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20 Minute Labs: Rainbow in a Straw

logo 20 minute labs | Yellow Scope


girl straw | yellow scope

Have you ever noticed that you can hold some of your drink in a straw if you put your thumb (or your tongue!) over the end? Then watch it come careening out when you let go? Kinda fun, huh?

What keeps the liquid in the straw? Well, when you cap a straw full of liquid with your thumb, you stop any air at the top from coming in or going out. Even though gravity is tugging on the liquid to fall out, it's prevented from dropping. If you let go, the air pressure from outside the straw can expand into the straw and the liquid pours out.

Let's put this fun science trick into use with an experiment! In this month's 20 Minute Lab, we'll hold a "rainbow" in a straw, using common items from home, while exploring the concept of density!


  • rainbow straw supplies
    6 small clear glasses or cups
  • 2 narrow, clear containers (travel shampoo bottles work great!)
  • food coloring (red, yellow, and blue minimum so you can make six colors)
  • 6 cups water
  • 1/2 cup salt
  • teaspoon
  • measuring cup
  • clear straw



  1. coloring in glasses | Yellow Scope
    Line up your 6 glasses and add food coloring to each one, so each cup will hold a different color:
    Red: 8 drops red)
    Orange: 6 drops yellow, 2 drops red
    Yellow: 8 drops yellow
    Green: 8 drops green, or 3 drops blue + 5 drops yellow
    Blue: 8 drops
    Purple: 6 drops red + 2 drops blue

  2. Using the measuring cup, add 1 cup of water to each glass.

  3. Stir the water and food coloring with your straw so the colors are uniform.

    rainbow straw lineup | Yellow Scope
  4. Let's first see what happens if we stop here and don't add salt.
    • Put your thumb over one end of the straw. Press down tight!
    • Dip the straw about half an inch into the purple glass.
    • Release your thumb so the water gets sucked up.
    • Put your thumb back over the end.
      rainbow straw thumb | Yellow Scope
    • Lift the straw out of the purple glass and move it to the blue glass.
    • This time, dip the straw in about a half inch deeper than you did before, so that you'll be adding blue water under the purple water in the straw.
    • Continue with the other colors - green, yellow, orange, red - as far as your straw will fit into the glasses (the straw may be too long to fit in the final two).
    What does your straw of colors look like? Kinda muddy right?

  5. Empty out your straw. 
  6. Okay, let's see now what happens when we add salt to the water. Add 1 tsp of salt to the purple cup, 2 tsp to the blue, 3 tsp to the green, and so on through 6 tsps of salt in the red. (Note: salt washes out the colors; purple suffers the most, so we found the best luck adding the least salt to it and having red take the most). Like so:

    table of salt | Yellow Scope
  7. Using your straw, vigorously mix the salt up in each glass until the salt disappears (you'll have to mix red extra hard due to all that salt!).

  8. Pour the orange and red solutions separately into the two tall containers. (This way, the straw will fit all the colors in one go!)

  9. Now repeat the procedures from above:
    final lineup rainbow straw | Yellow Scope
  • Cap the straw with your thumb.
  • Dip the straw into the purple glass about half an inch.
  • Release thumb to suck up water, recap with thumb.
  • Lift the straw out and go to the blue glass.
  • Dip straw in an added half inch.
  • Continue all the way through to red.

Check out the video to see how it worked for us! 


density | Yellow Scope 20 minute labWhy did the colored water without salt all mix together? Why did it stay in nice rainbow layers with the salt? The answer has to do with density, or how much 'stuff' can be packed into a given space.

The first experiment you did - with just water and no salt - is called a control. A control is when you do an experiment where nothing interesting is added or changed. We wanted to see what would happen without the salt. The water all had the same density - just a cup of water and 8 drops of food coloring. The same amount of 'stuff' in each one!

When the density is the same, there's nothing to prevent the water from mixing in the straw and the colors getting muddled.

straw | Yellow Scope 20 minute labAn experiment with a variable is when you add or change something to see what happens. In this experiment, salt is the variable. The different amounts of salt means that each container had a different amount of 'stuff' in it. Some with a little salt, some with a lot. In other words, each container had a different density.

More dense liquids sink. The first thing you added to the straw was the purple water - the one with the least salt and therefore, the least dense. Then you sucked up some blue, the next least dense - but more dense than the purple - which stayed happily below the less dense purple water. All the way to red, the most dense solution. They don't mix because their densities keep them stacked in their places!

Try starting from red (most dense) and going to purple (least dense) instead - what happens? 


Let us know how your experiments turned out! Share your photos and results with us on Facebook, Twitter, Instagram, or send us an email to We love getting your messages!

For more exciting experiments, check out our Yellow Scope science kits on the Shop tab of our website!

Chelsea Schuyler
Chelsea Schuyler