Sugar Density Rainbow

Age

Elementary School

Format

Hands-on

Materials

   Sugar
   Three small cups to mix solutions in
   One large (~8oz), clear cup that will be easy to see into (thinner is better)
   Three types of food coloring
   A tablespoon measuring spoon
   A plastic spoon
   A funnel
   A supply of clean water if you will be presenting multiple times
   A water basin to clean materials between performances
   A towel to dry off the measuring spoon 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. Demonstrators should wear goggles and lab coats, and the ST general safety box should be available to them. While the materials involved in this demonstration are not particularly dangerous, care should always be exercised to ensure the safety of the audience and to avoid messes. The sugar solutions should not be consumed!

Preparation

Make sure a large basin of clean water is available. You should use this water to clean the materials between performances. If the cleaning water becomes too saturated with food coloring, replace it. Make sure clean water is available for making the solutions.

Demonstration

In the three small cups, add one, two, and three tablespoons of sugar, respectively. Make sure the measuring spoon is dry before you insert it in the sugar! Be sure to remember which glasses have the most and least sugar. You could write out signs to place next to the glasses to help the audience remember which one is most dense. Add three tablespoons of water to each cup. Stir the sugar-water solutions very well (for about 30s each) to dissolve as much sugar as possible. Add about two drops of different food coloring to each cup. Because yellow food coloring tends not to be very strong, it should be added to the least-dense solution, if used at all, so that it's not overwhelmed by seepage from a higher layer.

Add the highest density solution to the large, clear cup. Now place the plastic spoon upside down in the clear cup so that the bottom of the spoon just touches the top of the solution. Slowly and carefully pour the medium-density solution over the back of the spoon on top of the high-density layer. This is necessary because it is important to avoid letting solutions plunge into previous layers, as the food colorings will mix and the rainbow layering will not be preserved. Repeat the process carefully with the low-density layer. If done correctly, the clear gas will have well-defined (though not sharp) layers of different density solutions of different colors.

What to Say

As you're preparing the sugar solutions, let the audience see that the sugar dissolves into the water. You may want to discuss saturation: ask the audience what would happen if you introduced a large amount of sugar into a small amount of water.

Optionally, you could discuss solubility. You could give examples of substances that do not dissolve in water.

When you're finished stirring in the sugar, show the audience the three cups. They saw that you used three times as much sugar in the first cup as the last, but the volume of the two solutions are similar. Use this to illustrate your discussion of density. Make sure to note that density is defined as mass divided by volume.

Ask the audience if they know which is more dense: pure water or salt water. Ask them if they've ever heard of the Dead Sea between Israel and Jordan, which has about 35% salt concentration, as opposed to about %3 for typical seas [3]. Explain that people can float in the Dead Sea effortlessly because the density of the water is higher than the density of the human body. You might want to use the examples of life preservers or rafts to illustrate that objects float if they are less dense then the surrounding liquid.

Before you layer the solutions, ask the audience if they think any of your three solutions could float on top of the others. Make sure they can guess the ordering correctly. Ask them why you need to add the solutions so slowly and carefully.

Finally, ask the audience if they think it would be easier to swim in very salty water than in a regular swimming pool. You could introduce the concept of viscosity to explain that, even if it is easier to float in saltwater, it would be more difficult to propel yourself. You could ask the audience to think about swimming in honey or mud. Make sure the audience understands that typical saltwater doesn't have high enough salinity for this effect to be significant for human swimmers, but that it might be important for microscopic organisms in seawater [2].