Sep'15

#Tinker: Color Changing Milk Experiment


by STEM Scouts



This post is an edited version of an experiment write-up originally appearing on Steve Spangler Science.

This experiment is simple, visually stimulating and quick to do. You likely have all of the necessary materials around your home. With just these materials, you will create an awesome chemical reaction and a beautiful explosion of color. It’s fun for all ages, but recommended for ages 4 to 12. Let’s get to it. Tinker away!

Experiment Materials

  • Milk (the higher the fat content, the better)
  • Dinner plate
  • Food coloring
  • Dish-washing soap
  • Cotton swabs

Experiment Procedure

  1. Pour enough milk in the dinner plate to completely cover the bottom to the depth of about 1/4 inch. Allow the milk to settle.
  2. Add a single or a few drops of each of the four colors of food coloring to the milk. Keep the drops clustered together in the center of the plate of milk.
  3. Find a clean cotton swab for the next part of the experiment. What do you think will happen when you touch the tip of the cotton swab to the center of the milk? It’s important not to stir the mix. Touch the tip of the cotton swab to the milk. Did anything happen?
  4. Now place a drop of liquid dish soap on the other end of the cotton swab. Place the soapy end of the cotton swab back in the middle of the milk and hold it there for 10 to 15 seconds. What happened this time?
  5. Add another drop of soap to the tip of the cotton swab and try it again. Experiment with placing the cotton swab at different places in the milk. Notice that the colors in the milk continue to move even when the cotton swab is removed. What makes the food coloring in the milk move?

How does it work?

Milk is mostly water but it also contains vitamins, minerals, proteins and tiny droplets of fat suspended in solution. Fats and proteins are sensitive to changes in the surrounding solution (the milk).

The secret of the bursting colors is the chemistry of that tiny drop of soap. Dish soap, because of its bipolar characteristics (nonpolar on one end and polar on the other), weakens the chemical bonds that hold the proteins and fats in solution. The soap’s polar, or hydrophilic (water-loving), end dissolves in water, and its hydrophobic (water-fearing) end attaches to a fat globule in the milk. This is when the fun begins.

The molecules of fat bend, roll, twist and contort in all directions as the soap molecules race around to join up with the fat molecules. During all of this fat molecule gymnastics, the food coloring molecules are bumped and shoved everywhere, providing an easy way to observe all the invisible activity. As the soap becomes evenly mixed with the milk, the action slows down and eventually stops.

Try adding another drop of soap to see if there’s any more movement. If so, you discovered there are still more fat molecules that haven’t found a partner at the big color dance. Add another drop of soap to start the process again.

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