Materials
- 4 clear cups
- Water
- Salt (2–3 tablespoons per cup)
- Spoon
- Food coloring (blue and yellow)
- Index card
Instructions
- Fill all four cups with the same amount of water.
- Add blue food coloring to two of the cups and yellow food coloring to the other two cups. Stir gently.
- Add 2–3 tablespoons of salt to the blue cups only
- Stir until the salt is completely dissolved. The yellow cups remain plain water.
First setup: little or no mixing
- Place an index card over the top of one yellow (plain water) cup.
- Carefully flip the yellow cup upside down and place it directly on top of a blue (salt water) cup.
- Slowly slide the card out from between the cups.
- Observe that there is little or no mixing between the yellow and blue water.
Second setup: mixing occurs
- Place an index card over the top of one blue (salt water) cup.
- Carefully flip the blue cup upside down and place it directly on top of a yellow (plain water) cup.
- Slowly slide the card out.
- Observe how the blue and yellow water mix!
What’s going on
- Salt water is denser than plain water because dissolved salt increases the mass of the water without greatly changing its volume.
- When denser blue salt water is placed on top of less dense yellow plain water, gravity causes the salt water to sink and the plain water to rise. This movement causes the two waters to mix.
- When plain water is placed on top of plain water, both liquids have the same density. Without a density difference, there is little movement, so the water remains mostly unmixed.
- This experiment demonstrates how density differences drive fluid motion and mixing.
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