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STEM Play: Rainbows in Milk

Have you seen the kid science experiment where you put food coloring in milk, then add dish soap to make “rainbows”? I had to show this to Mitch! He was pretty amazed.rainbow milk stem

I’ve got a vague notion that dish soap breaks up grease, that’s why we use soap on our dishes after all. But what does this little experiment teach? Google to the rescue!

I found this article on Color Changing Milk to explain the science! It’s from Steve Spangler, a science teacher turned internet star. 

The Rainbow Milk Experiment

You’ll need a shallow Bowl, a pack of Food Coloring (4 colors), Liquid Dish Soap and Cotton Swabs.

  • Pour milk into a shallow bowl — about a quarter inch will do.
  • Have your child add four drops of food color to the milk–one of each color. Place the colors close to each other for best results.
  • Dip a cotton swap into dish soap and tap the swab on the dye.
  • Sit back and watch with amazement!

(Get the full science-y experiment at Spangler’s site here.)


Where’s the Science?

Kids are going to have fun playing with dye and soap, but let’s add a little science to the mix.

According to Spangler, the secret is the chemistry between the soap and the milk fats:

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.

You can take this a step farther and turn the milk rainbow into a real science fair project. Try making rainbow milk with skim milk, whole milk or cream. What happens? Record the results! What happens if you just use water? Hmmmm…

rainbow milk just water

Nothing! With no fats to repel, the dish soap has no reaction to the dye. And most important, it got Mitch thinking about what I was trying to teach him about science experiments!

Mitch also liked to see what happens when you keep adding more dye and more soap…he thought it made some pretty nice art.rainbow milk