Maintaining Mass

Allyn Short
Type Category
Instructional Materials
Activity , Demonstration , Experiment/Lab Activity , Lesson/Lesson Plan , Answer Key
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.



Students will view a teacher demonstration then do an activity themselves.  They will compare and contrast the results and methods, to determine why the results were different.  Students will use knowledge about conservation of mass and an understanding of systems to determine why the teacher demo seems “broken”.   Students will then  design and run for themselves an “improved” version of the demonstration.  

Intended Audience

Educator and learner
Educational Level
  • Middle School
  • Grade 8
  • Grade 7
  • Grade 6
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 come to the realization that in a closed system, the mass does not change even when a chemical reaction occurs, but in an open system, the results may look different from that. Students do an activity, then modify a “broken” demo, to show that mass is conserved during chemical reactions in closed systems. The actions of individual atoms is not addressed during this lesson, though students do use quantitative analysis of the masses of products and reactants to draw conclusions.

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 follow specific instructions to do an activity, then evaluate the results of the teacher demo vs. their activity. They use their conclusions to revise the design of the teacher demo, and use their revised plan to carry out the experiment, in order to make the experiment useful for supporting the Law of Conservation of Mass.

Disciplinary Core Ideas

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

Comments about Including the Disciplinary Core Idea
Students are led to this understanding by analyzing the different setup and different quantitative results of the teacher demo vs. the student activity. They then apply their understanding by modifying the teacher demo, and using those modifications and the data they collect, to support the Law of Conservation of Mass. The lesson does not involve the actions of individual atoms and molecules, only of the masses of macro-scale substances.

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

Comments about Including the Disciplinary Core Idea
Students are specifically asked whether a chemical reaction has occurred, and to describe how they know. For this lesson to be useful for teaching the Law of Conservation of Mass, students must be able to do this. They will need to come into the lesson with prior knowledge about signs of a chemical reaction.

Crosscutting Concepts

This resource appears to be designed to build towards this crosscutting concept, though the resource developer has not explicitly stated so.

Comments about Including the Crosscutting Concept
This lesson demonstrates the difference between open systems (the original teacher demo) and closed systems (the student activity and student experiment). The language of systems is not used, though it is suggested as an add-on for high school during the “Explain” section questions and answers. The idea of open vs. closed systems could easily be highlighted in this lesson, as the teacher can use the language while drawing a parallel to the concrete ideas of the open vs. closed containers used. During the compare/contrast part of the lesson, the teacher can ask students about where the gases went during the teacher demo, and link that to the concept of the chemical reaction in the open flask being a sub-system of all the systems working in the classroom. The introductory activity involves video clips of a plant growing and a building being constructed, in order to make the point at the end of the lesson that the Earth and its materials is a wider system.

Resource Quality

  • Alignment to the Dimensions of the NGSS: The lesson centers around phenomena, beginning with video clips that lead to a question, and following up with a discrepant event (teacher demo that does not seem to support the Law of Conservation of Mass). Students use the practice of planning and carrying out investigations, combined with an understanding of open vs closed systems, to support the core idea of conservation of mass. The lesson does not overtly use the language of systems; a teacher will have to show students the connection to that vocabulary. The lesson also does not assess the entirety of a Performance Indicator, as it does not ask students to consider the chemical reactions on an atomic or molecular level.

  • Instructional Supports: Students are motivated by the opportunity to fix a “broken” demo themselves. Connections are made to buildings and the growth of plants. Students will build on prior knowledge of chemical vs. physical changes. They will discuss ideas orally with each other and informally with the teacher, and will also share their ideas in written form. The lesson does not include suggestions for differentiation, either for expansion or for additional support.

  • Monitoring Student Progress: The lesson includes many suggestions for formative questions and check-ins, involving ideas about conservation of mass as well as about designing an experiment. The lesson also includes a summative assessment. The conclusion that students write in the “Expand” portion of the lesson is likely to be a more useful and in-depth summative assessment than the multiple choice questions in the “Evaluate” section . Those questions can be useful as a quick post-lesson measurement, as they directly assess understanding of the core idea, and by implication the cross-cutting concept, but the practice is not addressed in them. Depending on their experience, students may need support about what to write in their conclusions, in order to show how their practice of experimentation showed support for the core idea of conservation of matter because of the crosscutting concept of systems.

  • Quality of Technological Interactivity: The lesson does not use interactive computer technology.