Density Rainbow

Physics, Chemistry:	Density, Buoyancy
Grade Range:	Elementary School
Format:	Hands-on

Elementary students love this demonstration, and are often very interested in watching the liquids sort. This demonstration has two methods to present it; one with a variety of solutions, and the other using only sugar solutions.

Materials

Method A

• Graduated Cylinder (500mL)
• Various Liquids, such as:
  • Liquid Soap
  • Water
  • Corn Syrup
  • Lamp Oil
  • Vegetable Oil
  • Glue
• Small objects to put in (Paper clips, bits of wood, plastic, etc.)

Method B

• 1 Large Beaker (600 mL)
• 5 Small Beakers (140-150 mL)
• Sugar
• Water
• Spoon
• Food Dye (Red, Yellow, Blue, Green)

Method C

• 1 Large Beaker (600 mL)
• 3 Small Beakers (140-150 mL)
• Hot Plate
• Ice
• Water
• Food Dye (Red, Green, Blue)
• Spoon

Safety Precautions

Please read the Liquid Chemical section of the Demonstration Safety page before performing this demonstration.

Demonstration

Method A

1. Set the graduated cylinder on a table with a bin or tablecloth underneath to catch any spills. Add 100 mL of water to the cylinder before the start of the event.
2. Let students add one of the other liquids in (but only a small amount) and watch where it ends up. Is it above or below the water? As more liquids are added, they will sort themselves by density, and students will be able to see some clear distinctions between the liquids!
3. Let students know that the density of something is equal to its mass divided by the volume it takes up. Give them a small object and ask them to predict where it will float in the cylinder before dropping it in. Was their prediction correct?

Method B

1. Measure out 100 mL of water into each of the small beakers. Add three drops of food coloring to four of the beakers, with each getting only one of the colors.
2. Add sugar to each of the colored beakers. Give two teaspoons to red, five to yellow, eight to green, and eleven to blue. Mix each one thoroughly, rinsing or wiping the spoon between beakers so that a beaker doesn't get too much or too little sugar.
3. In the big beaker, first add the blue solution. Then, to add the rest of the solutions in color order, ending with clear, by pouring them slowly over the back of the spoon to prevent splashing and mixing. If done correctly, the solutions will stack on each other, creating a rainbow!

Method C

1. Measure out 100 mL of water in each of the beakers. Start heating one on the hot plate, and add ice to another and let it cool down.
2. Add 2 drops of food dye to each of the beakers; red in the hot beaker, green in the room temperature beaker, and blue in the cold beaker.
3. Once the hot beaker is hot, turn off the heat plate. Add the blue water to the large beaker first. Then add the green water, pouring it over the back of a spoon to ensure it spreads out and doesn't mix much. Repeat for the red water, and if done correctly the solutions should stay mostly separated!
4. The solutions will mix over time as the temperatures become more balanced, so plan to reset this demonstration every 15 minutes or so.

Why This Works

Density is equal to the mass of a substance divided by the volume it takes up. Water has a density of 1kg/L, and can be used as a base for finding how dense other liquids are. Liquid soap and corn syrup are both more dense than water, so you will see them fall underneath it, while oils are less dense than water, so the lamp oil and vegetable oil would float on top. solid objects have differing densities as well. When the paper clips or other small objects are dropped in, they will sink below some layers and float on top of others! How well an object floats is what we call buoyancy. If we know the buoyancy of an object, then we can calculate the mass of the object.

The sugar water variation of this demo is a great way to show that liquids that look the same can have different densities. The sugar water does not take up much more volume, or space, than the regular water. However, the sugar water has more mass per volume than the regular water, which means that it has a higher density. The more sugar there is dissolved in solution, the more mass there is per volume, and therefore the density will get higher.

The temperature version of this demo shows the students how a substance can change in density if the temperature changes. If something is hot, then the atoms and molecules have more energy, and can take up more space, or volume, while having the same amount of mass, and there be less dense. Likewise, if a substance is cooled down, then it will take up less space, or volume, with the same amount of mass, and be more dense! By simply heating up or cooling down a solution, we can affect the density of it!

Additional Information

• This pairs well with the Cartesian Diver demonstration.