Summary

In this lab, students will measure how much carbon dioxide they exhale by reacting their exhaled breath with limewater (calcium hydroxide solution). Students will practice writing balanced equations and completing mass-to-mass stoichiometric calculations.

Grade Level

High School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-7: Use mathematical representation to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking

Objectives

By the end of this lab, students should be able to:

• Complete mass-to-mass stoichiometric calculations.
• Better understand the concept of limiting reagents.

Chemistry Topics

This lab supports students’ understanding of:

• Net ionic equations
• Stoichiometry
• Limiting reactant
• Concentration

Time

Teacher Preparation: 20 minutes

Lesson: 50-60 minutes, split over 2 days (to allow for the precipitate to dry)

Materials

For each student or small group (depending on whether you want students to take measurements on their own breaths or to use one group member’s data):

• goggles
• straw
• 250 mL beaker
  
• saturated calcium hydroxide solution (limewater)
• filter paper
• timer
• funnel
• balance (several balance stations can be set up around the lab for students to use when ready)
• drying oven (optional – if you are not using the oven, allow more time for precipitate to dry)
  

Safety

• Always wear safety goggles in the lab when carrying out an investigation.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• When working with acids and bases, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
• Remind students to exhale through the straw, do not inhale the limewater.

Teacher Notes

• This lesson assumes students are familiar with solving mass-mass stoichiometry problems and the concept of limiting reactant. They don’t need to solve for limiting reactant but should be familiar with the concept.
• The formulas of the compounds in this activity are not provided, so students should know how to write formulas from a compound’s name, or you could provide the formulas for students.
• The lesson will take at least two periods separated by at least one day to complete. The use of the drying oven on a “low” setting will require a longer time to dry the precipitate, but a higher setting runs the risk of scorching the paper. If you do not have a drying oven available, you can store the filter papers in a dry, well-ventilated area for a longer period (e.g., a week) before weighing the final mass and doing the calculations.
• The solubility of calcium hydroxide is 1.7 g/L at 20 ^oC. Since each student needs ~100 mL of solution, for a class of 20 students you would need to have 2 L of saturated calcium hydroxide solution. If you wish to prepare it yourself, dissolve 3.5 g in 2 L of room-temperature distilled water, stirring thoroughly, and allowing any undissolved material to settle. (You can be sure it’s saturated if a little bit of solid remains at the bottom.) Filter the undissolved solid out before making the solution available to students. (Keep in mind that calcium hydroxide exhibits retrograde solubility – unlike many other ionic compounds, its solubility decreases as the temperature increases.) You could also purchase commercially available limewater solution. Scale up for more than 20 students, or you could reduce the amount of material required if you only have one student per pair or small group actually exhale into the limewater and the whole group can use that person’s measurements. (Although this is less fun!)
• Limewater is basic (pH ≈ 12.5) and can cause skin corrosion/irritation, so instruct students to be careful not to inhale the limewater or get any on their skin. If they do get it on themselves, instruct them to rinse the affected areas thoroughly with water.
• Waste solutions are neutralized with vinegar and dumped down the drain. This can also be accomplished by the students; adding a couple drops of phenolphthalein indicator can add a “splash” to this otherwise mundane chore of environmentally responsible waste disposal.
• This lesson can be modified for a more “inquiry” method, where students are required to create their own procedure and justify what kinds of data they are collecting.
• For students who need more support or guidance (such as students in an ESOL program or with IEPs), portions of this lab can be modeled ahead of time; for example, the procedure of filtration, rinsing, and drying can be demonstrated via a teacher presentation prior to this lab. Small heterogeneous lab groups likewise can help students who may be struggling with the language in the lab understand what is expected of them. In terms of understanding the calculations, all students can benefit from prior instruction in mass-to-mass stoichiometry problems including direct instruction, small group practice, and student-led problem sets.