Stoichiometry Balloon Races

Regents of the University of Colorado
Type Category
Instructional Materials
Demonstration , Instructor Guide/Manual
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.


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The demonstration described in this resource involves a simple reaction between magnesium and hydrochloric acid.  The reaction is performed in flasks with balloons attached to capture the hydrogen gas that is produced by the reaction. Four separate flasks are used side-by-side running the same reaction, using differing amounts of each of the reactants in each flask.  Students observe the effect that the first two balloons are smaller than the others, but the final two are approximately the same size.  This result is only predictable from an understanding of the stoichiometric ratio of magnesium to hydrogen chloride in the balanced chemical equation.  Thus, the demonstration provides an opportunity to engage students in question of why the third flask that starts with more total reactants would not produce more hydrogen gas compared to the flask with a lesser starting amount.  This demonstration is a valuable experience for students to introduce the concept of limiting reactants in chemical reactions or alternatively, it could also be used for assessment of student understanding of stoichiometric relationships.  


It is important for the teacher to review the recommendations and safety notes in the Prep Notes page of the resource.  In the Prep Notes for the demonstration, sometimes it is suggested to use magnesium ribbon and other points mention magnesium turnings.  Magnesium turnings are a better choice as this allows for more surface area for the reaction to proceed at a faster rate so that there is sufficient time for students to spend time developing an explanation of the results.  However, this may cause the flasks to heat up which can impact the result of the demonstration as the higher temperatures will affect the size of the balloons.  It is important for all of the reaction mixtures to control the temperature of the flasks to control for this.  This can be achieved by setting the flasks in a cold water bath.  While this will slow the reaction somewhat, it will help ensure an expected result.  If materials are not available to perform the reaction or to support students not able to be in class for the demonstration, teachers may want to consider use of the video of the demonstration that is available on YouTube from the Berkeley Chem Demos group:

Intended Audience

Educational Level
  • Grade 12
  • Grade 11
  • Grade 10
  • Grade 9
  • High School
Access Restrictions

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

Performance Expectations

HS-PS1-7 Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

Clarification Statement: Emphasis is on using mathematical ideas to communicate the proportional relationships between masses of atoms in the reactants and the products, and the translation of these relationships to the macroscopic scale using the mole as the conversion from the atomic to the macroscopic scale. Emphasis is on assessing students’ use of mathematical thinking and not on memorization and rote application of problem-solving techniques

Assessment Boundary: Assessment does not include complex chemical reactions.]

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

Comments about Including the Performance Expectation
This demonstration gives students an opportunity to examine the idea of conservation of mass using the stoichiometry of the reaction to determine how much product is expected to be produced from a given amount of reactants. This demonstration would be a valuable lesson in a unit on understanding mathematical relationships represented by chemical equations to help students to gain a deeper understanding of the meaning of coefficients proportional relationships between masses of atoms in the reactants and the products, and the translation of these relationships to the macroscopic scale using the mole as the conversion from the atomic to the macroscopic scale. In order to achieve the aim of the performance expectation from NGSS, teachers need to allow time for students to analyze the mathematical relationships as they develop explanations for the results. See additional tips below for ideas.

Science and Engineering Practices

This resource was not designed to build towards this science and engineering practice, but can be used to build towards it using the suggestions provided below.

Comments about Including the Science and Engineering Practice
Observing the demonstration itself does not require students to use mathematics and computational thinking. The teacher will need to structure the lesson in a way that highlights this practice. After students have observed the reaction, the teacher should consider using the Predict-Observe-Explain strategy for structuring the use of the demonstration for learning. This strategy is explained here: After the demonstration, in the "Explain" section of the strategy suggested here, the teacher should ask students to construct an explanation for why the fourth flask with the largest amount of reactants available did not produce more reactants than in the third flask and expect that they use evidence from their observations along with stoichiometric calculations to support their ideas. Structuring the lesson in this way will provide a valuable opportunity for students to use computational thinking regarding the stoichiometric ratios to analyze the results of the demonstration they have observed.

Disciplinary Core Ideas

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

Comments about Including the Disciplinary Core Idea
Students will need to use knowledge of chemical reactions to predict what will happen when magnesium and hydrochloric acid are combined in the reaction flask. To emphasize this component of the Chemical Reactions disciplinary core idea from NGSS, teachers should ask students to write the balanced chemical equation on their own and then after the demonstration, ask them to use their knowledge of the ratios from the chemical equation to explain the result (see the tip above for the suggested Predict-Observe-Explain strategy to help structure the lesson).

Crosscutting Concepts

This resource is explicitly designed to build towards this crosscutting concept.

Comments about Including the Crosscutting Concept
To emphasize that matter is conserved in this reaction, a teacher may want to pose a question of how it is possible that the same amount of hydrogen gas is produced in both the third and fourth flasks, even though the last one started with more magnesium. Asking students to explain how matter is still conserved will help emphasize this key concept and help to build ideas of limiting and excess reagents. This question can be incorporated into small group or whole class discussions following the demonstration after students have developed their initial explanations of the observed results.

Resource Quality

  • Alignment to the Dimensions of the NGSS: This demonstration provides an opportunity for students to observe phenomena with a result that can only be explained through careful analysis. Students must rely on prior knowledge of the chemical properties of the reactants to predict the reaction that takes place, including the stoichiometric ratio between the two reactants and the resulting products. This is a valuable opportunity for students to use mathematical reasoning to explain the race results. To meet the expectations of three-dimensional learning with NGSS, it is important for a teacher to structure a lesson with this demonstration to be sure to emphasize the practices and crosscutting concepts as described above.

  • Instructional Supports: The surprising end result of the demonstration is engaging for students to extend their learning about stoichiometric relationships in chemical reactions. Teachers will need to consider how to provide differentiated support for all learners through opportunities for students to represent their ideas and make sense of the observations using mathematics. Developing an explanation for the results of the demonstration requires students to make use of knowledge of chemical reactions (single-replacement reaction here), chemical bonding (to correctly predict the formulas of products), and stoichiometry (to analyze the results using mole ratios). The teacher should consider students’ prior learning to determine appropriate supports that will allow students to engage with the focus practice here of using mathematics and computational thinking. For example, it may be beneficial to provide the balanced chemical equation for some students so that the can focus on using the stoichiometric ratios from the coefficients.

  • Monitoring Student Progress: No guidance is given for how to implement the demonstration and monitor student learning during the lesson. Teachers should consider using the Predict-Observe-Explain strategy for structuring the use of the demonstration for learning (as noted above in the tips provided). 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. In addition, a teacher may hold short conferences with each student/group, posing questions such as “what do you know about the reaction from the balanced chemical equation?” or “what does the observation of the size of each balloon on each flask indicate about what’s happening in the reaction?” and then following with further probing questions to uncover students’ thinking. The conferences provide an opportunity for the students to prepare to communicate their results in a whole class discussion and provides the teacher with data to use in planning appropriate ways to support students in practice of using mathematics and computational thinking to construct explanations in ways that connect to the disciplinary core ideas and crosscutting concepts.

  • Quality of Technological Interactivity: There is no use of technology needed to perform the demonstration.