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        Substances and Chemical Reactions

        Students gain experience in distinguishing between solids, liquids, and gases and how you know when a chemical change has occurred.

        Lesson Summary

        Overview

        Chemistry in elementary school? Many people think that chemistry is best introduced in the middle and high school years. That's certainly true if the discussion includes abstract concepts such as molecules and bonds. However, primary school is not too early to begin teaching students about chemical changes. In simplest terms, a chemical change (or chemical reaction) occurs when two substances are mixed together and transform into a third substance. Not all substances react chemically when mixed together, however. For example, some substances, when mixed together, remain simply a mixture of those two substances.

        In this lesson, students observe chemical reactions that produce obvious effects (as opposed to reactions in which the substances appear not to change at all). They begin by exploring a different substance every day for one week. They compare the substances and learn that substances can be solids, liquids, or gases. Next, through teacher demonstration (or direct, supervised student involvement), students watch what happens when sand and water are mixed together (no chemical reaction), and when several pairs of acids and bases are mixed together (a chemical reaction occurs). Students then get to build their own "film canister rockets," using baking soda and vinegar as rocket fuel. This lesson concludes with open-ended thinking when students are asked to determine where rust comes from.

        Objectives

        • Learn to distinguish between solids, liquids, and gases
        • Explain that a chemical change will occur when some substances are mixed together
        • Explain how you know when a chemical change has occurred

        Grade Levels: K-2, 3-5"

        Suggested Time

        • "Substance of the Day" activity: 30 minutes each day for one week (more if you like)
        • Rest of the lesson: two 45-minute blocks

        Multimedia Resources

        Use these resources to create a simple assessment or video-based assignment with the Lesson Builder tool on PBS LearningMedia.

        Materials

        • safety glasses
        • plastic cups
        • sand
        • water
        • newspaper
        • measuring cups
        • measuring spoons (Tbs.)
        • baking soda
        • lemon juice
        • vinegar
        • head of purple cabbage, shredded
        • powdered laundry detergent
        • cola
        • liquid soap
        • pickle juice
        • raw and cooked eggs
        • empty film canisters with lids
        • construction paper
        • tape
        • scissors
        • toilet paper
        • spoon
        • steel wool

        Substances to explore (suggestions)

        • water
        • wood
        • packing peanuts
        • plastic beads
        • vegetable oil
        • rocks
        • fur
        • metal objects (coins, wire, magnets, etc.)
        • helium balloon
        • fan
        • *Oobleck (cornstarch and water; see recipe in step 2)

        Substance investigation tools (suggestions)

        • magnifying glasses
        • scales
        • droppers
        • magnets
        • toothpicks
        • cups

        Before the Lesson

        • Collect several substances for students to explore (see suggestions above).
        • Make the Oobleck.
        • Boil a head of shredded purple cabbage in five cups of water. Strain the liquid ("juice") and let cool.
        • Make copies of the Acids and Bases: Making a Film Canister Rocket (PDF) handout and the Acids and Bases: Testing Rockets activity instructions (optional) for each team of students at the reading level.

        The Lesson

        Part I

        1. Explain to students that a substance is anything that is made up of "stuff" and takes up space. The substance might be in the form of a solid, a liquid, or a gas. Ask students for the definition of a solid, a liquid, and a gas. Ideally, their definitions will be based on their experience comparing and categorizing solids, liquids, and gases. Then point to a desk and ask:

        • Is this desk made out of a substance?
        • Is it a solid, a liquid, or a gas?

        Repeat the questions for a book, a window, tap water, hair, air, and so on. Then ask students to identify additional objects in the room and categorize them as solids, liquids, or gases.

        2. Plan to explore a "substance of the day" in your science area. Have "substance investigation tools" available, such as magnifying glasses, scales, droppers, magnets, toothpicks, and cups. Some possible substances to explore might be water, wood, packing peanuts, plastic beads, vegetable oil, rocks, fur, metal objects, a helium balloon, a fan blowing air, and *Oobleck (a slightly messy but really cool mixture of water and cornstarch; see recipe below). Ask students to observe the substances and explore them with the tools. Then have students report their findings to the class. You could also have students compare and contrast substances; for example, a Venn diagram that compares water and Oobleck would be very interesting.

        3. Tell students that they are going to watch what happens when different substances are mixed together. (Or supervise students as they mix these substances together themselves.) Begin by mixing water and sand together. Ask:

        • When water and sand are mixed together, do they change into something else? (Some students might say that a change has occurred because the mixture looks different from its ingredients.)
        • Can this mixture be separated back into its original parts? (yes) How? (The water could be filtered out of the sand, or the mixture, if left alone, will settle into two parts.)

        Now ask students to predict what would happen if the following substances were mixed together: plastic beads and vegetable oil; air and water (for example, air blown through a straw into a cup of water); and rocks and packing peanuts. (The substances do not change. They can be easily separated out from the mixture.)

        4. Show students the Acids and Bases: Testing Rockets video. Stop the video after the ZOOM cast members tell you about the ingredients. Ask your class if lemon juice, baking soda, and water are all substances. Categorize each one as a solid, liquid, or gas. Then ask students to predict what might happen when lemon juice and baking soda are mixed together.

        Tell students that sometimes when two (or more) substances are mixed together, a change occurs and another substance (or substances) is created. This is called a chemical reaction. Gas may form, heat may be produced, and color may change. These types of changes indicate that a chemical reaction has occurred. Also explain that some substances when mixed together produce no chemical reaction.

        5. Play the video again, and stop after the first cork pops off of the soda bottle. Ask:

        • Has a chemical reaction occurred? (yes)
        • What clues tell you that a chemical reaction has occurred?

        6. Start the video again, and play it until theZOOM cast members describe carbon dioxide. Stop the video when they say, "See the bubbles? Carbon dioxide is a gas." Ask:

        • How is the carbon dioxide similar to and different from the baking soda and the lemon juice?
        • Was the carbon dioxide there before the lemon juice and baking soda were mixed together?
        • Where did it come from? (It was produced when the lemon juice and baking soda mixed together; it is a result of a chemical reaction between the lemon juice and baking soda.)
        • Why do you think the creation of carbon dioxide made the cork pop off?

        7. Watch the rest of the video resource. Ask:

        • Was there a chemical reaction when the ZOOM cast mixed orange soda and baking soda? (no)

        8. Demonstrate for your class some other chemical reactions. First, spread out newspapers on your work surface, as these experiments can be messy. Then mix one half cup of vinegar and one tablespoon of baking soda together, and watch the chemical reaction that occurs. (If you wish, have students try it themselves.)

        Tell students that they just witnessed a chemical reaction -- the vinegar and the baking soda combined to make a new substance. The bubbles they saw forming were filled with carbon dioxide, a byproduct (result) of the chemical reaction. Ask students to predict what will happen when vinegar and powdered laundry detergent are mixed together. Then mix the substances and watch the reaction.

        9. Explore other types of chemical changes, as follows:

        • Pour one half cup of purple cabbage juice into one quarter cup of lemon juice. The color change you see is a result of a chemical change.
        • Have students add other liquids to purple cabbage juice to see whether or not the substances react chemically. In separate cups, have them combine one half cup of purple cabbage juice with one quarter cup of each of the following: vinegar, water, water to which two tablespoons of baking soda have been added, cola, liquid soap, and pickle juice. Some of these substances, when mixed with the cabbage juice, will cause a visible chemical change, and others will not.
        • Boiling or frying an egg is also an example of a chemical change. Examine raw and cooked eggs and talk about the change that occurs in an egg when it is cooked. Discuss the idea that sometimes chemical change can take longer than a few seconds to occur. For example, boiling an egg can take 10 minutes.

        10. Then demonstrate the sand and water again, but before you do, ask students to predict what will happen. Ask:

        • Do you think a chemical reaction will take place?

        11. Show students the Acids and Bases: Making a Film Canister Rocket video. Then distribute the Acids and Bases: Making a Film Canister Rocket (PDF) handout and have students make their own. Be sure to have students wear safety glasses and stand back a safe distance when launching their rockets. Ask students to describe to you what is happening. Then ask:

        • Why does the film canister separate from its lid and launch into the air like a rocket?

        Check for Understanding

        Dunk a piece of steel wool into a shallow cup of water. (Do not use steel wool pads made of stainless steel as they will not rust!) After wetting the steel wool, adjust it so that part of the steel wool is exposed above the water. Cover the cup. Ask students to draw a picture of the steel wool set-up. Then let it sit in the classroom for two weeks. Ask students to check on the steel wool occasionally and finally draw a picture of it on day 14. The part not still covered by water should be covered with rust. (Even the submerged part should have rusted, though, probably not as visibly as the exposed part.) Ask students if they think this means a chemical reaction has occurred. Have a class discussion about their ideas. Ask:

        • What substances were being mixed?
        • What substances are next to the steel wool? (air and water)
        • Do chemical reactions always happen quickly?

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