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: https://www.youtube.com/watch?v=U-Uo9-wvhQk