Unit Plan 4 (Grade 8 Science): Physical vs. Chemical Changes

Focus: Investigate how temperature, state, and particle motion change during interactions, and use models to predict and describe what happens when thermal energy is added or removed. Compare physical changes driven by thermal energy (like melting, boiling, condensation) with chemical changes, using particle models to clarify the difference.

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 deepen their understanding of physical vs. chemical changes by focusing on what happens at the particle level when substances are heated or cooled. Building on prior work with properties and chemical reactions, they now concentrate on thermal energy and how it affects particle motion, temperature, and state (solid, liquid, gas). Through simple heating/cooling investigations, temperature–time graphs, and particle diagrams, students develop models to predict and describe what happens in common situations, such as melting ice, boiling water, and condensation. Along the way, they contrast these physical changes with chemical changes, clarifying the differences in what happens to particles.

Essential Questions

• How does adding or removing thermal energy change particle motion, temperature, and state in a substance?
• What is the difference between a physical change and a chemical change when we think about particles?
• How can models (particle diagrams and graphs) help us predict and describe what will happen during heating or cooling?
• Why might temperature sometimes change during physical changes and sometimes during chemical reactions, and how can we tell which is which?
• How does understanding thermal energy and particle motion prepare us to explain more complex phenomena (like phase changes and reaction energy) later?

II. Objectives and Standards

Learning Objectives — Students will be able to:

1. Describe how particle motion and spacing differ in solids, liquids, and gases and how these change when thermal energy is added or removed.
2. Collect and interpret temperature–time data for simple heating and cooling processes (e.g., ice melting, water heating to near boiling, cooling water).
3. Develop and revise particle models and state diagrams that show changes in motion, temperature, and state during heating and cooling (e.g., melting, freezing, evaporation, condensation).
4. Use models to distinguish between physical changes (state changes, no new substances) and chemical changes (new substances formed), linking to prior unit work.
5. Explain, using particle motion and thermal energy, why temperature sometimes plateaus during phase changes (e.g., melting, boiling).
6. Create a final model-based explanation (diagram + written reasoning) that predicts and describes how particles will behave in a given thermal scenario (MS-PS1-4).

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

• MS-PS1-4 — Develop a model that predicts and describes changes in particle motion, temperature, and state when thermal energy is added or removed.
  • In this unit, students create and refine models (particle diagrams, temperature–time graphs, and state diagrams) for heating and cooling processes and use them to explain physical vs. chemical changes.

Success Criteria — Student Language

• I can describe how particles move and how close together they are in solids, liquids, and gases.
• I can use temperature–time data and graphs to describe what happens when a substance is heated or cooled.
• I can draw or revise a particle model that shows how motion, spacing, and state change when thermal energy is added or removed.
• I can explain why a change is a physical change (same substance, different state or energy) or a chemical change (new substance) using particle-level reasoning.
• I can create a model that predicts what will happen in a new heating/cooling scenario and explain my prediction.