Liquid Layers Experiment
- Using the 9 oz cups, put 1 tsp of salt into Cup #1, 2 tsp of salt into Cup #2, 3 tsp in #3, etc. up to #6. Writing a number on each cup may help data collection.
- With the salt in each cup, add 9 oz of warm (even hot) water. Stir the solution until all of the salt has dissolved completely. You’ll get a work out on #6! All the salt needs to dissolve.
- Use the Color Fizzer tablets dye the solutions in each cup a different color. You can mix colors in plain water first, but don’t mix the tinted salt water solutions. If you want to make a rainbow of colors then add two red tablets to Cup #1, a red and a yellow tablet to Cup #2, two yellow tablets to Cup #3, a yellow and a blue tablet to Cup #4, two blue tablets to Cup #5, and a blue and a red tablet to Cup #6. Whatever you decide for you colors, make them as distinctive as you can so they show up in the straw. Now carefully transfer the liquids to the six Baby Soda Bottles and place them in order from 1-6 in the Baby Soda Bottle Rack. This will give you a bit more room to create your Salt Water Density Straw.
- Grab a clear drinking straw and hold the straw near one end, wrapping four fingers around the straw and placing your thumb over the straw’s opening. With your thumb off of the straw’s opening, dunk the opposite end of the straw into the “1 tsp” solution. “Cap” the straw firmly with your thumb and remove the straw from the solution. Now that you have the first solution in the straw, dip the end of the straw into the “2 tsp” solution. This time, dip the straw deeper than you did into the first solution. After you’ve dipped the straw, lift your thumb and replace it. Remove the straw and you should have the first and second colored solutions in a stack inside of the straw. Continue the dipping process until you have all six colored solutions inside the straw. It’s a density column of salt water!
When you’re ready, remove your thumb to empty the straw and start all over again!
- Table salt
- White teaspoon scoop
- Color Fizzer tablets
- Clear drinking straws
- Six 9-oz Cups
- Six Baby Soda Bottles with caps
- Baby Soda Bottle Rack
- Adult supervision
How Does It Work?
Density is the measurement of how much “stuff” is packed into a measured space. That’s how we get the equation for density: Density = Mass (the stuff) ÷ Volume (a measured space). Nearly every substance and material imaginable has a different density. This is especially true for the six solutions you made using salt and water.
By increasing the amount of salt in the solution but keeping the amount of water constant, you create solutions that have increasing densities. The more salt that is mixed into a measured amount of water, the higher the density of the solution. As the Density Straw reveals, a solution with a low density stacks on top of a solution with a higher density.
So, density explains why the solutions stack on top of each other inside the straw, but what keeps the solutions in the straw? That has to be water wizardry! You could expect the solutions to just fall out of the straw as you lift the straw from a solution. However, thanks to cohesion (similar molecules attracting each other) and adhesion (different molecules attracting each other), there is surface tension sealing the water at the bottom of the straw. The surface tension is strong enough to help hold the solutions in the straw… as long as air pressure inside the straw is lower than all the air pressure outside the straw. Gravity tugs the solutions downward which creates a slight vacuum in the empty part of the straw. That lowers the air pressure inside the straw which is why you need your thumb to cap the straw. This prevents air pressure from equalizing in the straw. Remove your thumb, air pressure equalizes, and gravity simply moves the colored solutions out.
Take it Further!
Now that you have mastered the Liquid Layers experiment, try different ways to experiment with it. What happens if you try adding the layers to the straw in reverse order? How about if you go completely out of order and add colors from #1, then #3, then #5? You can even try using a different sized straw to change up the experiment. Just remember that your thumb needs to fit over the end to create a complete seal in order for this to work. If you can’t block the opening, you won’t be able to pick up the water!
This dramatic salt water density change can be experienced in real life. While humans will (sort of) float in an ocean, we really float in bodies of water like Utah’s Great Salt Lake and the Dead Sea in Israel and the West Bank. They are so salty that it’s nearly impossible to sink in them! Just be sure to rinse off really well when you get out.
Science Fair Connection
Creating the Liquid Layer straw is pretty cool, but it isn’t a science fair project. You can create a science fair project by identifying a variable, or something that changes, in this experiment. Let’s take a look at some of the variable options that might work:
- Try using something besides salt. Would sugar work the same?
- Instead of using water, try different liquids to see if you can layer them.
That’s just a couple of ideas, but you aren’t limited to those! Try coming up with different ideas of variables and give them a try. Remember, you can only change one thing at a time. If you are testing different liquids, make sure that the other factors are remaining the same!