What You Need



To help students understand the transfer of energy that occurs in a combustion reaction.


This lesson is part of the Energy in a High-Tech World Project, which examines the science behind energy. Energy in a High-Tech World is developed by AAAS and funded by the American Petroleum Institute. For more lessons, activities, and interactives that take a closer look at the science behind energy, be sure to check out the Energy in a High-Tech World Project page.

Students in grades 6-8 are beginning to understand the connections among the kinds of energy (potential, kinetic, radiant, thermal, chemical, electrical, electromagnetic, nuclear, heat, and work). They may understand that substances or objects may possess various forms of energy, and that energy changes often involve a transfer of energy in the form of heat or work. (National Science Education Standards, p. 154.)

Students may associate energy with force, i.e., kinetic energy or work, but they may not completely understand the first law of thermodynamics, which is that energy can’t be created and it can’t be destroyed in ordinary physical and chemical transformations (in nuclear reactions, the transformation between energy and matter must be considered). It can only change form.

In this lesson, students will gain a better understanding of what energy is, how it can be changed into other forms of energy, and how physical and chemical processes contribute to that. They also will learn what happens to energy when materials are heated in a combustion system, as in an auto gasoline-powered engine.

Prior to beginning this lesson, students should be able to define atoms and molecules in simple terms. They also should understand that atoms and molecules are always moving, and that adding heat increases the speed at which they move.


To start this discussion about energy, print and distribute The Mysterious Everything student sheet. Use The Mysterious Everything teacher sheet to read “The Mysterious Everything” to your students in class. (Please note: The student sheet has most of the same text as the teacher sheet but without the answers to the questions, which you will ask when you see the placeholders that signal you to stop reading. The idea is for students to read along with you and try to solve the mystery.)


Introduce to students the idea that energy and transformations of energy are everywhere and all around us. Energy takes many different forms, such as potential and kinetic, solar, thermal, chemical, electrical, and nuclear.

Ask students to go to the About Combustion student esheet to explore how energy changes during combustion. Using this esheet, students will read about what happens to energy when materials are heated in a combustion system, as in an auto engine.

First, students should go to and read How Car Engines Work: Internal Combustion on the How Stuff Works website. They should study the illustration of the engine parts so they understand how combustion works in a gasoline-powered engine. They should use the About Combustion student sheet to answer the following questions once they have finished reading about how car engines work:

  • Why is a car engine an internal combustion engine rather than an external combustion engine? (Because it is a more efficient way to transfer energy as opposed to an external combustion engine, which burns fuel—usually coal, wood, or oil—to first generate steam, which then does the work. Also, a smaller engine is required; with an internal combustion engine, the fuel is burned inside the engine.)
  • What are the four strokes in the combustion engine in this article? (They are the intake, compression, combustion, and exhaust strokes.)
  • Describe how these four strokes work. (As the piston goes down [as a result of momentum from the previous cycle], the intake valve opens to let air and gasoline enter the cylinder. This is the intake stroke. The intake valve then closes as the piston moves back up, compressing the air/fuel mixture. This is the compression stroke. The compressed mixture is ignited by the spark plug causing an explosive combustion [the chemical reaction of oxygen with fuel]. The hot gases from the combustion expand, driving the piston down—the combustion stroke—and back up—the exhaust stroke—as the exhaust valve opens, pushing the exhaust gases out the tailpipe. The piston then falls back down as the intake valve opens, beginning the intake stroke of the next cycle.)

Students should then go to Energy: The Quick Tour to further explore how energy is transformed in internal as well as external combustion engines. Students should answer these questions once they have finished:

  • What is it in the combustion process that causes gas to expand? (The right mix of air and fuel, and an ignition source is required for combustion; chemical energy from the combustion reaction is released, increasing the energy of the hot gases in the cylinder, causing them to expand.)
  • How does the transformation of energy into mechanical energy in a car engine differ from the transformation of energy into mechanical energy in a power plant, which is an external combustion system? (In the power plant, combustion of the fuel [such as coal] is used to heat water, which generates steam under pressure that expands to do work.)

Next, they should go to The Conservation of Energy and the First Law of Thermodynamics. While reading this article, students should think about the following questions and come to class prepared to discuss the answers:

  • What is the first law of thermodynamics? (Energy can change from one form to another, but cannot be created or destroyed.)
  • Where does the energy from the combustion process go? (It is turned into mechanical energy [work]. In the car, it’s what powers the wheels. The rest of it is dissipated as heat to the atmosphere—from emission of the hot exhaust gases out the tailpipe, from the engine block itself, or from the coolant as it passes through the radiator.)

A combustion reaction is a chemical reaction. To see how the chemical reaction works, ask students to go to Combustion Reaction and click on the play button. Ask students to answer this question:

  • What is happening in this chemical reaction? (The fuel/air mixture is compressed, a heat source is applied [spark], and the gasoline explodes. Gasoline is made up primarily of hydrocarbons. During combustion, the carbon atoms of the hydrocarbons in gasoline bond primarily with oxygen atoms from the air to form carbon dioxide [CO2] and some carbon monoxide [CO]. Hydrogen atoms of the hydrocarbons combine with oxygen atoms to form water molecules [H2O].)

For an optional, hands-on activity about combustion, you may want to try a simple demonstration. Read the Demonstration I on Fire Wars: Teacher Demonstration at NOVA Teachers website. Be sure to conduct the experiment yourself with students watching from a safe distance. The demonstration includes some preliminary questions about how a fire burns, so lead into the activity by asking students the questions listed in the procedure section.


Ask students to write in their own words an explanation of the combustion process in an internal-combustion engine and the chemical result. Ask them to compare that process to an external-combustion engine and explain how the two processes differ.


The following Science NetLinks lessons can help extend the ideas in this lesson:

  • Converting Energy is about how energy can be measured and includes an exercise for insulating a pyramid
  • The Transfer of Energy 1: Thermochemistry can be used to teach students about heat and chemical reactions using a hot/cold pack experiment
  • The Cold Car Start Science Update discusses how and why chemical reactions slow down in cold temperatures, affecting a combustion reaction

The Energy Story is an energy education website of the California Energy Commission that has simple information about heat energy, how it is measured, and how we get energy from food.

Funder Info
American Petroleum Institute
This content was created with support from the American Petroleum Institute.

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Lesson Details

Grades Themes Project 2061 Benchmarks National Science Standards