodine and which don't. Then when you sit down to your Thanksgiving dinner, you can know whether to also give thanks that you're helping combat iodine deficiency.
Micronutrients, such as iodine, are types of nutrients that people need in small amounts. Iodine is important for a person's thyroid to function normally. (The thyroid is a gland in the neck that makes key hormones.) It is found in small amounts in other foods, including saltwater fish, seaweed, shellfish, yogurt, milk, eggs, cheese and a handful of other edibles. If a person doesn't consume enough iodine, they can become iodine deficient. The lack of this micronutrient can cause different medical problems (usually due to hypothyroidism caused by a thyroid that does not make enough hormones). These conditions include goiter (a visible swelling of the thyroid) as well as serious birth defects. In fact, iodine deficiency is the most common preventable cause of mental retardation.
Iodine (in the form of iodide) is added to table salt to help prevent iodine deficiency. Since the 1980s there have been efforts to have universal salt iodization. This has been an affordable and effective way to combat iodine deficiency around the world, but not all salt contains iodine, however. You'll investigate whether different salts have iodine by mixing them with laundry starch, which forms a blue-purple–colored chemical with iodine. (Vinegar and hydrogen peroxide are added to the salt solution to help this chemical reaction take place.)
• Disposable plastic cups that are 10 ounces in size or larger. (Alternatively, you could use smaller cups and scale down the activity.)
• Distilled water
• Measuring cups
• Measuring spoons
• Laundry starch solution, also called liquid starch (Alternatively, you could make a suitable starch solution by dissolving one cup of starch-based biodegradable packing peanuts in two cups of water.)
• Iodine antiseptic solution (optional) (Use either an iodine tincture or povidone-iodine solution, found in the first aid section of grocery stores and drugstores. If the iodine doesn't come with a dropper, you'll also need a medicine dropper.)
• Disposable plastic spoons
• At least three different types of salt to test—for example, plain (noniodized) table salt, iodized table salt, pickling salt, rock salt, kosher salt, "lite" salt and sea salt (If you aren't using the iodine antiseptic solution, include iodized table salt.)
• 3 percent hydrogen peroxide
• White vinegar
• If you are using an iodine antiseptic solution, you can prepare a positive control cup so you know what the reaction between iodine and starch should look like. To do this, pour one half cup of distilled water into a disposable cup, add one half teaspoon (tsp.) of laundry starch solution and then add five drops of the iodine antiseptic solution. Be careful when handling the iodine because it can stain.
• Stir well with a disposable plastic spoon. What happens to the liquid when the iodine is added?
• Pick one of the types of salt you want to test and measure out four tablespoons (tbsp.) into a clean, plastic, disposable cup. Add one cup of distilled water to the salt and stir well for about a minute with a clean, disposable plastic spoon. You do not need all of the salt to dissolve.
• Then add one tbsp. of white vinegar, one tbsp. of hydrogen peroxide and one half tsp. of starch solution. What do you think the purpose of the starch is?
• Stir the salt solution well with the disposable plastic spoon and then let the solution stand for a few minutes. What happens to the solution after you stir it? Does it become a blue-purple color?
• Repeat this process using the other, different types of salt you want to test. For each type, be sure to use a different, clean disposable cup and spoon. Do any of the other salt solutions become a blue-purple color?
• Based on your results, which salts do you think contain iodine (in the form of iodide) and which do not? Do your results agree with the labeling on the salt packages, which often say whether the salt contains iodide or not?
• Extra: Try this activity with even more different types of salts. For some ideas, see the Materials list above. Which types of salt contain iodine and which do not? Do your results agree with their labeling?
• Extra: In this activity you added vinegar because it is an acid and helps the chemical reaction take place. Try testing the iodized salt solution again but this time leave out the vinegar. Does the reaction still take place, turning the solution a blue-purple color? If the reaction occurred, did it take a longer amount of time to happen?
• Extra: Temperature often affects chemical reactions. You could try this activity again, but test an iodized salt solution at different temperatures (by heating or cooling the distilled water). How does changing the temperature of the solution change how the color-changing reaction takes place?
Observations and results
Did the iodized table salt solution change to a blue-purple color when you mixed in the starch? Did the "lite" table salt similarly change color whereas most of the other salt types did not?
In this activity you should have seen that the iodized table salt and the "lite" table salt solutions both changed to a blue-purple color (as did the iodine antiseptic solution, if you used it). This indicates that iodide is present in these types of salts. You likely saw no color change for the solutions made using the noniodized salt, rock salt, kosher salt or sea salt because these varieties do not typically contain iodide.
The starch solution was used in this activity because it forms a blue-purple–colored chemical when combined with iodine. Because the solution’s original pH needs to be changed for this chemical reaction to effectively take place, vinegar (an acid) is also added. Hydrogen peroxide is used to turn the salt's iodide into iodine, which the starch reacts with.
Be sure to thoroughly wash any measuring spoons or other utensils that came into contact with the solutions made in this science activity. You can dispose of the solutions by pouring them down the drain.
More to explore
Micronutrient Information Center: Iodine, from the Linus Pauling Institute, Oregon State University
Micronutrient Deficiencies: Iodine Deficiency Disorders, from the World Health Organization
Testing for Iodide in Table Salt ( pdf ), from Stephen W. Wright, Journal of Chemical Education
Determining Iodide Content of Salt, from Science Buddies