Freezing Balloons

Contributor
Division of Chemical Education, Inc. of the American Chemical Society
Type Category
Instructional Materials
Types
Instructor Guide/Manual , Animation/Movie , Demonstration
Note
This resource, vetted by NSTA curators, is provided to teachers along with suggested modifications to make it more in line with the vision of the NGSS. While not considered to be "fully aligned," the resources and expert recommendations provide teachers with concrete examples and expert guidance using the EQuIP rubric to adapted existing resources. Read more here.

Reviews

Description

The demonstration featured in this blog and video presents a puzzle for students to explain observations of the difference in behavior of four balloons - three filled with helium and the final one with air - when cooled in liquid nitrogen. Students are not told what is in the balloons before the demonstration. They must use their understanding of gas laws and intermolecular interactions to revise their model for behavior of gases in order to explain the observed phenomenon. An explanation of the phenomenon observed in the demonstration is provided by the author here: http://jchemed.chem.wisc.edu/blog/solution-chemical-mystery-4-case-misbehaving-balloon The demonstration referenced in this blog is based on “Freezing Balloons,” published in a 1991 issue of Chem13 News. The original demonstration is presented here: https://www.chem.wisc.edu/deptfiles/genchem/demonstrations/Gen_Chem_Pages/10liquidsnsolidpage/freezing_balloons.htm A key difference in the demonstration published in Chem13 News and the one shown in the blog is that the original demonstration showed the difference in a balloon filled with helium and a second filled with carbon dioxide, rather than air used in the video shown in the blog. Teachers could use the simple directions presented here to do the demonstration for the class as an alternative to watching the video.

Intended Audience

Educator and learner
Educational Level
  • Grade 12
  • Grade 11
  • Grade 10
  • Grade 9
  • High School
Language
English
Access Restrictions

Free access - The right to view and/or download material without financial, registration, or excessive advertising barriers.

Performance Expectations

HS-PS1-3 Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.

Clarification Statement: Emphasis is on understanding the strengths of forces between particles, not on naming specific intermolecular forces (such as dipole-dipole). Examples of particles could include ions, atoms, molecules, and networked materials (such as graphite). Examples of bulk properties of substances could include the melting point and boiling point, vapor pressure, and surface tension.

Assessment Boundary: Assessment does not include Raoult’s law calculations of vapor pressure.

This resource is explicitly designed to build towards this performance expectation.

Comments about Including the Performance Expectation
This demonstration gives students an opportunity to observe the behavior of real gases as the temperature of a sample decreases: one that behaves ideally and the other that does not. To fully address the PE, students need to critically analyze the observed behavior to infer the strength of electrical forces between the gas molecules in each sample. The demonstration could be extended to include lab work or use of an online simulation to test students’ hypotheses. This would help students connect the macroscopic observations with understanding of the molecular level model of gases that explains their behavior.

Science and Engineering Practices

This resource appears to be designed to build towards this science and engineering practice, though the resource developer has not explicitly stated so.

Comments about Including the Science and Engineering Practice
The phenomenon observed in this demonstration provides an opportunity for students to apply their understanding of ideal gas model and the limitations of the model to explain the difference in behavior of the air and helium filled balloons. It is important for a teacher to listen closely to students’ ideas as they work to explain the observation from the demonstration to identify any misconceptions students may hold about the structure of matter and properties of gases. It may be beneficial to have students sketch pictures to show what they think is going on in each of the balloons when it is cooled in the liquid nitrogen. Students could then present their sketches and explain their ideas to each other in small groups or in a large class discussion.

This resource is explicitly designed to build towards this science and engineering practice.

Comments about Including the Science and Engineering Practice
The blog presents a simple question to consider: “How do you think this experiment works?” In order to answer the questions, students must consider what they know about the behavior of gases and why the molecules of carbon dioxide in air would interact differently than atoms of helium. Explanations would include reference to the assumptions inherent in the ideal gas law to consider which of the samples best follows the expected result (in this case, helium) and why the other sample does not (here, it is the air sample).

Disciplinary Core Ideas

This resource is explicitly designed to build towards this disciplinary core idea.

Comments about Including the Disciplinary Core Idea
To further emphasize the connection between the bulk scale observations and the forces between the gas particles, students should be asked to include drawings or descriptions at the molecular level in their explanations. As suggested above, students could present their sketches and explain their ideas to each other in small groups or in a large class discussion. Teachers then could ask probing questions to help students relate the observed behavior to infer the strength of electrical forces between the gas molecules in each sample.

Crosscutting Concepts

This resource appears to be designed to build towards this crosscutting concept, though the resource developer has not explicitly stated so.

Comments about Including the Crosscutting Concept
The ideal gas model is based on an assumption that gas particles do not interact. This assumption can be used to accurately predict the behavior of a helium filled balloon at low temperatures as there is minimal interaction between helium atoms. However, the intermolecular forces between molecules in air are strong at low temperatures, meaning the molecules move much closer together than can be predicted by the Charles’s Law for an ideal gas. The information provided in the instructional materials from the blog explain this in greater detail, but there is no suggestion for how to make this connection more explicit for students. Teachers will need to structure discussion opportunities to allow for the connection of the demonstration to a deeper exploration of the differences in ideal and real gases.

Resource Quality

  • Alignment to the Dimensions of the NGSS: This demonstration provides an opportunity for students to directly observe a phenomenon that is explicitly connected to the strength of electrical forces between particles. It provides a strong connection between the disciplinary core ideas, practices, and crosscutting concepts for three-dimensional learning. With some guided structure, the demonstration can be used in a lesson sequence to help students develop a deep understanding of the behavior of gases based on a complex understanding of the structure of matter and intermolecular interactions.

  • Instructional Supports: The author of the bog does not provide suggestions for using the demonstration as part of an instructional sequence. Teachers must plan how they will move students from describing their observations from the demonstration to making evidence-based explanations for the observed behavior of the balloons. Ideas for structuring lessons for students to express, clarify, and represent their ideas to provide evidence-based explanations and give/receive peer feed back including how to support students in incorporating all relevant ideas and forms of evidence they have encountered during the unit are available from Tools for Ambitious Science Teaching website: http://ambitiousscienceteaching.org/pressing-evidence-based-explanations/

  • Monitoring Student Progress: Teachers should consider using the Predict-Observe-Explain strategy for structuring the use of the demonstration for learning. This strategy is explained here: http://arbs.nzcer.org.nz/strategies/poe.php The use of this strategy can help to elicit student thinking before and after the demonstration to support teachers in planning instruction and allow sufficient opportunities for providing ongoing feedback to students. This strategy will work well with the tools available from the Tools for Ambitious Science Teaching website referenced above.

  • Quality of Technological Interactivity: The blog includes a video that is available on YouTube. This provides limited interactivity. Teachers could use a tool like EduCannon, https://www.educanon.com/, or other app to edit the video to include questions and pauses for students to record their observations and begin to make sense of those obserations. The video could also be posted for students to view independently through Google Classroom or other student management space to flip the learning experience and allow more time for discussion of the observed phenomenon.