Limiting and Excess Reactants Mg and HCl Balloon Demo

Feb 24

Demos are a great way to start a unit. For our stoichiometry unit, I usually start with the demonstration between Mg and HCl. I found this demo on the U of Oregon website, but they have been working on updating resources to make them usable since the retirement of Flash. And each year, I take more time than I need to try and remember how much of each reactant I used, so I thought I would document it here for myself and others to come to when they need an easy limiting and excess reactant demo.

Set Up and Materials

3 flasks, each containing 100 mL of 1 M HCl

3 balloons, filled with 0.6 g, 1.2 g, and 2.4 g of Mg powder or filings respectively
Universal indicator
Ring stands and clamps

Diagramming the HCl

I give each student a piece of blank copy paper. Since we are past our reactions unit at this point, I ask them to write the reaction we will be observing at the top of their copy paper.

I then ask students to draw three flasks, beakers, boxes (if they’re not artistically inclined), on the copy paper with some space above the flasks. As the year progresses, I ask my students to work on representing the reactions and demos we see on the particulate level. I start this demo by pouring 100 mL of HCl in three separate flasks. I ask students to represent the HCl on the particulate level. At this point in the year, I am hoping they represent HCl as a strong acid, which would be ionized and that for every H+, there is 1 Cl-. Some students may be there, some may not represent the acid as ionized. Each flask will look the same, as each flask contains the same amount (and concentration) of HCl.

Adding the Universal Indicator

After we have diagrammed the HCl, I get a dropper full of universal indicator. I show students that the indicator is currently green. I ask students to make a hypothesis about what they think will happen after I add the indicator to the HCl solution in each flask. I then add the indicator to the first flask and we observe that the flask will turn red (the universal indicator is red in acidic conditions). I don’t tell the students why it turns red, we just observe that it does.

Adding the Mg to the Flask

Before adding the Mg, I ask students to look at the chemical reaction they wrote and tell me what we are going to observe produced. The students should be able to tell me that hydrogen gas will be produced.

I take the first balloon with 0.6 g of Mg in it, and place it on the top of the first flask. I then shake the Mg from the balloon into the flask below and we watch the reaction. You can ask students to provide signs of a chemical reaction while this is occurring. I have them draw a balloon over their first flask and draw what is inside the balloon at the particulate level. Most students will draw that the hydrogen is what is in the balloon, but they may not draw the hydrogen as diatomic.

I then proceed to place the second balloon with 1.2 g Mg on the 2nd flask, and the 2.4 g Mg on the 3rd flask. I ask students to draw similar balloons on the 2nd and 3rd flasks. The 2nd and 3rd flasks should produce more hydrogen, so I have students ponder the question “how could we represent that these balloons are larger on the particulate level?” Ideally, students would draw more H2 in the larger balloons.

Side note:

I don’t tell them how much Mg is in each balloon, just that it is Mg. Throughout the entire demo, I don’t tell them much. The reason for that is that I want them to come up with questions that we will address as we go through our unit.

Questions about the Demo

After the students have diagrammed the demo, and we have let it sit for a few minutes, I have students come up with questions about the demo. Each student gets 3 post-its and I ask them to write down three questions that they have about the demo (1 per post-it).

After everyone at their table has come up with 3 questions, I ask them to share those questions at their table (read them out), and then pick the top 2-3 that they think are most interesting, most important, etc.

Each table then reads aloud to the class what questions they picked, and I have them place the questions on a central location in the classroom. We will refer to these questions as we go throughout the unit.

Tips

Use different colored balloons for the 3 different amounts of Mg. That’s more for you than for the students-helps you keep track of the different amounts.

Collect your students' diagrams to get an idea of where they are in terms of diagramming reactions on the particulate level.

Conclusion

This balloon demo is a simple yet powerful way to introduce limiting and excess reactants in a visually engaging manner. By encouraging students to observe, predict, and generate their own questions, you set the stage for deeper exploration throughout the stoichiometry unit. The use of particulate-level diagrams strengthens their understanding of molecular interactions, and the class discussions help bridge the gap between macroscopic observations and atomic-scale reasoning. As the unit progresses, students can refer back to this demo to solidify their grasp of key concepts, making it an excellent anchor for learning.

In my next blog post, I’ll be talking about how I am going to add some CER practice as we revisit this demo in the hopes of developing my students’ CER skills for a lab during our stoichiometry unit. Thanks for reading. Happy teaching!