We’re deep in the middle of winter here, which means we have been taking the opportunity to learn about all things cold, dark and snowy. And so as part of our polar study, we made this super cool Northern Lights tray, which doubles as a science experiment on water surface tension (more on that later).
Create the Magic of the Northern Lights
First, you will need some sort of shallow tray or plate. A round-shaped tray works best.
Next, pour in some milk (you can use expired if you have it!), enough to cover the bottom and an inch or two high. Place several drops of food coloring on the surface of the milk. You can use different colors (we used common aurora colors like green, blue, purple, etc.)
Next, get out some Q-tips and a little cup or dish containing dish soap. You don’t need a lot of soap, just enough so that you can dunk your Q-tips in.
Now, watch the magic happen. Instruct your students to tap each dot of food coloring with a soap-coated Q-tip, and watch the northern lights appear! ✨
You can continue to add food coloring and repeat this activity as long as you wish, although the milk will continue to get darker and darker. It really is cool to watch – your students will want to do this again and again!
We paired this activity with some printed pages from our Arctic Journal, along with some wonderful books about the Aurora Borealis.
The Science Behind It
We did this activity along with our study of the polar night and the arctic circle, BUT I’ll never pass up an opportunity to discuss how things work – and this little activity is a great way to kick off talking about a phenomenon known as surface tension.
What is surface tension?
Surface tension is defined as the tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid.
In other words, the molecules on the surface of a liquid (like milk) are pulled more tightly to each other than the molecules under the surface. This unseen force is called surface tension, and it results in a “skin” on the surface of a liquid, allowing it to resist external forces. (Think about pond skaters on the surface of the water.)
The surface tension of the milk keeps the food coloring drops from spreading. When the dish soap is added, it binds to the fat molecules in the milk, reducing the surface tension. This is why it looks like the colors are “running away” from their point of origin.
You can repeat this experiment with plain water, black pepper and a toothpick dipped in soap. Fill a shallow container with water, instead of milk, and pour in some black pepper. Have your students tap the surface of the water with a toothpick coated in soap, and watch the pepper scurry away!
Want to teach your kids more about the northern lights and the polar night? Check out our new Arctic Journal!