Measuring Changes

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SWBAT explain how thermometers work.

Big Idea

The physical properties of a substance can change with temperature. How do bulb thermometers measure temperature?

The Need for the Lesson

In the previous lesson, Racing Molecules, we learned that the kinetic energy of water can be measured and the speed calculated. Is this how thermometers work? How do thermometers work? Students harbor many misconceptions about how temperature is measured.  

Investigation Preparation & Summary

5 minutes

Students will measure temperature by creating their own thermometer - an experience of thermal expansion first hand. This is the tendency of mattter to change in volume in response to a change in temperature. (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 how changing the temperature of the water will change the volume of fluid inside the thermometer. (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.) (SP3 - Planning and Carrying Out Investigations)

Students will evaluate their findings by creating a graph to compare the temperature of the water to the height of the water in the straw portion of their thermometer (SP4 - Analyzing and Interpreting Data) (SP5 - Using Mathematics and Computational Thinking). 

Students will answer questions about the graph and use information from Racing Molecules to provide an explanation for why the volume changes. (SP7 - Engaging in Argument from Evidence) and (SP8 - Obtaining, Evaluating and Communicating Information)

A complete materials List can be found in the resources section.

The link for the ruler template can be found here.

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.

Mastery begins with practice. It is important that students clearly understand the expectations for Turn/Talk/Record and demonstrate their ability to engage as scientists. My students often express that one of the things they like about science class is that they feel trusted. Turn/Talk/Record releases the ownership of learning back to the students, trusting them to engage in appropriate discourse.

Today's questions are:

      1. When do people use thermometers? List as many examples as you can.
      2. Which thermometer is recording a higher temperature? How can you tell?
      3. Why does the height of the line change when the temperature changes?

I stop students before they start the procedure to share out answers to the first three questions. This helps me assess their readiness to move onto the lab in this lesson.

Setting Up the Thermometer

Setting up the test thermometer is a bit tricky so I circulate around the room to help students with placing the clay around the neck of the vial. We want to seal the vial and push enough clay into the neck of the vial so the air pressure increases - sending water up the straw. Some students may require additional clay to make the seal and increase the air pressure. Students need to be patient and attend to precision so that all leaks are sealed.

As students often grab the straw to move their their homemade thermometer, be sure to remind students that they should move the vial by holding it at the rim.  

Adding the graph helps students visualize the relationship between the volume of fluid and the temperature of the fluid.

Before we began this lesson I administered a probe to check for student understanding. In the video below, I share the most common answers from my students and highlight their misconceptions. Probes are an excellent tool for determining what students know so the lessons can target misconceptions. Student sample probe responses are in the resource section.

Connecting the Learning

5 minutes

What are other types of thermometers and how do they work?

Students suggest digital thermometers, ear thermometers and the crystal thermometers used in aquariums. I share with students a short description about how each one works. 

Digital thermometers measure change in electrical resistance.

Ear thermometers measure thermal radiation coming from your body.

Liquid crystal thermometers reflect different colors of light at different temperatures.

In each case the thermometer measures change. A change that results in an observable pattern so we can count on the thermometer as reliable tool for measurement.

There is a lot more to learn about measuring heat than what we have covered in this lesson.