Grade Level: 6th – 8th grade
Subject Matter: Physical Science/Chemistry
National Standards: NS.5-8.1, NS.5-8.2
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.
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.
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.
Click on image above to see full size and download a pdf of this data sheet.
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?
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.
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?
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.
• 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?
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?