Milk, Food Coloring, and Detergent: An Active Learning Activity

In searching for quick science videos I stumbled across the Sick Science! YouTube channel. The videos posted there are short and to the point, they are of very high quality, and they show how to do a wide variety of eye-catching demos. Beware – if you like science demos, the site is rather addictive.

One of the prettiest demos I found (Color Changing Milk) shows a beautiful rainbow of colors that bursts from the center of a dish of milk when food coloring and detergent are added. It’s a wonderful early-semester video because it is a striking way to demonstrate the relationship between hydrophobic and hydrophilic molecules.

Color changing milk

A dish of milk + food coloring + detergent = a dazzling display of molecular motion.

Best of all, the video ends with an invitation to test other variables, including sample questions. You could use these as a starting point to have students ask questions, design experiments, make predictions, carry out the experiments, and interpret their results. It would be an inexpensive and memorable active learning activity for lecture or lab. Not a bad payoff for a one-minute video!

The Sick Science! videos are brief, so they don’t offer much in the way of explanation. I will be honest; after I watched Color Changing Milk I wasn’t sure if I could explain it to students myself, and I actually like and appreciate chemistry. Of course this uncertainty could be good or bad, depending on your teaching objectives.

But if you want to give your students (or yourself!) some help with what’s going on at the molecular level, you can find a somewhat lengthy (12-minute) explanation of the phenomenon at the Spangler Effect YouTube site. The narrator’s explanation includes a model using yellow sticks (fat molecules) and matchsticks (detergent molecules), as shown in the screenshot below.

Fat-detergent model

The Spangler Effect video uses matchsticks to represent detergent molecules and yellow sticks to represent fat molecules. But where’s the water?

But I think this explanation still doesn’t quite go far enough. In my opinion, he also needs to add models for water molecules and show how milk’s fat globules are suspended in water. (The diagram below, from a physics lab at Emory University, could be helpful in filling out the story.) It would then be easier to show how the detergent disrupts the fat globules. Once liberated, the fat molecules “race” to find each other, and that molecular motion causes the burst of color.

Diagram of fat emulsion

This diagram of one droplet in an emulsion may help students visualize how fat globules are suspended in milk. Detergent disrupts these globules.

Toward the end of the video, the narrator substitutes watered-down Elmer’s Glue for the milk in the dish. (Like milk, glue contains both hydrophilic and hydrophobic molecules.) If you add food coloring and detergent just like before, then let the glue dry, you end up with a round “stained glass” disk that you can pop out of the dish and show your friends.

If you use these videos in your own class, please post a comment and let us know how it goes!

This entry was posted in Active learning, Assignments, Collaboration, Engaging students, Learning at home and tagged , , , , , , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s