Rainbow Density Lab

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About: I'm a middle school science teacher going on 17 years in the classroom. I've taught 6th, 7th, and 8th graders. I'm constantly looking to improve my instruction and Instructables is one of the places I sear...

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

Students are engaged in a hands-on experience that encourages real-world connections and forces them to follow the directions. The experiment also allows the flexibility for students to create their own experiments at the conclusion of the lab. The lab usually takes between 45 minutes - 1 hour to complete (excluding extension questions).

A few of the concepts that can be tied to the lab include; the difference between a solvent and a solute, what it means for a solution to be saturated, density, how liquids with different densities interact, investigate other ways to stack solutions, and students will be able to identify where liquids of different densities might come into contact in nature.

LAB SHEET

Step 1: Materials

Food Coloring

Water

Salt

5 small beakers

100 mL graduated cylinder

Spoon or stirring rod

Scale or electronic balance

Pipette

Masking tape

Pen / Marker

Step 2: Procedure

1. Label each of the 5 beakers 1-5.

2. Into each beaker, add salt and water in the amounts identified on the attached worksheet.

3. Stir each solution well to dissolve ALL salt.

4. Add two drops of food coloring in the amounts identified in the table on the worksheet.

5. Gently pour about 20 mL of the solution from beaker 1 into the graduated cylinder. Helpful hint: When using the pipette to transfer the solutions, pull the solution off of the top of the beaker (if you pull from the bottom and there are granules of salt that had not completely dissolved, gravity will pull them to the bottom of the graduated cylinder and mix your layers.).

6. Using the pipette, carefully layer 20 mL from beaker 2 on top of the 20 mL of blue liquid that is already in the grad. Add the layer by SLOWLY running the solution down the inside of the glass of the grad. Do NOT squirt the solution directly into the solution at the bottom of the cylinder.

7. Repeat for beakers 3-5.

Step 3: Final Product

The solution should remain layered as long as it is undisturbed. Place the best examples in a sunny window to leave on display. Evaporation could start to influence the layers, but then you just have an additional teaching tool.

Inevitably, some students will not follow the instructions. There is enough solution in each beaker to complete the experiment twice (50 mL of the solution but only need 20 mL of each color). Students can either restart the experiment if they made a mistake, or develop their own experiments with the remaining solutions if they did the experiment perfectly the first time.

Follow up questions include making connections to nature and an inquiry as to what other solutions/liquids might be used to recreate this layered experiment. Have fun and stay curious!

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    5 Discussions

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    Alex in NZ

    15 days ago

    Ermagerd, I recognise those food colours. My mum had exactly those bottles back in the 1970s! This is a great demonstration of haloclines, but the food colouring is what made it for me :-)

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    Biodynamicgreen7550

    Answer 25 days ago

    I connect it to fresh water rivers running into oceans. We talk about brackish water and how the layers are mixed by wind and currents. I also connect it to different densities between warm and cold and talk about weather fronts in the atmosphere and upwelling in the oceans.

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    Tami Kruse

    26 days ago

    It actually didn’t work. I will try it again and see if it works.

    1 reply
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    BiodynamicTami Kruse

    Reply 25 days ago

    The first drops of the next layer need to go really slowly. Make sure to run the solution down the inside of the glass with a pipette. Good luck!