Lesson Objectives

  • Explain the difference between chemical and physical changes.
  • Describe what happens to atoms in molecules involved in a chemical reaction.
  • Measure the masses of reactants and products in a chemical reaction and note that they are the same.


Engage: Activate Prior Knowledge; Generate Interest

Lead a group discussion in which students brainstorm a list of chemical changes that they might already know about. Make a list of students’ responses on the board or on chart paper under the title “Possible Examples of Chemical Changes.” Note that students might include some physical changes in the list. At the end of this session, refer back to the list and ask students to review their responses and make changes as needed. Before the end of the lesson, make sure that all changes that remain on the list are correctly identified as chemical changes.

Stimulate interest by having students watch Introduction to Chemical Reactions (00:48) to see a few examples of chemical reactions and to be introduced to the definition of a chemical reaction. Ask students to recall the definition of a substance. Students might recall that a substance, sometimes called a pure substance, is a type of matter with a fixed makeup of atoms. Substances can be either elements or compounds. Remind students that each substance has a unique set of physical and chemical properties. Ask students to recall the difference between chemical and physical properties. Tell students that in this lesson, they will learn about how substances are changed into new substances.


Explore: Allowing Students to Experience Content

Have students complete the Exploration Fizz, Foam, Flames to observe signs of a chemical reaction and contrast it with a physical change. During the Exploration, students might be confused by the use of the burning match. Help students to understand this by asking them some questions: How can you snuff out, or extinguish, a flame? (blow on it, or cover it) What is removed by doing this? (oxygen, which it needs to burn) Tell students that flames cannot burn in the presence of a gas called carbon dioxide. Ask students what they think happens when a burning match is extinguished when it is placed in a flask? (the oxygen is replaced with carbon dioxide). Where did the carbon dioxide come from? (the reaction creates carbon dioxide gas) Can you see this in the reaction? (yes, the liquid in the flask bubbles). Ask students which reaction did not produce a chemical reaction? (water and baking soda)

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Have students watch and take notes on the video segment Traits of Chemical Reactions (1:47) to learn about changes that occur during a chemical reaction and to learn the definitions of reactants and products. During the first part of the video, students observe a burning match. Ask students to describe the starting materials (red substance, white wood) and the ending materials (black substance).
Students then observe the reaction between baking soda (sodium bicarbonate) and vinegar. Write the following chemical reaction on the board or on chart paper: sodium bicarbonate + vinegar > carbon dioxide + sodium acetate + water.
Ask: What are the products and reactants of this reaction?

Have students take notes on the video segment Changes in Matter (00:50-1:31) to be introduced to the conservation of mass in a chemical reaction. Write the reaction for combustion on the board (CH4 + 2O2 ?2H2O + CO2). Ask students to identify the number of each type of atom in the reactants and the products. Ask students how this shows that mass is conserved. Students might wonder why it seems like matter “disappears” during a fire. To answer this, have students look at the products. Ask students what happens to water when it is hot enough to boil (as it would be in a fire)? (it turns to a gas, water vapor) What are the properties of CO2 (carbon dioxide)? (it is a gas) Ask students if matter really “disappears” during a fire. (no – matter changes to new substances that are gases, which escape into the atmosphere)

Hands-On Activity
Students will complete the Hands-On Activity Balloon-Filling Bottle.

Explain: Firm Up Understanding; Allow Students to Explain What They Know

Briefly review concepts from the previous session. Ask: What is a chemical reaction? What do you call the starting substances in a reaction? (reactants) What do you call the ending substances in a reaction? (products) Ask: What happens to atoms in a chemical reaction? (atoms in reactants rearrange to form products) If you were to measure the mass of reactants before a chemical reaction, how would it compare to the mass of products after a reaction? (the masses would be the same).

Have students review glossary terms chemical change, physical change, chemical reaction, substances, conservation of mass.

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Students should record the definitions in their science journals along with the following terms:
  • reactants: the substances that combine in a chemical reaction
  • products: the new substances that are formed in a chemical reaction

Elaborate: Allow Students to Apply What They Know

Have students work through the Science Sleuth activity, The K!Zing! Caper using the Case Sheet


Project Ideas: To help your students apply their understanding of chemical reactions, you may wish to have your students complete some or all of the following projects. The time required to complete each project will vary; some may require students to work outside the classroom.

  • Extend the Exploration Fizz, Foam, Flames. Determine the chemical formulas for vinegar and baking soda. Then, write unbalanced equations for the reactions that occur. Then balance the equations.
  • Extend the Hands-on-Activity: Have students research to find the formulas for the reactants and products of the reaction between vinegar and baking soda. Students should find that the reaction occurs as two separate reactions. Carbon dioxide is formed by when a product of the first reaction decomposes into water and carbon dioxide. Have students write out both steps of the reaction.
  • Extend the Hands-on-Activity: Try using warm or cold water or undiluted vinegar to test the effect of temperature and concentration on reaction rate.

Evaluate: Check for Understanding

Have students complete the Brief Constructed Response (BCR) item titled Chemical Reactions. You may also wish to assign the on-line concept assessment and use the results in the student reports to guide you in assigning any remediation to students.

In order to evaluate students’ understanding of science investigation, have them review the video, MythBusters: Chemical Reaction: Skunked!. Provide them with a copy of the student observation sheet and the critique sheet so they can critique how well the Mythbusters conducted the science investigation. Use their results to monitor how well they are grasping the basic process of science investigation.

Review Sheet: Students may review the information in this section using the Chemical Reactions Review Sheet.