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


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.

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.


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.


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 ( This video provides an accessible explanation as to why CO2 causes 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.