Heat Transfer - Convection Part 2

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Objective

SWBAT use a variety of models to explain transfer by means of convection currents.

Big Idea

This series of mini labs helps to introduce students to the complexity of convection.

Getting Started

In the second part of two lessons on convection, students will observe a change in the volume of a liquid at different temperatures in one activity and then observe the mixing of colored hot and cold water to see convective mixing taking place. 

For the first activity, you will need enough ice water for each group, about 600 ml per group along with hot water for each group. Note that you will also need crushed ice and hot water of the second activity, so prepare plenty. 

Before starting the lab create one filled test tube for each group:

  1. Mix a small amount of water and a few drops of green food coloring in a beaker
  2. Pour the green water into each test tube until it is almost completely full.
  3. Push the end of the eye dropper into the hole on the large end of the rubber stopper.
  4. Place the rubber stopper on the glass bottle; press down to seal tightly. Some green water should rise up into the eye dropper. 
  5. Place these in a test tube holder until ready to use. 

 

For the second part of the lesson you will need a pair of pill bottles for each group of students with 2 1/4" holes drilled into the lid. 

You could set up these two activities as stations and have students rotate through both in one class period or it could stretch across two classes. 

Engage

10 minutes

As students enter I have the following "Do Now" projected on the overhead for them to complete in their journals or you may have photocopies prepared for them on their desks. 

 

 

The intent of using the Temperature probe here is both to clarify student thinking up to this point and elicit their ideas on how a thermometer works -- as most don't realize what it is they are measuring when they read a thermometer (the fluid in the thermometer is heating and expanding due to heat transfer from the substance being measured). It builds nicely off of the preview lesson on conduction and radiation and further explores the workings of a thermometer. 

The discussion that follows gives me the opportunity to listen in on their thinking about why we wait to take the measurement from the thermometer until after the reading stops rising or falling. My experience over the years is that students rush to make their temp readings. 

This probe comes from Uncovering Student Ideas in Science, Volume 3: Another 25 Formative Assessment Probes by Page Keeley, National Science Teachers Association; 2008. 

Explore

30 minutes

Think about how you want to manage this activity before it is taught. If you think your students require more time, you will want to split this lesson over two days. If the first activity is all set up when kids come into the class, it goes very quickly (10 min). You can have students move on to the second part and then go back and complete the follow up questions for homework. But if your students are struggling with making meaning of this first activity, having them go back and respond to these questions as homework won't "grow" their understanding. Think about, and do, what is best for you and your students. 

As students begin, remind them of lab safety as them are working with hot water. Goggles are recommended. 

In the first activity - Volume Change Caused by Temperature Change, ask each student to color in the images of the test tubes on the lab sheet to show the change in level of the liquid in each of the hot/cold baths and at room temp.  

As you walk around the room dialog with your students as to what is happening with the molecules of the liquid as it heats and cools.  You are laying an important foundation here for the change in density of fluids as the volume changes when heated and cooled. 

 

In the second activity - Sinking and Floating Waterexercise caution when having the students fill the pill bottles with hot water. I did this for them and had them use test tube tongs to carry and submerge the pill bottles into the cool water. Safety first!

Before they begin, make sure each student has a blue and a red pencil so that they can record this data (visually as drawings) as they watch the movement of the water. It is a good idea to place a white backdrop behind the beaker. This enables the "scientist" to get a better view of what is happening. See the image below. 

 

 

In the second part of this lab, they add the second bottle with ice to the same beaker and observe the mixing adding their observations. 

 

Explain

10 minutes

In activity one, students should make the connection on their own that they have created a thermometer. Some might ask why we don' t fill thermometers with water. Push this thinking back on them and ask them the range of temperatures that we measure with thermometers and what happens to water when it cools to 0 C. If you haven't chatted with them about what is in a liquid thermometer, this is good lesson push their thinking on properties of liquids like alcohol or mercury and why thermometers are filled with them. 

As you move through the follow up questions engage them in science talk about how the change in temperature affects the volume of the water. Bring them back to the definition of density and what they know about mass and volume. 

Connect this understanding about volume change/density change with temperature change and what they observed in the second activity. 

You might use some focus questions such as;

  • What have you determined so far?
  • What do you think happened?
  • What did this demonstrate?
  • What is going is going on when......?

Your goal here is to build their understanding and confidence about how heat changes the way fluids interact keeping the bigger focus of this learning moving towards Earth science phenomena such as air masses, ocean currents and the like. 

Here is a clip of my students sharing their understanding: