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Rainbow Density Experiment

Build an entire rainbow in a glass and explore density with just one special ingredient…salt! Learn how to do this colorful experiment below.

What you need:

  • Salt

  • Water

  • Food coloring

  • Measuring cup

  • Teaspoon

  • 5 plastic cups

  • 1 tall glass (we used a graduated cylinder)

  • Pipette

How to build a rainbow in a glass:

  1. Start with 1/2 cup of five different colors of water: red, orange, yellow, green, and blue.

  2. Add 1/2 tsp of salt to the orange, 1 tsp of salt to the yellow, 1 1/2 tsp salt to the green, and 2 tsp salt to the blue. Make sure to not add salt to the red.

  3. Add the blue water to your tall glass. We used a graduated cylinder and added 5 mL of each color.

  4. Using a pipette, gradually add the green water, then yellow, then red.

What’s going on?

Salt makes water more dense, and it will stay at the bottom of the container because the less dense (because it’s less salty) water wants to float on top. By adding the most salt to the blue water and adding less in reverse rainbow order, we’ve made the colors stack and created this cool display. You can use different colors, but just make sure you put the most salt in the color you want on the bottom and use slightly less salt as you go up!

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