Thermal (Brownian) Motion: Atoms and Molecules Are Always Moving

Interactive, scaffolded model

Overview and Learning Objectives

In this activity, students will see that, when a small particle is surrounded by water molecules (or by other atoms/molecules), the resulting motion looks random. The particle appears not to move in straight lines. However, this apparently random movement is due to collisions with many other atoms or molecules, all moving in straight lines until they collide.

Students will be able to:

• replicate Brown's experiment in which he discovered Brownian motion;
• explain that the apparent random motion of atoms and molecules is responsible for many cellular processes;
• analyze how temperature influences the motion of particles.

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Assessment

http://www.concord.org/~barbara/workbench_web/pdf/Thermal_Motion_Assess.8.07.pdf

Rubric

http://www.concord.org/~barbara/workbench_web/pdf/Thermal_Motion_Rubric8.07.pdf

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Classroom Practice

As a fundamental science activity, it will be useful to have students take the time to understand how the models work, and how to save and print their reports for you to collect.

Research on student understanding of kinetic molecular theory has shown that students have a great many alternative ideas about what the world is made of. For instance, students of all ages have trouble understanding that matter is made of discrete particles that are in constant motion and have empty spaces between them. (Novick and Nussbaum, 1978)

Understanding that the overall movement of molecules dissolved in water is random due to many collisions with constantly moving water molecules will set students up for the next Stepping Stone in which they will explore osmosis and diffusion.

You may want to distribute the text in the "Additional Info" section as background prior to having students run the activity.

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Central Concepts

Key Concept:

Particles are constantly and randomly hit by other particles around them; if they receive more hits from one side than the other, there will be a net force causing them to move.

Additional Related Concepts

Concept Map Available

Biology

• Motion

Physics/Chemistry

• Atom
• Brownian Motion
• Random kinetic motion

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Textbook References

• Biology: Concepts and Connections (Pearson) 5th Edition - Chapter 4: A Tour of the Cell

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Benchmarks and Standards

AAAS

• THE PHYSICAL SETTING: ENERGY TRANSFORMATIONS - Although just as much total energy remains, its being spread out more evenly means less can be done with it (Full Text of Standard)
  
  
• THE PHYSICAL SETTING: MOTION - Whenever one thing exerts a force on another, an equal amount of force is exerted back on it (Full Text of Standard)
  
  

NSES

• Life-Science: Matter, energy, and organization - 1 All matter tends toward more disorganized states (Full Text of Standard)
  
  
• Physical-Science: Energy Conservation / Entropy - 4 Everything tends to become less organized and less orderly over time (Full Text of Standard)
  
  
• Physical-Science: Motions and Forces - 1 Objects change their motion only when a net force is applied (Full Text of Standard)
  
  

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Extensions and Connections

An activity that introduces students to models and their limitations as well as the idea that an atom moves in a straight line until it collides with something is called Superballs are Like Atoms (http://molo.concord.org/database/activities/130.html).

A colleague in Italy, Enrica Giordano, writes: "We also made a video of milk and latex particles in water under the microscope. Projecting it on a screen you can draw particles trajectories, take measurements and make some simple calculations to verify at a first level the Einstein relationship linking time and deplacement."

A good historical review of Brownian Movement can be found at http://www.sciences.demon.co.uk/wbbrowna.htm.

See also: http://www.phy.ntnu.edu.tw/java/gas2D/gas2D.html for a Brownian Motion applet from the University of Taiwan.

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Additional Info

Additional Background

Random Kinetic Motion

To us a cell looks very small, but to a molecule the cell is more like a large, very crowded city in the year 2700. Surrounding molecules bombard a particle constantly from all directions, every second.

Work in living cells never stops, as there are always machinery and structures to be built and repaired, and chemical reactions to make these changes possible. Small and large molecules move continuously about the cell, and must meet each other before being joined together with various strong bonds or weak attractions.

For example, enzymes find their receptors through random motion. Proteins find each other in order to self-assemble. The movement of water molecules help untangle DNA.

Like building things from Lego blocks and magnets, first you have to bring blocks close to each other, and then the right ones with fitting surfaces "click" together into a wall or a machine. Often the fit needs a bit of work. When you screw in a light bulb, you have to move it about, "feeling" the position. So a molecule not only moves with direct motion, but it also vibrates, and rotates around some bonds. All of these movements allow the best fit between molecules to be made.

In short, scientists have found that heat energy makes all particles move around the cell nonstop in an unpredictable fashion (scientists call it a "random walk"); the more heat is around, the faster they move, reaching speeds of up to 500 miles an hour. It also explains why every living creature needs a certain temperature range for comfortable living. When molecules move too slowly in our cells, construction work slows; very fast-moving molecules will complicate the construction as well.

A botanist Robert Brown was the first who described a jittery non-stop movement of tiny microscopic particles suspended in a vacuole inside a pollen grain. Brown found that the "movement without end" could be seen not only in different living plants, but also in dead pollen and in fine powdery dust that ruled out that the random "movement without end" was due to pollen being "alive." Using our Molecular Workbench you can model and explore Brown's "movement without end."

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Macro Micro Link

With a microscope you can see this kind of motion with particles such as pollen in solution. Particles are getting hit, but not equally on all sides.

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Activity Credits

Created by CC Project: Molecular Logic using Molecular Workbench

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Requirements

• Java 1.5+ - Java 1.5+ is available for Windows, Linux, and Mac OS X 10.4 and greater. If you are using Mac OS X 10.3, you can download MW Version 1.3 and explore within it instead.

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