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Systems and Scale | Activity 2.5

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

Students complete a quiz to assess their understanding of the hierarchy of scales and their ability to apply the key facts about molecules and then discuss their answers to the questions.

Materials 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


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 Handoutt for each group, but this is optional.


1. 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).


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.


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.


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.

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.