What is a Chemical Reaction?

American Chemical Society - Middle School Chemistry
Type Category
Instructional Materials
Model , Simulation , Demonstration , Lesson/Lesson Plan
This resource, vetted by NSTA curators, is provided to teachers along with suggested modifications to make it more in line with the vision of the NGSS. While not considered to be “fully aligned,” the resources and expert recommendations provide teachers with concrete examples and expert guidance using the EQuIP rubric to adapted existing resources. Read more here.



The phenomenon of a burning candle is used to introduce the idea of reactants and products.  Students use a paper model of atoms in a combustion reaction to see that atoms are not created or destroyed, only rearranged.  They move on to counting atoms and using charts to expand to other reactions.  The lesson is accompanied by animations and a reading, and features a video explanation for teachers.

Intended Audience

Educator and learner
Educational Level
  • Grade 8
  • Grade 7
  • Grade 6
  • Middle School
Access Restrictions

Free access - The right to view and/or download material without financial, registration, or excessive advertising barriers.

Performance Expectations

MS-PS1-5 Develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved.

Clarification Statement: Emphasis is on law of conservation of matter and on physical models or drawings, including digital forms, that represent atoms.

Assessment Boundary: Assessment does not include the use of atomic masses, balancing symbolic equations, or intermolecular forces.

This resource is explicitly designed to build towards this performance expectation.

Comments about Including the Performance Expectation
The purpose of this lesson is for students to visualize the rearrangement of atoms during chemical reactions, through the use of a paper model and of charts. The students follow directions to assemble and use a model, but do not develop their own models; therefore this lesson is useful for working toward this PE but is not suitable as an assessment.

Science and Engineering Practices

This resource is explicitly designed to build towards this science and engineering practice.

Comments about Including the Science and Engineering Practice
Students build on the learning from a model (a chart showing atoms in the reactants and products during the chemical formula for the combustion of methane) and revise to show the combustion of propane and of butane. The variables in this case are the proportions of each atom in the reactants and in the products, and the relationship is 1:1 in reactants and in products, no matter what the reaction. Though atoms are not observable during chemical reactions, their rearrangement is what transforms the reactants into products.

Disciplinary Core Ideas

This resource is explicitly designed to build towards this disciplinary core idea.

Comments about Including the Disciplinary Core Idea
This lesson is meant to show students that atoms are conserved during a chemical reaction. The lesson does not discuss mass, so students should do some macro-level activities involving the masses of products and of reactants. Reactions in which one or more reactants or products are gases can be useful, as students often have a misconception that gases “don’t weigh anything”. Mass activities could be done before this lesson, so the students can use knowledge from this lesson to explain why they got the results they did with their mass experiments.

This resource is explicitly designed to build towards this disciplinary core idea.

Comments about Including the Disciplinary Core Idea
The purpose of the activity is for students to use a model that allows them to physically rearrange the atoms of reactants to form atoms of products, and to use a mathematical model (a chart) to see that the atoms are the same on both sides of the equation. The lesson does not specifically focus on the properties of reactants vs. products, though this is alluded to in the introductory phenomenon, when students and teacher discuss the reactants and the products of a candle-burning reaction.

Crosscutting Concepts

This resource is explicitly designed to build towards this crosscutting concept.

Comments about Including the Crosscutting Concept
The model, charts and animation of “Combustion of Methane” are designed to help students visualize this idea.

Resource Quality

  • Alignment to the Dimensions of the NGSS: The lesson begins with an anchoring phenomenon, and uses the practice of modeling to address the DCI of conservation of matter through the lens of the CCC of atoms being conserved. The lesson does not specifically address mass, nor do students develop their own models, so this lesson alone will not suffice to address the Performance Indicator.

  • Instructional Supports: The learning is three-dimensional, and anchored by a familiar phenomenon. The lesson has limited opportunities for students to discuss and justify their ideas - some discussion and questions that call for brief answers. The lesson does have supports that can be used to scaffold or extend learning - animations and a chapter reading are included. Teachers may need to offer additional support with new vocabulary, especially on the student handout, where students may not be familiar with words like “coefficient”, “subscript”, and “conserved”.

  • Monitoring Student Progress: The charts and questions on the student activity sheet may be used as a formative assessment. A summative assessment is not necessary as this is the first of a series of lessons in this chapter. A “Test question bank” is provided for the entirety of the chapter, but the questions relating to this lesson are not sufficient. Many questions are closed-ended (multiple choice, true-false, fill-in) and the short answer questions are about content knowledge, and do not address the skill of modeling.

  • Quality of Technological Interactivity: This lesson does not involve interactivity. Computer technology may be helpful, as the lesson includes a Flash animation that could be projected for the class.