While looking for labs to go with Chapter 18: Equilibrium in Modern Chemistry, I had a hard time finding one for which I had the required ingredients.  The Modern Chemistry curriculum had this lab, Strike a Balance, which has the students come up with their own experiment to determine if your body is getting rid of excess carbon dioxide (produced by exercising) in your breath.  One of the students knew that carbon dioxide added to water would make it slightly acidic. So we could use a pH indicator in water to see when the carbon dioxide concentration goes up or down.  I had a couple of different pH indicators out and let the students decide which one to use.  I also had out a 0.1 M solution of NaOH to make the water slighly basic before they blew into it which would then neutralize it.  This could be useful depending on the indicator they chose to use.IMG_4345

In the photo above you can see a flask of tap water (roughly 100 ml) with a few drops of Bromothymol Blue indicator.  Note the pipette in the beaker with the indicator, this helps to make sure we don’t cross contaminate with the pipettes and protects my counter.  An interesting thing we found is that using distilled or filtered water makes a big difference. One group used filtered water and found the Bromothymol Blue indicator turned the water yellow, showing it was acidic, so they couldn’t get it to change color by blowing since that just makes it more acidic. So tap water works best for this experiment.

Students did different experiments, some timed how long they had to blow into the flask to change the color of the indicator after just sitting around, and then did it again after 5 or 10 minutes of exercise.  Some tried walking, then running, some did kempo (martial arts) forms, and some tried holding their breath.  Most found that after excercising they had to blow for a shorter time before the indicator changed color, indicating a higher concentration of carbon dioxide in their breath. FullSizeRender 7 The one anomaly was a student who is an Irish dancer and  therefore used to a lot of exercise.  The concentration of carbon dioxide in her breath didn’t seem to change after she exercised but she didn’t really get her heart rate up either.  I think we would have had to make her dance for quite awhile to see a change.

I found this other lab write up on the Royal Society of Chemistry‘s website which explained the chemistry in a bit more detail and with specific procedures which helped me as the teacher figure out how to do this and make it work with the indicators that I had.

Before class students were to watch Crash Course Chemistry 28: Equilibrium.  

But while preparing for class I found these videos which are also useful for this topic.

I also found this website , Chemistry LibreTexts, which has a lot of nice examples for calculating equilibrium constants, particularly when dealing with liquids and solids.