Note: This video is designed to help the teacher better understand the lesson and is NOT intended to be shown to students. It includes observations and conclusions that students are meant to make on their own.
The teacher will use a small candle flame to demonstrate a chemical reaction between the candle wax and oxygen in the air. Students will see a molecular animation of the combustion of methane and oxygen as a model of a similar reaction. Students will use atom model cut-outs to model the reaction and see that all the atoms in the reactants show up in the products.
Students will be able to explain that for a chemical reaction to take place, the bonds between atoms in the reactants are broken, the atoms rearrange, and new bonds between the atoms are formed to make the products. Students will also be able to explain that in a chemical reaction, no atoms are created or destroyed.
Be sure you and the students wear properly fitting goggles. Be careful when lighting the candle. Be sure that the match and candle are completely extinguished when you are finished with the demonstration.
Get the entire lesson plan and Student Activity Sheet for “Lesson 6.1 - What is a Chemical Reaction?"
Supplement in-class learning with interactive, multimedia-rich Google Forms lesson modules, perfect for reinforcing key chemistry concepts and scientific investigation skills.
Tell students that in previous chapters they have studied different aspects of physical change. When atoms and molecules speed up or slow down, that is a physical change. When they change state from liquid to solid or from gas to liquid, that is a physical change. When a substance is dissolved by water or some other solvent, a new substance has not really been formed. The ions or molecules can still come back together to form the original substance.
Let students know that in this chapter they will explore what happens during a chemical change. In a chemical change, the atoms in the reactants rearrange themselves and bond together differently to form one or more new products with different characteristics than the reactants. When a new substance is formed, the change is called a chemical change.
Explain that in most chemical reactions, two or more substances, called reactants, interact to create different substances called products. Tell students that burning a candle is an example of a chemical reaction.
Expected Results
The wick will catch on fire and the flame will be sustained by the chemical reaction.
The following question is not easy, and students are not expected to know the answer at this point. However, thinking about a candle burning in terms of a chemical reaction is a good place to start developing what it means when substances react chemically.
Students often say that the string or wick is burning. It is true that the string of the wick does burn but it’s the wax on the string and not so much the string itself that burns and keeps the candle flame burning. Explain that the molecules that make up the wax combine with oxygen from the air to make the products carbon dioxide and water vapor.
Point out to students that this is one of the major characteristics of a chemical reaction:
In a chemical reaction, atoms in the reactants combine in new and different ways to form the molecules of the products.
Students may be surprised that water can be produced from combustion. Since we use water to extinguish a fire, it may seem strange that water is actually produced by combustion. You may want to let students know that when they metabolize or “burn” food in their bodies, they also produce carbon dioxide and water.
Remind students that air is a mixture of gases. Explain that when something burns, it reacts with the oxygen in the air.
Ask students to make a prediction:
Expected Results
The flame goes out.
Note: Some curious students may ask what the flame is made of. This is a great question and not trivial to answer. The flame is burning wax vapor. The light of the flame is caused by a process called chemiluminescence. Energy released in the chemical reaction makes electrons from different molecules move to a higher energy state. When the electrons come back down, energy is released in the form of light.
Explain to students that wax is made of long molecules called paraffin and that paraffin is made up of only carbon atoms and hydrogen atoms bonded together. Molecules made of only carbon and hydrogen are called hydrocarbons. Tell students that you will use the simplest hydrocarbon (methane) as a model to show how the wax, or any other hydrocarbon, burns.
Project the image Methane and Oxygen React.
Show students that there is methane and oxygen on the left side of the chemical equation and carbon dioxide and water on the right side. Explain that the molecules on the left side are the reactants and the ones on the right side are the products. When the candle was burning, the paraffin reacted with oxygen in the air to produce carbon dioxide and water, similar to the chemical reaction between methane and oxygen.
Explain to students that the chemical formula for methane is CH4. This means that methane is made up of one carbon atom and four hydrogen atoms. Show students that the other reactant is two molecules of oxygen gas. Point out that each molecule of oxygen gas is made up of two oxygen atoms bonded together. It can be confusing for students that oxygen the atom, and oxygen the molecule, are both called oxygen. Let students know that when we talk about the oxygen in the air, it is always the molecule of oxygen, which is two oxygen atoms bonded together, or O2.
Note: Leave this equation projected throughout the activity in the Explore section of this lesson. Students will need to refer to it as they model the chemical reaction.
Give each student an activity sheet.
Download the student activity sheet, and distribute one per student.
The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan. The activity sheets are formative assessments of student progress and understanding. A more formal summative assessment is included at the end of each chapter.
Students will record their observations and answer questions about the activity on the activity sheet. The Explain It with Atoms and Molecules and Take It Further sections of the activity sheet will either be completed as a class, in groups, or individually, depending on your instructions. Look at the teacher version of the activity sheet to find the questions and answers.
Question to Investigate
Where do the atoms in the products of a chemical reaction come from?
Materials for Each Student
Prepare the Atoms
Build the Reactants
After you are sure that students have made and written the formula for the reactant molecules, tell students that they will rearrange the atoms in the reactants to form the products.
Build the Products
Tell students that in a chemical reaction, the atoms in the reactants come apart, rearrange, and make new bonds to form the products.
Represent the Chemical Equation
