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Exploding Colors of Milk
At this time of year I like to push my students to think deeper and connect more science ideas. I know they always push back when I do this in the spring. Her minds are outside in the warm weather. We do get outside a lot. However, all the hard work we did by experimenting, thinking, arguing from evidence, writing, rewriting and arguing ideas will go to waste if I do not give them more opportunities to put that skill to work. So, we do the “exploding colors of milk” lab.
This lab is simple to set up but conceptually complex to explain scientifically. You simply take a flat plate covered about 1 cm deep with whole milk. Place 5 or 6 drops of food coloring on the milk and then place one drop of liquid dish soap in the center of the milk. The food colors will fly to the outer edge of the plate and continue moving until the soap is dispersed throughout the plate. This phenomenon is all tied up with surface tension, diffusion polar and non polar molecules. This always produces an “ah ha” moment in the classroom.
I let the kids experience the “ah ha” first then backtrack to give them both new background and refresh their memories about what they already know. I begin with surface tension. This is where the forces attracting one molecule to another at the surface are stronger than those deeper in the liquid. To get a better idea of what this is you can use the old activity where you count the number of drops of water you can fit on the head of a penny before the water rolls down the edges of the penny. Most pennies will hold more than 40 drops. This is because of surface tension. The water will look like a dome on top of the penny. This is a good chance to break out some math skills and plot the different numbers from lots of trials or from the class.
Then, we talk specifically about what surface tension is. For high school hydrogen bonding fits here but for most it is just attraction between molecules of water.
At this point we will discuss what we know and remember about polar molecules and non-polar molecules. The key idea is that the atoms in polar molecules do not share the electrons equally where non-polar molecules do. With that said I have the students chart which ingredients in whole milk are made up of which type (polar or non-polar) of molecules. The only ingredients in milk we think of as non-polar are the fats. All else is pretty polar except for this unique molecule that keeps the milk from separating. If you are older like me you may remember shaking the milk container because if you did not the cream would be at the top as fats float. This unique substance is called an emulsifier. This molecule consists of a non-polar end and a polar end on the same molecule. The non-polar end is attracted to the non-polar parts of milk and the polar end keeps the polar substance in milk hanging around the fats.
It is time for a little experiment and you can do a simple one where you give the kids oil and water in a test tube or small jar with a lid and ask them to make them mix and stay mixed. Or, you can do something more exotic like the ocean in a bottle activity.
The key idea here is that oil and water will not mix. Polar materials will mix with other polar materials and non-polar materials will not mix with polar materials. If a substance dissolves in water it is polar.
Now they know about the big players in the milk system and now it is time to look at the detergent molecule. This molecule, like the emulsifier, is both polar and non-polar. It removes stains by imbedding the non-polar part of the molecule in the greasy stain (non-polar) and letting the polar end of the molecule hang out on the outside. When enough of these detergent molecules do this around a stain the thing begins to look like a “Koosh” ball with a non-polar center and a polar outside. That polar outside lets the stain/Koosh ball travel down the drain with the water in the washer.
Now comes the thinking part. I ask the students to generate ideas about what causes the colors in the milk experiment to keep moving. They seem to know that the initial movement is the surface tension breaking and that the continuing movement is not surface tension again but something different. Most students break into one of two camps. One thinks the phenomenon is simply the diffusion of the thick soap drop through the milk from a region of high concentration to low. The other camp thinks the fats have something to do with the phenomenon.
The students design experiments. One might try different amounts of soap and time how long the colors move. Another might try cream, whole, 2 percent and skim milk. The chosen experiment is not the big part of the plan here. I want to look at how each student approaches the problem and how well they apply what they know to solve the problem and how they use what they know and found in the experiment to support or refute their ideas about why the colors move.
So, if you are looking for some spring activities that have some inquiry and some high interest you might want to grab a jug of milk, some food coloring a little detergent and begin some rich explorations.