Connecting Molecular Motion to Thermometers

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SWBAT Apply their understanding of molecular motion and thermal energy to explain how a thermometer works

Big Idea

Understanding how a thermometer works will deepen your students understanding of molecular motion.


My students now know that molecules move, based on their investigations with food coloring in a beaker of water, so I now want them to develop an understanding of how a thermometer works.  In particular, I want students to realize that when something hot touches the bulb of the thermometer that the alcohol molecules begin to warm up, expand and, as a result, take up more volume in the tube.  I also want them to see and explain that the opposite occurs when they place the thermometer in something with a lower temperature.

This lesson walks students through the process of developing an ability to follow a procedure.  Often, especially as we navigate the NGSS, we are told that students should develop their own investigations.  On the contrary, scientists follow procedures in many studies that they try to replicate.  We certainly want to avoid 'cookie-cutter' labs that take their inquisitiveness out of the equation, but this activity intends to have students experience the factors that make a thermometer work.  We will then build off of this experience to develop deeper conceptual understanding of molecular motion and the relationship (Crosscutting Concept) between matter and energy.  

Interesting thought following this lesson: Have kids put a thermometer in upside down to observe what happens, especially if the alcohol doesn't touch the liquid.  Later, they can place enough water into the cup to just touch the temperature reading level and observe what happens.

How Does a Thermometer Work?

5 minutes

As a review, I ask students: "Do water molecules always move at the same rate?" I want students to recall that temperature affects molecular motion. 

Here are some questions that may help students put their prior learning experiences together to answer this question:

  • How does the movement of food coloring in water connect to the movement of molecules?
  • What does states of matter have to do with water molecules?

This jogs their memories and establishes a baseline to build off of today in class.  

Experimenting with Thermometers

25 minutes

Having materials, such as thermometers and hot and cold water and cups available and ready to go helps maintain time on learning, so plan ahead.  I then give students thermometers and they complete pages 32 and 33 in this worksheet.  

This is an actual "exploration" of the thermometer. Here students are using a magnifying glass to closely examine a thermometer.

Students follow the procedure on the worksheet and after about 10-15 minutes we review their findings.  Here they are using their finger to warm the bulb of the thermometer. 

I try to maintain my distance as students are following the procedure on the worksheet, because I want them to develop their ability to follow steps and encourage them to work together through anything that confuses them.

Modeling Understanding

25 minutes

Now that students have collected observations from their thermometer activity, it is time for them to develop a conceptual understanding.  The NGSS places a significant emphasis on modeling as a form of representing student understanding, stating: 

Modeling can begin in the earliest grades, with students’ models progressing from concrete “pictures” and/or physical scale models (e.g., a toy car) to more abstract representations of relevant relationships in later grades, such as a diagram representing forces on a particular object in a system. (NRC Framework, 2012, p. 58) 

Students record a picture of their model at different temperatures in their science notebooks.  I want students to model their understanding that molecules speed up, spread out and take up more space when they heat up; opposite occurs when molecules cool down. 

I also want them to think about the implications of attractive forces.  As I circulate and assess student models I ask them to explain the effect that temperature has on molecular motion.  I am looking for them to state that more thermal energy means that the molecules will be further apart and have weakening attractions, due to the distance between the molecules.  Note: Strength of attraction doesn't change. 

Student Reflections

10 minutes

Now that students have explored what actually happens in a thermometer, I want them to reflect on their previous beliefs from the do now activity.  They record what they have learned in their science notebooks.  

I then lead a discussion by asking students to share their reflections.  I ask: "Would anyone like to share how their ideas have changed since the beginning of class--do water molecules always move at the same rate?"  I am looking for students to defend their ideas with observations from the concrete examples that they experienced, as well as apply their understanding of volume.  As we embark on this journey of implementing the NGSS, we must remember that it must all come back to the evidence collected and be applied to what students already know and experience in life and science class.  In this case, I am using a concrete example to help explain an abstract concept that occurs at the submicroscopic word.  I want students to be able to explain that the liquid moves up the inner tube in the thermometer, because the molecules are spreading out.  If the molecules weren't spreading out then the liquid wouldn't move up the tube.