Physical changes are reversible and do not produce a new substance. Chemical changes result in the production of a new substance and cannot be reversed.
1. W 401 PHYSICAL AND CHEMICAL CHANGES Savannah Webb, Former 4-H Extension Agent, Maury County Jennifer Richards, Assistant Professor, 4-H Youth Development MANAGEMENT OF APHIDS AND BYD IN TENNESSEE WHEAT 1
2. Tennessee 4-H Youth Development Physical and Chemical Changes Skill Level Introduction to Content Beginner A physical change is any change in a Learner Outcomes substance’s form that does not change The learner will be able to: its chemical makeup. Examples of Explain the difference between a physical and chemical change. physical changes are breaking a stick or melting ice. Educational Standard(s) Supported 5.PS1.4: Evaluate the results of an A chemical change, or chemical reaction, experiment to determine whether the occurs when atoms of a substance are mixing of two or more substances result rearranged, and the bonds between the in a change of properties. atoms are broken or formed. During a chemical reaction, the structure or Success Indicator composition of the materials change. Learners will be successful if they: Identify the five factors that indicate a When a chemical change is complete, the chemical change has occurred. resulting substance(s) is/are different from Give at least two examples of a physical change. the original substance(s). As a result of a Classify a change as either physical chemical reaction, new substances with or chemical. new properties are formed. Time Needed 15-20 Minutes Introduction to Methodology In this lesson, students will learn about Materials List physical and chemical changes. Physical Playdough, M&M’s, 6 percent hydrogen changes will be demonstrated with peroxide, 16 oz. water bottle, dry yeast, warm water, liquid dish soap, food coloring, pie playdough and/or M&M’s Candy. The pan, funnel, small cup or bowl. chemical change will be demonstrated by making “Elephant’s Toothpaste.” 3 Author Webb, Savannah. UT Extension Maury County.
3. Terms and Concepts Introduction Tips for Engagement Physical Change — Change in form or appearance that can be undone. Introduction of basic concepts Chemical Change — Formation of new substances that cannot be undone. is necessary before beginning Catalyst — A substance that increases the rate of a chemical reaction demonstrations. This helps without itself undergoing any permanent chemical change. students mentally process what Exothermic Reaction — A chemical reaction that releases energy in the they are seeing. While doing form of heat or light. demonstrations, point out at Endothermic Reaction — A reaction in which the system absorbs energy. each stage how what is happening makes the reaction a chemical or physical reaction. Setting the Stage and Opening Questions Have students watch the YouTube video “Everything is changing! (Parody of bruises by Train). A song about physical and chemical changes.” (youtube.com/watch?v=RG8VHqHgF44 ) Use video as a springboard to start discussion on chemical and physical changes. Begin discussion by having students repeat the differences in the two changes heard in the song. Provide a more detailed explanation of physical vs. chemical changes. Write differences on the board. Have students write differences in their science journals or on a clean sheet of paper. o Physical Change — A change that does not result in the production of a new substance. Physical changes result only in a change in the form (i.e., solid, liquid, gas) or appearance of a substance, but not the chemical composition of the substance. The substance will still ultimately be the same. Once a physical change occurs, it can be reversed. Example — You take an ice cube out of the freezer and put it in your glass. Whoops! You forget to pour your drink. You come back a couple of hours later and find your ice has melted. This change in the state of water is an example of a physical change. When your ice melted, it became water. Can you reverse this change by refreezing the water? Yes! o Chemical Change — A change that results in the production of a new and different substance. These changes are often indicated by one of these factors: color change, a gas is given off, a new substance is formed, heat is given off (. . . we call this an exothermic reaction), and/or heat being absorbed (. . . we call this an endothermic reaction). Chemical reactions, once they occur, are irreversible. Example — I eat an apple. When I am finished with the apple, I put the core in the compost pile. Over time the apple rots and becomes compost material. This is an example of a chemical change because you cannot change the compost back into an apple core. Reshow the YouTube video to reinforce the concepts of physical and chemical changes. After video is shown, have students quickly illustrate in their science journal notes the examples of physical and chemical changes that were in the song. Ask students: “What are some examples of physical changes? What about chemical changes? What makes a physical change different from a chemical change? Are physical changes reversible? What about chemical changes?” Experience Demonstrate a couple of physical changes. o You have a pile of brown M&M’s and a pile of red M&M’s. You slowly add the red M&M’s to the brown M&M’s pile. Ask students, “Did this change result in the formation of a new substance?” No! The M&M’s are still M&M’s after adding the red ones. The only difference is the appearance has changed. “Could you reverse this change?” YES! o You take playdough and roll it out into a snake-like shape. You decide to cut the snake in three smaller pieces. Ask students to identify what type of change this is. This is a physical change because you changed only the shape and size of the snake.
4. Demonstrate a chemical change with the Elephant’s Toothpaste Experiment. o Empty a full packet of dry yeast into a separate cup or bowl. o Add 4 tablespoons of warm water and stir until the yeast and water Life Skill(s) from TIPPs are well mixed (30 seconds). for 4-H o Use a funnel to pour ½ cup of 6 percent hydrogen peroxide into the empty soft drink or water bottle that is placed in an empty pie pan. 5th Grade o Add a large squirt (1 tablespoon) of dishwashing liquid to the hydrogen peroxide in the soft drink bottle. Swirl the bottle to mix Participate in 4-H club the detergent into the hydrogen peroxide. meetings by saying pledges, o Add 4-6 drops of food coloring to the soft drink bottle. Swirl the completing activities and bottle to mix the food coloring into the solution. being engaged. (Head) o Use the funnel to pour all of the yeast solution into the bottle. Quickly remove the funnel and observe the reaction! Allow students to touch the bottle to feel any changes taking place and to touch the foam that forms from the reaction. o Rinse all the materials and the pie pan after the demonstration. Share Have students share about the following question. “What did you observe in the chemical reaction?” Process Ask students: “What is the difference between an endothermic and an exothermic reaction?” Have students brainstorm about this question: “How do we know a chemical reaction occurred in the Elephant’s Toothpaste experiment?” Ask the students: “What was the purpose of the yeast in the experiment?” Apply Share with the students: “The rate at which hydrogen peroxide decomposes depends on temperature, the concentration of the hydrogen peroxide, and light. So what special precautions are taken with hydrogen peroxide to keep it from spoiling?” References You Be the Chemist: Lesson Plans for Making Chemistry Fun. The Chemical Education Foundation. Retrieved July 23, 2015. http://www.tangischools.org/cms/lib3/LA01001731/Centricity/Domain/857/Experiments.pdf Elephant’s Toothpaste. Ruben Meerman. ABC Science Online. Retrieved July 23, 2015. http://www.abc.net.au/science/surfingscientist/pdf/teachdemo26.pdf
5. Supplemental Information Physical and Chemical Changes What is happening in the Elephant’s Toothpaste Experiment? The solution will begin to rapidly produce foam with a much greater volume than the original ingredients. The tiny bubbles that are produced are oxygen gas (not air). The hydrogen peroxide breaks down into water and oxygen gas. The reaction occurs naturally, but yeast is added as a catalyst. A catalyst is a substance that helps to increase the rate of a reaction. (Note: A catalyst is never consumed during a reaction.) The soap is used to help us “see” the reaction. Bubbles of oxygen released by the reaction become trapped in the soap, creating a foam. The reaction occurs so quickly, releasing so much gas and creating so much foam, that the foam begins to flow out of the bottle. The result of this reaction looks like toothpaste being squeezed out of a tube. The foam will be warm to touch because the underlying chemical reaction is exothermic (releases heat). What is hydrogen peroxide? Hydrogen peroxide (in a much stronger 6 percent concentration) is a chemical well-known to hairdressers as a bleaching agent. It is also used to bleach paper and many other industrial chemical processes. In hair dressing bleach, the liquid is incorporated into a thick gel, which clings to hair preventing contact with the scalp. The chemical symbol H2O2 for hydrogen peroxide is very similar to water’s H2O. The H2 represents two hydrogen atoms while the O2 represents two oxygen atoms. Hydrogen peroxide molecules are very unstable and naturally decompose into water and oxygen gas. The chemical equation for this decomposition is: 2 H2O2 --> 2 H2O + O2. The equation above represents two hydrogen peroxide molecules decomposing into two water molecules and one oxygen molecule. The decomposition of hydrogen peroxide also releases a small amount of heat, so the reaction is exothermic. Hydrogen peroxide is stored in opaque plastic or brown glass bottles to minimize exposure to light, which accelerates the natural decomposition of hydrogen peroxide Why does yeast accelerate hydrogen peroxide decomposition? Hydrogen peroxide is a natural byproduct of metabolism. All known animals that metabolize oxygen produce a natural enzyme called catalase, which catalyzes the decomposition of hydrogen peroxide into harmless water and oxygen gas. Catalase is found in every organ in the body and in particularly high concentrations in the liver. Hydrogen peroxide is harmful to living things because it is a strong oxidizer that can cause damage to living cells at the molecular level. Yeast is a fungi that also produces the catalase enzyme. Adding yeast to hydrogen peroxide rapidly increases (catalyzes) the decomposition of hydrogen peroxide into water, oxygen gas and heat (you will notice that the foam produced feels warm). W 401 02/17 Programs in agriculture and natural resources, 4-H youth development, family and consumer sciences, and resource development. University of Tennessee Institute of Agriculture, U.S. Department of Agriculture and county governments cooperating. UT Extension provides equal opportunities in programs and employment.
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