In this lesson, students will create a particulate model of matter that explains energy changes and transfer during a physical process, such as the crystallization of a solid from a supersaturated solution.

Grade Level

High and middle school

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

  • MS-PS1-4: Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.
  • MS-PS3-2: Develop a model to describe that when the arrangement of objects interacting at a distance changes, different amounts of potential energy are stored in the system.
  • HS-PS3-2: Develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles (objects) and energy associated with the relative position of particles (objects).
  • Science & Engineering Practices
    • Developing and using models
    • Constructing explanations (for science) and designing solutions (for engineering)
    • Engaging in argument from evidence
  • Crosscutting Concepts
    • Cause and effect: Mechanism and explanation
    • Systems and system models
    • Energy and matter: Flows, cycles, and conservation

AP Chemistry Curriculum Framework

This demonstration supports the following unit, topic, and learning objective:

  • Unit 6: Thermodynamics
    • Topic 6.1: Endothermic and Exothermic Processes
      • ENE-2.A:Explain the relationship between experimental observations and energy changes associated with a chemical or physical transformation.
    • Topic 6.3: Heat Transfer and Thermal Equilibrium
      • ENE-2.C: Explain the relationship between the transfer of thermal energy and molecular collisions.


By the end of this lesson, students should be able to

  • Use the particulate model of matter to explain different exothermic and endothermic physical processes based on changes in the kinetic and potential energy of the particles that make up a system.
  • Use the particulate model of matter to predict whether a physical process would be exothermic or endothermic based on the analysis of changes in the kinetic and potential energy of particles in the system.
  • Represent changes in the energy of a system during a physical process using different types of visual representations.

Chemistry Topics

This lesson supports students’ understanding of

  • Heat
  • Exothermic
  • Endothermic
  • Physical Change
  • Intermolecular Forces
  • Kinetic Energy
  • Potential Energy
  • Temperature


Teacher Preparation: 20-30 minutes

Lesson: 60-120 minutes, depending on application.


  • Reusable Hand-warmers (e.g., HotSnapZ)


  • Always wear safety goggles when handling chemicals in the lab.
  • Students should wash their hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup and disposal of materials.

Teacher Notes

  • Course Sequence Suggestions:This activity was originally designed to follow an experience with particulate energy during phase changes. The original intent was an opportunity to extend student models to a new scenario. Click here to view the preceding lesson.
  • Introduction to energy during a chemical reactions unit: Dissolution and crystallization of ionic compounds acts as an interesting bridge between the worlds of physical and chemical changes. This lesson was originally designed as an opportunity to extend the model students developed on energy in phase changes. You may wish to do so immediately or use this lesson to begin conversations on the role of energy in chemical changes. Instead of waiting to explore this idea in a more traditional energy unit, consider using this lesson during a unit on chemical reactions and reaction types. This lesson is not intended to develop a formal, quantitative understanding. Instead, use this lesson to begin the cycle of modeling energy in chemical changes early in the year. This phenomenon can be revisited during a more quantitative look at chemical energy (ex. Enthalpy) and again during when the multi-faceted process of dissolution is detailed (see below), where a more correct conception of this phenomenon lies.
  • Energy and phase changes: A more traditional location for this lesson, which was originally designed as an opportunity to extend the model students developed on energy in phase changes. Many students will approach the situation as a simple change in state of matter, but some will begin to question whether the true nature of the phenomenon lies somewhere else. Use this lesson as a transition to chemical energy and bond energies. By replacing the traditional line between physical and chemical changes with a gradient, students begin to see that these categories are manmade in an attempt to describe and categorize what we observe. Removing that barrier reveals that both physical and chemical changes simply involve a rearrangement of particles, just on different scales. The categories of kinetic, potential (phase) and chemical energy simply allow us the vocabulary to describe the diverse ways energy manifests itself on the particle-level.
  • Solutions: This lesson can be used to introduce heat of solution. Alternatively, if students have experienced this lesson prior to this point, challenge their model by demonstrating that some dissolution processes are endothermic and some are exothermic. This new data presents a limit for their model, which will prompt students to modify or replace their old model to explain this new data. ‘What particle-level differences might account for different energy exchanges when a salt goes into solution?’ Even if the scope of a course does not include an in-depth consideration of factors like lattice energy, solvation and hydration, this experience can be valuable. Not only can students practice the iterative process of modeling building, but leaving students with questions can foster a curiosity about chemistry.
  • Background Information for Teachers: The energy changes that we observe when a system undergoes a physical change can be explained by using the particulate model of matter. A reusable hand warmer contains a supersaturated solution of sodium acetate. Sodium and acetate ions in this solution are far apart, separated by water molecules. When particle rearrangement is induced by clicking on a metal disk, some ions with opposite charge move towards each other and arrange into small crystals. The kinetic energy of the ions increases as they rearrange and get closer under the action of attractive forces. The increase in the average kinetic energy of particles is felt as an increase in the temperature of the system. While the solvent-solute interactions are more complex than what is described here, the scenario provides an avenue for students to apply their particulate model of matter to a more complex situation.

    This lesson is rooted in the scientific modeling process. To learn more about modeling in chemistry, read this brief primer. The second half of the lesson makes use of a system of bar graphs for having more quantitative discussions about energy with students. For an introduction into how to use these types of bar graphs, view this screencast.

Instructional notes and answers, along with a list of additional teaching resources can be found here.

  • Student Prerequisite Information: Particulate model of matter, including the understanding of interactions between particles (intermolecular forces) and the relationship between temperature and average kinetic energy of particles in the system.

    Kinetic and potential energy, including the understanding of how kinetic and potential energy change when particles move as a result of their interactions.

    This lesson is intended to extend student models of energy as developed in lessons on thermal energy transfer and energy in phase changes.

For the Student


The reusable hand warmers we’re considering contain a supersaturated aqueous solution of the ionic compound sodium acetate (CH3COONa). The main species present in this solution can be represented as shown below:

  1. Draw a particulate representation of the supersaturated solution of sodium acetate that is present before the hand warmer is activated. Describe in writing what your image seeks to represent.

  1. Activate the hand warmer by bending the metal disk inside the plastic packet several times. Observe and feel any changes that may occur. Describe in writing all changes that you observed.

  2. Draw a particulate representation of the contents of the hand warmer after bending the metal disk. Describe in writing what your image seeks to represent.

  1. Using the two particulate representations before and after the activation, build an explanation for any changes in temperature that you perceived in the system. Your explanation should be based on the properties and behaviors of the molecules and ions that comprise the system.

  2. Describe how the kinetic energy and potential energy of the molecules and ions in the system change when the hand warmer is activated.

  3. Using bar graphs similar to those used in Parts 1 and 2 qualitatively represent the kinetic energy and the potential energy of the particles before and after the activation.

  4. How do you think the particles in the hand warmer behaved similar to the melting ice we considered previously? How do you think the two situations are different?

  5. The models you’ve built suggest a different form of potential energy, commonly known as chemical potential energy. How could this form of energy be defined? Build a detailed explanation of how chemical potential energy in the supersaturated solution of sodium acetate is transformed into kinetic (thermal) energy as a result of the activation.

  6. The packaging provides the following directions for reuse: “Fully immerse the hand warmer in a pot of boiling water for 20 minutes until all of the crystals in the pack have disappeared.” Does your model predict this phenomenon? Explain.