Liquid Layers - Straw Stack of Color The secret is density… and a steady hand. email to friend
print this page

1. Fill four of the plastic cups 3/4 full with water
2. Use food coloring to color each cup a different color – blue, red, green and yellow. You’ll want the colors to be fairly dark, so add 15-20 drop of food coloring to each cup.
3. Add 1 tablespoon of salt to the blue water and stir.
4. Add 2 tablespoons of salt to the red water and stir.
5. Add 3 tablespoons of salt to the green water and stir.
6. Add 4 tablespoons of salt to the yellow water, and you guessed it, stir the water. Not all of the salt will dissolve immediately… and that’s okay. Over time the salt will completely dissolve, but you don’t have to wait for that to happen to get started.
7. This last step in preparing the solutions is very important… Add a little water to each cup so that the water level is the same in all four cups.
8. Let’s practice the straw-dropper technique using plain water in the fifth cup. Place one end of the straw into the water – about an inch – and place your index finger over the other end of the straw. Pull the straw out of the water and notice how the water stays in the straw. If you release your finger, the water will fall out of the straw. Remember doing this as a kid with your drink? Hey, maybe you're still a kid and you’re an expert! Make sure you can do this well before moving onto the next step.
9. It’s time to layer some liquids. Place the empty straw into the blue water (about an inch below the surface). Seal the other end of the straw with your index finger and remove the straw. There should be about an inch of blue water in the straw. Keep your finger firmly pressed against the top so the blue water doesn’t fall out.
10. Without releasing your finger, lower the straw into the red liquid about an inch lower than the blue liquid in the straw. Slowly release your finger from the top of the straw and the red liquid will push the blue layer up to the level of the water in the blue cup. Press your finger firmly on top of the straw and remove the straw. Look… you have two layers! Don’t get so excited that you release your finger from the top of the straw – you’ll have to start all over again! Also, be sure to hold the straw straight up and down (vertically) because tilting the straw will cause the liquids to mix and you’ll have to start again.
11. Lower the straw with the two colored layers into the green saltwater solution about an inch lower than the red solution in the straw. Slowly release pressure with your finger and the green solution will push the red and blue layers up about an inch. Seal the top of the straw with your index finger and move onto the yellow solution.
12. Lower the straw into the yellow solution (the suspense is killing you… it feels like your finger is going to fall off… but you continue!). Lower the straw about an inch below the top of the green layer and release your finger. The yellow liquid will push the top layers up. Put your index finger over the top of the straw one last time and remove the straw from the water. To everyone’s amazement, you have four layers of colored water in your straw! The on-lookers who have gathered to see what you’re doing start chanting, “We’re not worthy! We’re not worthy!”
13. All good things must come to an end. When it feels like your index finger is going to fall off, release the pressure and your masterpiece will fall into the fifth cup. The crowd yells, “Do it again!” and you can’t resist the temptation.

How does it work?

There’s really no trick to layering liquids as long as you understand the concept of density. In simplest terms, density is the quantity of something per unit measure (assuming that everything is at the same temperature and pressure). For example, you added 1 tablespoon of salt to the blue water, but you added 4 tablespoons of salt to the yellow water. So, the yellow solution has a greater density of salt than the blue water. The density of the yellow solution is greater than the green solution, which is greater than the red, which is greater than the blue. By increasing the amount of salt in each cup of water (and keeping the volume or the amount of water in each cup the same), each liquid had a different density. The solution with the highest density (yellow) stayed at the bottom of the straw while the solution with the least amount of salt (and the lowest density) remained at the top.