Molecular Motion Teacher Guide



Molecular Motion


Physical Science

Grade Level


Activity Name(s)

Brownian Motion


Being Prepared

This activity uses computer modeling software that allows students to test and retest as they work. Working in small groups of 2 and no more than 3 encourages students to discuss and refine their ideas as they work.

Getting Started

Brownian Motion uses 3 different models. The model in Data Collection I has start, stop, and reset buttons. In Data Collection II students can manipulate the trajectory of an atom by increasing or decreasing the "steering force". In Data III students will look at atoms that they add to their model. They can observe the path of the atoms to see how they react to each other.

Diffusion uses 2 different models. In Data Collection I students model what happens when a drop of dye is added to water. To add the dye simply click on the model once it starts running. In Data Collection II and III students will look at the movement of different size molecules as the temperature is raised or lowered.

Suggested Timeline

Each activity should take 45-50 minutes to complete.

Thinking about the Discovery Questions

The activities in this unit allow students to investigate motion at the molecular level.

Brownian Motion explores the question "What causes Brownian motion?"

Diffusion explores the question "Would the smell of perfume make it across a room faster if it were hotter or colder in the room?". The activity builds on the understanding of the random movement of molecules to explore the process of diffusion. Students will use a model that allows them to see differences in movement in hotter and colder rooms.


There are number of misconceptions that students may come to the activities with. One of these is that atoms or molecules of a liquid, solid or, gas are not moving when the the substance itself is still. The assumption is that if movement can't be seen then the molecules making up the substance are not moving. Another misconception may be that the average distance between the atoms or molecules of a substance remains the same when the temperature of the substance changes.

Learning Objectives

  • NGSS
    • Performance Expectations
      • 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-PS2-5. Conduct an investigation and evaluate the experimental design to provide evidence that fields exist between objects exerting forces on each other even though the objects are not in contact.
      • MS-PS3-4. Plan an investigation to determine the relationships among the energy transferred, the type of matter, the mass, and the change in the average kinetic energy of the particles as measured by the temperature of the sample.
    • Disciplinary Core Ideas
      • MS-PS1 Matter and its Interactions
        • PS1.A. Structure and Properties of Matter
          • Gases and liquids are made of molecules or inert atoms that are moving about relative to each other. (MS-PS1-4)
          • In a liquid, the molecules are constantly in contact with others; in a gas, they are widely spaced except when they happen to collide. In a solid, atoms are closely spaced and may vibrate in position but do not change relative locations. (MS-PS1-4)
          • The changes of state that occur with variations in temperature or pressure can be described and predicted using these models of matter. (MS-PS1-4)
        • PS1.B. Chemical Reactions
          • Substances react chemically in characteristic ways. In a chemical process, the atoms that make up the original substances are regrouped into different molecules, and these new substances have different properties from those of the reactants. (MS-PS1-2),(MS-PS1-3),(MS-PS1-5)
        • PS3.A. Definitions of Energy
          • The term “heat” as used in everyday language refers both to thermal energy (the motion of atoms or molecules within a substance) and the transfer of that thermal energy from one object to another. In science, heat is used only for this second meaning; it refers to the energy transferred due to the temperature difference between two objects. (secondary to MS- PS1-4)
          • The temperature of a system is proportional to the average internal kinetic energy and potential energy per atom or molecule (whichever is the appropriate building block for the system’s material). The details of that relationship depend on the type of atom or molecule and the interactions among the atoms in the material. Temperature is not a direct measure of a system's total thermal energy. The total thermal energy (sometimes called the total internal energy) of a system depends jointly on the temperature, the total number of atoms in the system, and the state of the material. (secondary to MS-PS1-4)
      • MS-PS2 Motion and Stability: Forces and Interactions
        • PS2.B: Types of Interactions
          • Forces that act at a distance (electric, magnetic, and gravitational) can be explained by fields that extend through space and can be mapped by their effect on a test object (a charged object, or a ball, respectively). (MS-PS2-5)
      • MS-PS3 Energy
        • PS3.A: Definitions of Energy
          • Temperature is a measure of the average kinetic energy of particles of matter. The relationship between the temperature and the total energy of a system depends on the types, states, and amounts of matter present. (MS-PS3-3),(MS-PS3-4)
        • PS3.B: Conservation of Energy and Energy Transfer
          • The amount of energy transfer needed to change the temperature of a matter sample by a given amount depends on the nature of the matter, the size of the sample, and the environment. (MS-PS3-4)
    • Cross Cutting Concepts
      • Cause and effect
        • Students classify relationships as causal or correlational, and recognize that correlation does not necessarily imply causation. They use cause and effect relationships to predict phenomena in natural or designed systems. They also understand that phenomena may have more than one cause, and some cause and effect relationships in systems can only be described using probability.
      • Scale, proportion, and quantity
        • Students observe time, space, and energy phenomena at various scales using models to study systems that are too large or too small. They understand phenomena observed at one scale may not be observable at another scale, and the function of natural and designed systems may change with scale. They use proportional relationships (e.g., speed as the ratio of distance traveled to time taken) to gather information about the magnitude of properties and processes. They represent scientific relationships through the use of algebraic expressions and equations.
    • Practices
      • Developing and using models
        • Use and/or develop a model of simple systems with uncertain and less predictable factors.
        • Develop and/or use a model to predict and/or describe phenomena.
        • Develop a model to describe unobservable mechanisms.
        • Develop and/or use a model to generate data to test ideas about phenomena in natural or designed systems, including those representing inputs and outputs, and those at unobservable scales.
      • Planning and carrying out investigations
        • Conduct an investigation and/or evaluate and/or revise the experimental design to produce data to serve as the basis for evidence that meet the goals of the investigation.
        • Collect data to produce data to serve as the basis for evidence to answer scientific questions or test design solutions under a range of conditions.
        • Collect data about the performance of a proposed object, tool, process or system under a range of conditions.
      • Analyzing and interpreting data
        • Use graphical displays (e.g., maps, charts, graphs, and/or tables) of large data sets to identify temporal and spatial relationships.
        • Analyze and interpret data to provide evidence for phenomena.
      • Constructing explanations and designing solutions
        • Construct an explanation that includes qualitative or quantitative relationships between variables that predict(s) and/or describe(s) phenomena.
        • Construct an explanation using models or representations.
        • Construct a scientific explanation based on valid and reliable evidence obtained from sources (including the students’ own experiments) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.
        • Apply scientific ideas, principles, and/or evidence to construct, revise and/or use an explanation for real- world phenomena, examples, or events.
        • Apply scientific reasoning to show why the data or evidence is adequate for the explanation or conclusion.
        • Apply scientific ideas or principles to design, construct, and/or test a design of an object, tool, process or system.
      • Engaging in argument from evidence
        • Construct, use, and/or present an oral and written argument supported by empirical evidence and scientific reasoning to support or refute an explanation or a model for a phenomenon or a solution to a problem.
      • Obtaining, evaluating, and communicating information
        • Evaluate data, hypotheses, and/or conclusions in scientific and technical texts in light of competing information or accounts.
        • Communicate scientific and/or technical information (e.g. about a proposed object, tool, process, system) in writing and/or through oral presentations.
  • NSES
    • Physical Science - Motion and Forces
    • If more than one force acts on an object along a straight line, then the forces will reinforce or cancel one another, depending on their direction and magnitude. Unbalanced forces will cause changes in the speed or direction of an object's motion.
    • Physical Science - Transfer of Energy
    • Energy is a property of many substances and is associated with heat, light, electricity, mechanical motion, sound, nuclei and the nature of a chemical. Energy is transferred in many ways.

Discussion: Setting the Stage

  • What makes up the air around you?

    The air consists of gas molecules, water vapor, dust, and other small particles.

  • Do gas molecules move in the air? What is a common example that tells you that they move?

    Yes. One example is when you feel the wind blowing across your face.

  • Imagine that a drop of peppermint oil was placed in one corner of the room. After a few minutes you can smell it on the other side of the room. Explain what you think is happening.

    As gas molecules in the air bounce around in the gaseous state. The gas molecules move some of the peppermint oil molecules around by colliding with them and spreading out across the room.

Discussion: Formative Questions

  • How would you describe the movement of the different molecules as you ran the models?

    The movement appears to be random. This is because they aren't moving in an empty space. They bounce off of each other, constantly changing the direction of the movement.

  • Besides temperature what else might affect the rate of diffusion?

    The amount of different types of molecules is one factor. The more molecules (concentration) in the air, the more collisions and bouncing would occur. Another would be resistance to movement, though we would only find this when a substance is highly pressurized.

Discussion: Wrapping Up

  • How is air like a liquid?

    Both take the shape of the form they are placed in both are made of atoms and molecules.

  • If all things were kept equal, except the number of perfume molecules in the air what differences might you expect to see in how fast diffusion takes place?

    As the number of perfume molecules in the air increases so does the rate of diffusion.

  • Does diffusion happen in other substances?

    We can see diffusion in water. If you have ever place hot water in a clear mug and then place a tea bag in the mug you can watch the gradual movement of the tea through the mug. You can speed the process of diffusion by stirring the water.

Additional Background

Brownian motion is the random motion of particles suspended in a fluid (a liquid or a gas) resulting from their collision with the quick atoms or molecules in the gas or liquid. The term "Brownian motion" can also refer to the mathematical model used to describe such random movements, which is often called a particle theory. Robert Brown discovered that particles suspended in water moved, though he wasn't able to determine how they were able to move. Einstein published a paper in 1905 that explained that the Brownian motion was caused by the motion of water molecules even in still water.

Diffusion refers to the process by which molecules intermingle as a result of their kinetic energy of random motion. Consider two containers of gas A and B separated by a partition. The molecules of both gases are in constant motion and make numerous collisions with the partition. The tendency toward diffusion is very strong even at room temperature because of the high molecular velocities associated with the thermal energy of the particles (


Brownian Motion

  1. Explain how Brown's particles, which were surrounded by water molecules, appear to be moving "randomly" yet also follow Newton's first law at the same time.

    As the particles hit the water molecules they bounce off. It appears to be random because you can't see the water molecules. Newton's first law fits since it said "An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.". In this case the unbalanced force is caused by the water molecules the pollen comes in contact with.

  2. Describe the motion of a single atom in the second model. How is this different from the motion that Brown observed of a particle in water?

    The atom moves in a straight line with the trajectory changing when it contacts another surface, Brown observed the particles moving but not in a clear direction.

  3. Brown found the strange movement of tiny particles so fascinating that he stared at them for hours. They never stopped bouncing around. Assuming the motion of the particles he saw was due to water being made from atoms, what do you notice about the atom in the second model that would help to explain his continual observation of particle movement?

    You can not adjust the movement to make the single atom stop. If this happened then the particles Brown observed would not move.


  1. How does the smell of perfume make its way throughout an entire room? Explain the molecular motion of the perfume.

    The molecules that make up the perfume are in motion. As they move that come into contact with other molecules that are also in motion. This changes the direction they are moving slightly allowing the perfume to make its way across the room.

  2. Once the perfume molecules reach the other side of the room, what do the perfume molecules do?

    The molecules continue to move around the room.

  3. If there were an L shaped room and you were standing around the corner from where a bottle of perfume spilled, would you smell the perfume? Why or why not?

    You would eventually smell the perfume because there is nothing blocking the molecules from moving around the corner of the L.

  4. How does a change in concentration influence the diffusion of perfume?

    The the greater the concentration the quicker the diffusion across the room.

Further Investigation

Place vanilla extract in a balloon and tie it off. Discuss why you can smell the vanilla. Try other scents and see if some diffuse better than others.