Make a Chemical Clock

Make a Chemical Clock

Activity Type:

In this activity, students will perform three experiments using household ingredients to observe and record color changes, indicators that a chemical reaction has taken place. Students also will observe a chemical clock reaction and explore how reaction times can be sped up or slowed down.

Grade Level: 6th – 8th grade
Subject Matter: Physical Science/Chemistry
National Standards: NS.5-8.1, NS.5-8.2

[attach bubbles]


In the Science Friday video, “Flaming Bubbles”, Theo Gray, author of Mad Science, demonstrates that the right mixture of hydrogen and oxygen can result in an explosion – and a chemical change. A chemical change or chemical reaction occurs when the original substance is changed into a different substance. In chemistry, the relative amounts of reactants, or ingredients, also can affect the speed at which a reaction happens.

Warning: TalkingScience does not advise trying Theo's experiment at home!


Activity Materials
Clear plastic cups, three for each student
Bottle of three percent hydrogen peroxide, available in drug stores
Bottle of iodine tincture, also available in drug stores
Liquid starch in plastic dishes
Vitamin C tablets (500 or 1000 mg, non-chewable)
Pipettes or droppers, one per student
Measuring spoons (1 tablespoon, 1 teaspoon, 1/2 teaspoon)

Chemical reaction or change – a change that leads to the transformation of one substance into a different substance.
Physical reaction or change – a change in which the physical properties (state, size, shape) of a substance are altered, but the chemical make-up remains the same.
Reactant – a substance that reacts with another in a chemical reaction.


What To Do
Prep: For each student or group of students, prepare a Vitamin C solution ahead of time by crushing 1,000 mg of vitamin C and dissolving in five tablespoons of water.

1. Begin the lesson by having students watch the Science Friday Web video, “Flaming Bubbles”. Begin a discussion with the students on what constitutes a chemical reaction and review various examples, such as the way that mixing vinegar and baking soda produces carbon dioxide. How is a chemical reaction different from a physical reaction? Review examples of physical reactions, such as ice melting or water boiling.

2. Tell students that they will conduct a series of experiments to observe chemical and physical reactions using basic household ingredients. Review the importance of following proper safety procedures. Although the chemicals being used are non-toxic, students should not taste any of the solutions. Inform students that iodine can stain clothing, and that they should follow each step carefully in order to ensure accurate results.

3. Hand out copies of the Observation Chart below, one for each student, and have students observe and record any chemical or physical reactions for each of the experiments.

Click on image above to see full size and download a pdf of this data sheet.

Experiment 1

1. Hand out to each student a clear plastic cup, filled with one cup of water. Have students add 10 drops of iodine tincture into the cup. What happens to the iodine? Did a chemical reaction take place? Students should note their observations on the chart in the appropriate category.

2. Hand out the liquid starch and ask students to describe the color of the iodine and water solution, and the color of the liquid starch. What do they predict will happen when the liquid starch is added to the iodine and water solution?

3. Have students add four drops of liquid starch to the iodine/water solution and stir. Have students write their observations on the chart. Why did the solution change into a different color? Is this a chemical or physical reaction?

Experiment 2

1. Hand out another clear plastic cup, and have students label it with their markers as Cup A. Tell students that they are going to use this solution for Experiment 3, and that they should follow carefully the next set of instructions.

2. Have students place two tablespoons of water and one-half teaspoon of the pre-prepared vitamin C solution into Cup A. Explain to the students what is in the vitamin C solution. What color is the solution?

3. Have students predict what will happen if iodine is added to the solution. Then have them add one teaspoon of iodine tincture. After adding the iodine, have students continue stirring the solution until it turns from a dark color to clear. Ask students to describe or explain what happened. What does the color change tell us about how vitamin C affects the color of iodine?

4. Set aside Cup A to be used for the next experiment.

Experiment 3

1. Hand out a third clear plastic cup, and have students label it as Cup B. Combine in Cup B two tablespoons of water, two tablespoons of hydrogen peroxide, and one-half teaspoon of liquid starch. Are there any reactions?

2. Have students predict what will happen if Cup A and Cup B are mixed together. What are the ingredients of Cup A and Cup B? What did the first experiments tell us about some of those ingredients? Will the resulting reaction be the same now that other substances are involved?

3. In order to make sure all the contents are fully mixed, have students mix the liquids by pouring Cup A into Cup B, and then pouring all the liquid back into Cup A. Observe the liquid carefully for the next one to two minutes.

4. Discuss what happened to the solution with students. Be sure to have students take into account what they know about iodine when it reacts individually with vitamin C and with starch. How long did it take before a reaction occurred? Why did the reaction take a few minutes? How is this reaction similar to the first experiment? How is it different?

What's Happening?
A physical reaction or change occurs when the physical properties of the substance change, but the substance itself remains the same. Examples of physical changes include a crushed can, melted butter, or a bottle of juice that has been frozen. All of these substances may appear different from their original shape, but the can is still a can, the butter is still butter and the juice is still juice. In a chemical change or reaction, the molecular structure of the substance changes into a new substance. Examples of a chemical change include iron rusting, milk turning sour, or a gas explosion.


Distinguishing chemical changes from a physical change can be difficult. Color change is often used as an indicator of a chemical change or reaction.


In the first experiment, the iodine and water solution are a dark brown color until the starch is added. Then the solution changes to a dark bluish-black color. This happens because the iodine bonds with the starch to create a new compound. A chemical change has taken place, as indicated by the color change.


In the second experiment, the vitamin C in Cup A changes the iodine into a new compound, called iodide, that is clear in color.


In the third experiment, there are two different reactions happening at the same time. The vitamin C causes the iodine to become iodide, which makes the solution clear. This causes a small amount of the vitamin C to be consumed. The hydrogen peroxide, however, causes the iodide to turn back into iodine without getting used up. This causes the iodine/iodide to go back and forth between the vitamin C and the hydrogen peroxide until the vitamin C is used up. Because the hydrogen peroxide reacts much more quickly than the vitamin C, not a lot of iodine is produced. So the solution stays clear. However, once the vitamin C is used up, all the iodide goes back to iodine. And as in the first experiment, iodine mixed with starch creates a deep bluish-black solution.


This type of reaction is often called a chemical clock, because the time it takes the chemicals to react happens very predictably, like a regular clock.


Topics for Science Class Discussion
• What are some ways the reaction could be sped up or slowed down?
• How does the amount of water affect the length of the reaction?
• What would happen if you changed the ratios of the reactants?
• What would happen if you added more vitamin C to the bluish-black product?


Extended Activities and Links
Do Experiment 2 again, this time using a thermometer to measure the temperature of the liquids before hand. Then see how long the reaction would take if the water were hotter or colder. Create a chart and write down your results. How does the temperature affect the reaction speed?


In chemistry, the concentration of a solution means how much of a substance is in water. For example, a highly concentrated salt solution has a lot of salt in only a small amount of water, whereas a low concentration has a small amount of salt in a lot of water. Create a chart and do the experiment again with half as much water, and then twice as much as the original experiment. How do the concentrations of the liquids affect the reaction time?


Divide students into teams. Each team will research and present a physical change experiment and a chemical change experiment. Set up the teams’ final experiments in various stations around the room, so that students can rotate and determine which experiment demonstrates physical change and which demonstrates chemical change.


Explore more physical vs. chemical change activities:


Try some more chemistry experiments using household ingredients:


This lesson plan was created by the New York Hall of Science in collaboration with Science Friday as part of Teachers Talking Science, an online resource for teachers, homeschoolers, and parents to produce free materials based on very popular SciFri Videos to help in the classroom or around the kitchen table.


The New York Hall of Science is a science museum located in the New York City borough of Queens. NYSCI is New York City's only hands-on science and technology center, with more than 400 hands-on exhibits explore biology, chemistry, and physics.