Activity 2.5 - Using a Digital Balance and BTB (30 min)

Target Student Performance

Students (a) practice using two key tools for investigation—digital balances and BTB—with accuracy and precision and (b) describe how they can use these tools to detect matter movement and matter change.

Resources You Provide

BTB, blue (1 cup per group of four students)
clear plastic cups (1 per group of four students)
digital balance (1 per group of four students)
paper clips (10 per group of four students)
safety glasses (1 per group of four students)
straws (1 per group of four students)

Resources Provided

Recurring Resources

Learning Tracking Tool for Systems and Scale (1 per student)
Assessing the Learning Tracking Tool for Systems and Scale
(Optional) BTB Color Handout
BTB Information and Instructions Handout

Setup

Prepare the materials listed above for each group, including diluted BTB solution. Use the instructions on the BTB Information and Instructions Handout for details about how to prepare the BTB. Print one copy of the 2.5 Class Results for Investigation Tools 11 x 17 Poster and post it to the wall in an accessible location, or prepare to display the 2.5 Class Results for Investigation Tools Spreadsheet. You may want to print one copy of the BTB Color Handout for each group, but this is optional.

1. Use the instructional model to show students where they are in the course of the unit.

Show slide 2 of the 2.5 Investigation Tools PPT.

2. Introduce students to the two investigation tools.

Introduce the investigation tools to students and explain that since atoms are too small to see, scientists have developed special methods to investigate things about the world they can’t know from using their eyes only. Today they will explore two of these tools: a digital balance and Bromothymol Blue (BTB).

3. Have students practice using the digital balance in small groups.

Show students slide 3 of the PPT.

Accommodation: Print out slide 3 so each group has a copy of the instructions.

Divide students into groups of four. Tell students to follow the instructions on the slide, and to determine the mass of at least four different items: a paper clip, a straw, a cup of BTB, and a pencil.
Tell students to record their results on the 2.5 Class Results for Investigation Tools 11 x 17 Poster or the 2.5 Class Results for Investigation Tools Spreadsheet.
Have students compare their results across groups for the digital balance.

4. Have students practice using BTB in small groups.

Show students slide 4 of the PPT and tell one student in each group to use the soda straw to blow through BTB while the other students watch and record their observations.

Have each group record results on the 2.5 Class Results for Investigation Tools 11 x 17 Poster or the 2.5 Class Results for Investigation Tools Spreadsheet.
Have students compare their results across groups for the BTB using the (optional) BTB Color Handout as a reference guide.

5. Have students look for patterns and sources of error in their measurements.

Show students slide 5 to help students compare results of mass and color from different groups. Have them answer the questions on the slide. Have students compare and discuss the results from different groups, look for patterns, and identify possible sources of error. This activity will be the first time they do this, and you can judge the sophistication of their thinking.

If students are discussing…

…how the last digit on the balance (.01 g) is sensitive to slight movements and wind currents, and therefore it may not be reliable, then students are discussing levels of precision.
…the differences between the objects that are similar for all groups (paper clip and straw) and the objects that vary from one group to another (pencil and cup of BTB), then the students are discussing real variation in objects.
…that the balances themselves may be inconsistent, then they are discussing variation among balances. One way to check this is to mass the same object on different balances.
…variations in BTB color depending on how long and how hard different students blew through the solution, then they are discussing time dependence.

Accommodation: Create a slide with the discussion points and descriptions mentioned above to help facilitate the discussion about error.

6. Discuss key conclusions about digital balances and BTB

Discuss key conclusions about using digital balances and BTB for investigations:

Digital balances measure mass very accurately. The last decimal place (.01 g) shows tiny differences that can be affected even by air currents.
Use Slide 6 and the optional BTB Color Handout to remind students that BTB detects concentrations of CO₂ in the air. They can keep the handout to use in future investigations.

7. Discuss key conclusions about digital balances and BTB

Show slide 7 of the 2.5 Investigation Tools PPT. Discuss with students how a digital balance can be used to measure matter moving into or out of a system. Highlight that the mass of the system can be measured before and after a change happens in a system. Discuss the two possible conclusions students can draw from their observations:

If the mass of the system increases, then matter must have moved into the system (remember the facts about atoms)
If the mass of the system decreases, then matter must have moved out of the system.

8. Discuss matter change question as it relates to BTB

Show slide 8 of the 2.5 Investigation Tools PPT. Discuss with students how BTB can be used to measure matter change in a system. Highlight that the BTB in a closed container can be observed before and after a change happens in the system. Discuss the two possible conclusions students can draw from their observations:

If the BTB changes from blue to yellow, then a chemical change may be producing CO₂
If the BTB changes from yellow to blue, then a chemical change may be using CO₂ as a reactant.

9. Have a discussion to complete the Learning Tracking Tool for this activity.

Show slide 9 of the 2.5 Investigation Tools PPT.

Have students take out their Learning Tracking Tool for Systems and Scale.
Have students write the activity name in the first column, "Zooming Into Air."
Have a class discussion about what students did during the activity. When you come to consensus as a class, have students record the answer in the second column of the tool.
Have a class discussion about what students figured out during the activity that will help them in answering the unit driving question. When you come to consensus as a class, have students record the answer in the third column of the tool.
Have a class discussion about what students are wondering now that will help them move towards answering the unit driving question. Have students record the questions in the fourth column of the tool.
Have students keep their Learning Tracking Tool for Systems & Scale for future activities.
Example Learning Tracking Tool

Activity Chunk

What Did We Do?

What Did We Figure Out?

What Are We Asking Now?

Zooming Into Air

"Zoom into" air and explore how the world can be studied at multiple scales, including the atomic-molecular scale.

We can learn about the world at different scales.

· Three facts about atoms are: 1) Atoms last forever, 2) Atoms make up the mass of all materials, 3) Atoms are bonded to other atoms in molecules.

How can we use atoms and molecules to explain ethanol burning?

10. Have students complete an exit ticket.

Show slide 10 of the 2.5 Investigation Tools PPT.

Conclusions: What information can BTB provide?
Predictions: How do you think you could use a digital scale and BTB to figure out what happens when ethanol burns?
On a sheet of paper or a sticky note, have students individually answer the exit ticket questions. Depending on time, you may have students answer both questions, assign students to answer a particular question, or let students choose one question to answer. Collect and review the answers.

The conclusions question will provide you with information about what your students are taking away from the activity. Student answers to the conclusions question can be used on the Driving Questions Board (if you are using one). The predictions question allows students to begin thinking about the next activity and allows you to assess their current ideas as you prepare for the next activity. Student answers to the predictions question can be used as a lead in to the next activity.

We expect that individual students, and the class as a whole, will often have difficulty identifying sources of error and finding patterns in the data. Yet these skills are essential if they are to interpret the results of their investigations meaningfully. In particular, you will be working with students to identify patterns across multiple replications - for example, when different groups in your class all conduct the same investigation. The students will have to decide what claims about patterns are justified. Listen for their responses to the discussion in the final step to determine if they need more support in identifying sources of error or finding patterns.

Differentiation

Modifications

Have students figure out that BTB is an indicator for CO₂ by having them compare what happens to blue BTB when they blow bubbles in BTB through a straw, compared to making bubbles with a turkey baster. Ask students what the difference is between the air they exhale and the air in the bubbles from the turkey baster.

Tips

Collect results from the different groups and compare their measurements. Discuss threats to accuracy of measurement. Make sure that each student in every group has a chance to find the mass of something on the digital scales.

Extending the Learning

If students have questions about how or why the BTB changes color, show the first 6.5 minutes of the Colorful Indicators: MIT Chemistry Behind the Magic video (https://www.youtube.com/watch?v=nAhIrh9vM4A&t=2sO). This video provides an accessible explanation as to why CO₂causes the BTB to change color.

Have students see how long it takes the BTB to change colors for a resting student and for that same student after exercising.

Have students weigh other objects and use BTB to look for evidence of CO₂ (or other acidic materials) in other liquids.