Using Food and Drink to Describe Osmosis

Written by Jennifer Wiatrowski, Pasco-Hernando Community College

I use two different analogies to relate osmosis to the real world:

a. I use two large beakers (beakers A and B), a jug of water and lemonade mix or fruit punch mix (these are dry powders). I then tell the class I am going to mix two classes of a refreshing beverage (nice in a hot climate like Florida). I tell them that I will make the first beverage very sweet, and pour in a large amount of lemonade or punch mix. The second beverage will not be nearly as sweet and I pour in a small amount of the powder. I then fill each of the beakers to equal volumes. I then ask the class if the two beakers have equivalent solutions. (They say no!). So, although the two beakers appear to have equal volumes, the amount of water is varies between the beakers. This shows students that the amount of water in a given space is influenced by the amount of solute. I then ask the students to imagine connecting the beakers together with a selectively permeable membrane and then ask them which way the water would flow  (from beaker A to B or B to A?)

b. I then talk about Martha Stewart. Martha always says that you should never toss your salad until your guests arrive or you are ready to serve it (I was recently informed by my students that “toss the salad” has taken on additional meaning……so this gets a good laugh out of the class). So, at the beginning of class, I take a bag of salad and some dressing and mix them. I then put it aside for awhile. In the meantime we talk about hypertonic and hypotonic. I then ask to think about the scenario with the salad and the dressing. They eventually reason that the salad is hypotonic to the dressing (or the dressing is hypertonic to the salad) and the consequence of this is a watery salad as the dressing pulls water out of the greens. This would make Martha HORRIFIED.  To finish, we take look at the salad we mixed earlier.