Rainbow Water and Floating Grapes

Materials

  • 3 Beakers (or clear cups)
  • Test Tube (or similar)
  • Red, Yellow, and Blue Food Coloring (any 3 colors will work)
  • Salt
  • Sugar
  • 3 Grapes
  • Tweezers
  • Pipette

Take a Guess (Hypothesis)

Which is denser: saltwater or sugar-water? Are either of them more or less dense than regular water?

Directions

Part 1

  1. Fill each beaker with water (about 100mL).
  2. In one beaker, add 1 drop of red food coloring + 2 tsp (8g) of sugar. Mix until dissolved.
  3. In the second beaker, add 1 drop of blue food coloring + 2 tsp (8g) of salt. Mix until dissolved.
  4. In the third beaker, add 1 drop of green food coloring. Mix.
  5. You now have red sugar-water, blue saltwater, and green plain water. Drop a grape into each beaker. Do they sink or float? Record your observations.
  6. Use your tweezers to take the grapes out when you are done, and throw them away.

Part 2

  1. Using your pipette, fill the test tube ⅓ of the way with blue saltwater.
  2. Verrryyyy slowly use your pipette to add red sugar-water to the test tube, then do the same with the green plain water. (This part takes some patience! If you dropped slowly enough, you should see different colored layers in your test tube.)

Conclusion

Did you guess correctly? Order the 3 liquids from most dense to least dense.

How It Works

Part 1

Grapes are denser than water, so they sink.

Density = Mass / Volume

When you add salt to the water, there are more particles (more matter) in the cup of water. There is more mass, but the volume stays the same.

In other words, when you add salt to water, it increases the density of water. Saltwater is denser than a grape, so it floats.

Sugar-water has more particles in it than plain water. It is denser than plain water. However, sugar-water is still less dense than a grape, so the grape sinks.

Part 2

Both The First Great Lesson and our further work on density showed us that denser particles sink, and less dense particles float.

If you are careful, you can get sugar-water to float on top of saltwater, and plain water to float on top of sugar-water. If you pour too quickly though, all of the particles will mix into one solution. You can try this with Hot and Cold Water too!

Dive Deeper (Do Some Further Research)

What is a solution? How are solutions related to density?

What are some other objects that will sink in plain water, but float in saltwater?

  • Are those objects more or less dense than grapes?
  • Create your own experiment to find out! (Hint: try using different amounts of salt in your solution).