This "Rainbow in a Bottle" is a layered solution using different densities of salt water. The materials are super cheap and the end result is impressive.

This project is something I do with my 7th-grade students in my science class. You can find the link to that instructable (with lab sheet) HERE.

This "rainbow" should take between 45 minutes - 1 hour to complete depending on the size of your bottle. If you don't have the exact materials listed in the next step, I've included alternatives that you probably have at home.

Step 1: Gather Materials

A small, clear bottle

Food Coloring

Water

Salt

5 small beakers (or drinking cups)

Spoon or stirrer

Scale or electronic balance (or 1/2 teaspoon)

Pipette or medicine dropper

Step 2: Water

First, measure the volume of your bottle.

Next, divide the volume of your bottle by 5. This should give you a good estimate of how much water you'll need for each color.

I used 6 small glass beakers and added 80 milliliters of water to each one. If you don't have beakers or measuring cup with mL... 1/3 cup equals 79 mL.

Step 3: Food Coloring

Using Red, Yellow, Green, and Blue food coloring, you can create the full spectrum of color found in a rainbow.

Red: Add 6 drops of red

Orange: Add 3 drops of yellow and 3 drops of red

Yellow: Add 6 drops of yellow

Green: Add 6 drops of green

Blue: Add 6 drops of blue

Violet: Add 3 drops of red and 3 drops of blue

Step 4: Add Salt

Decide on the order of your colors. The color with the most salt will be the densest and must be on the bottom.

If you want your colors to be in reverse order from what I've done, just swap the salt amounts. If you don't want your rainbow to be in the order of the visible light spectrum, just swap the colors. It might be neat to have warm colors and cool colors alternating.

Violet: 15 grams (2.5 teaspoons)

Blue: 12 grams (2 teaspoons)

Green: 9 grams (1.5 teaspoons)

Yellow: 6 grams (1 teaspoon)

Orange: 3 grams (0.5 teaspoon)

Red: No salt

Stir, Stir, Stir. Stir each solution for a minute or two. Make sure the salt dissolves completely. Any granules of salt that aren't dissolved have the potential to mix/ruin your layers because they will sink to the bottom of your bottle.

Tip: Next time I do this experiment, I'll leave the violet salt-free. After adding the salt, my vibrant violet became muddled and it was difficult to see the layered effect with the blue.

Step 5: Add the Solution

Add each of your colored solutions to the bottle. Start with the saltiest color (the densest) on the bottom of the bottle. Fill the bottle about 1/6 of the way if you're using 6 colors (or 1/5 if using 5 colors).

For the next layer, you must use the pipette. Carefully layer the next color by SLOWLY running the solution down the inside of the glass of the bottle. Do NOT squirt the solution directly into the bottle because this will cause the colors to mix. This step takes patience and a steady hand.

Repeat for each of your colors being careful to go from saltiest (densest) to least salty (least dense).

Tip: A smaller/narrow bottle works best for this. A thicker bottle works but will take longer and won't look as impressive due to the light having to pass through more solution.