Racing Molecules

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SWBAT calculate the approximate speed of molecules in warm and cold water.

Big Idea

Temperature is the measurement of the average energy of particles in a system. but just how fast are they moving? Faster than a speeding car? Faster than the speed of sound? Faster than the speed of light?

The Need for the Lesson

Students know that all matter is made up of atoms and atoms have energy. They are constantly in motion. Just how much faster are atoms in warm water compared to atoms in warm water? And if we could measure their speed how fast do they move - faster than a car, the speed of sound, the speed of light?

Students will measure the difference in the speed of atoms in water of different temperatures.

Investigation Preparation & Summary

10 minutes

Students will be able to measure the average kinetic energy of water molecules at different temperatures (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.) (SP2 - Developing and using models)

Students will investigate the rate at which dye tablets dissolve in cold, warm and hot water. (SP3 - Planning and carrying out investigations)

Using and equation developed by scientists students will be able to estimate the average kinetic energy of water molecules (MP4 - Model with mathematics) with the real world application of math. 
(SP5 - Using mathematics and computational thinking), (8.EE.A.2 Use square root and cube root symbols to represent solutions to equations of the formx2 = p and x3 = p, where p is a positive rational number. Evaluate square roots of small perfect squares and cube roots of small perfect cubes. Know that √2 is irrational.)

Students will compare the calculated speeds to other know speeds: cars, light and sound. (SP4 - Analyzing and interpreting results) (SP7 - Engaging in argument from evidence)

In conclusion, students will explain the difference between hot and cold water at the molecular level (SP8 Obtaining, evaluating, and communicating information)

A complete materials list can be found in the resource section.

Students in Action

45 minutes

We begin this lesson with the Turn/Talk/Record strategy we used in Checking Temperatures. The students will turn to talk - engage in scientific discourse - with their elbow partner. 

The questions they are talking about create a need for the lesson:

Students will put some warm water in one cup and cold water in another, by looking at the cups can you tell which one if filled with the warm water?

      1. How might you figure out which cup had warm water in it without touching the water?
      2. What do you think is going on inside the warm water that makes it different from the cold water?

I stop students and ask them to share out their answers.  They would like a thermometer to figure out which cup has warm water and which cup has cold water.

Students prepare three cups of water. One cup is cold, one warm - about room temperature and the third is warmer but not too hot to touch. They measure the temperature of the water in each cup using the Celsius scale.

The students place food coloring in each of the cups at exactly the same time. Observations are made after 30 seconds, 1 minute and 2 minutes. Students record their observations using both words and a sketch. 

Asking students to sketch their findings helps focus their attention on the details.

Student Calculations

Using a mathematical relationship discovered by scientists students calculate the speed the water molecules in each of the cups. The speeds are compared to a car, the speed of sound and the speed of light.

We follow-up with a few questions to clarify our understanding.

Why does the food coloring spread? The water molecules are moving. What do we call the energy of motion? Kinetic energy. Which water sample had the highest kinetic energy? The warm water had the highest kinetic energy. How do you know? The warm water pushed the food coloring molecule around the cup the fastest. What other way can you tell that the warm water molecules have more kinetic energy? Temperature is a measure of average kinetic energy.

I have included a short video (link here in the resource section or click on the video below) showing the experiment using liquid food coloring instead of food coloring tablets. My students wanted to know if the liquid food coloring would act the same as the tablets. And they wanted to know if the color of the food coloring made a difference in the movement of the molecules. Audio was not included in the video so we could discuss the observations in class.

Connecting the Learning

10 minutes

Extending Our Learning

Which has more heat energy? In this video from Bill Nye the science Guy, we learn that all things hot and cold have heat energy. The amount of heat energy is related to the size of the object. Although the match feels warmer, it cannot melt the larger ice sculpture.

Closing a lesson with a video where students see the application of new knowledge applied to another example deepens their conceptual understanding.