Unit Plan 5 (Grade 8 Science): Conservation of Mass in Reactions

Focus: Model how atoms rearrange but are conserved in chemical reactions, so that the total number of atoms and the total mass of substances stays the same before and after a reaction (in a closed system). Connect particle models, mass measurements, and simple reaction representations to show that matter is not created or destroyed.

Grade Level: 8

Subject Area: Science (Physical Science • Matter & Its Interactions • Modeling)

Total Unit Duration: 5 sessions (one week), 50–60 minutes per session

I. Introduction

Students become “mass detectives,” investigating what really happens to atoms and mass during chemical reactions. Building on their understanding of properties, chemical changes, and particles, they measure mass before and after safe reactions and create particle models to show how atoms rearrange into new substances while the total number of each type of atom stays constant. They also explore why mass sometimes seems to change (e.g., gas escaping) and how closed systems help reveal true conservation of mass.

Essential Questions

• What happens to atoms during a chemical reaction?
• How can we show that the total number of atoms stays the same even when substances change form?
• Why does the mass of reactants and products stay the same in a closed system, and why might it look different in an open system?
• How can models, data tables, and mass measurements help us understand conservation of mass?
• Why is the idea that matter is not created or destroyed important for understanding chemical reactions and real-world processes?

II. Objectives and Standards

Learning Objectives — Students will be able to:

1. Describe chemical reactions as processes where atoms are rearranged into new molecules/structures, not created or destroyed.
2. Measure and record the mass of reactants and products before and after a reaction (in a simple closed-system setup) and compare values.
3. Develop and use particle models to show how atoms rearrange during a reaction while the total number of each type of atom stays the same (e.g., using colored tokens, drawings, or digital models).
4. Explain why mass might appear to change in open systems (e.g., gas escaping, products leaving the container) and how a closed system reveals true conservation.
5. Use models and data to describe how the total number of atoms does not change in a chemical reaction and how this relates to conservation of mass (MS-PS1-5).
6. Write or present a claim–evidence–reasoning (CER) that uses both mass data and particle models to support conservation of mass in at least one reaction.

Standards Alignment — 8th Grade (NGSS-based custom)

• MS-PS1-5 — Develop and use a model to describe how the total number of atoms does not change in a chemical reaction and mass is conserved.
  • In this unit, students build particle models and collect mass data for reactions in (mostly) closed systems to show that atoms and total mass remain the same.

Success Criteria — Student Language

• I can explain that during a chemical reaction, atoms are rearranged to form new substances, but no atoms disappear or appear from nowhere.
• I can measure mass before and after a reaction and explain why the total mass stayed the same (or why it might have seemed to change).
• I can draw or build a model showing atoms before and after a reaction and prove that the total number of each type of atom is the same.
• I can describe how open vs. closed systems affect whether we see mass conservation in our measurements.
• I can write a CER that uses data and models to show that mass is conserved in a reaction.