March, 2015:

Kids, you can use simple kitchen materials to make a colorful density column. This project uses colored sugar solutions with different concentrations. The solutions will form layers, from least dense, on top, to most dense (concentrated) at the bottom of the tower.

You will need a tall clear glass or jar (even better would be a jumbo test tube or graduated cylinder), pipettes or droppers, sugar, measuring spoons, food coloring or tablets, and 4 small cups.

Fill your cups with 1 cup of warm water and add food coloring. You want a different color for each density (for example blue, yellow, red, and green). Add 2, 4, 6, and 8 tablespoons of sugar to the 1st , 2nd, 3rd, and 4th cups. Label each cup with the amount of sugar added. Stir the water until the sugar is completely dissolved. You may need to supersaturate the sugar water solution to get all of the sugar to dissolve. Place the cup in the microwave for 20-30 seconds to warm the water and dissolve more sugar. Continue stirring until all of the sugar is gone.

Start with the cup with the most sugar (most dense). Using a pipette, dropper or back of a spoon, begin adding a first layer of sugar water to the jar. Carefully drip the next dense layer onto the surface of the first. The best technique is to place the pipette right above the surface of the first layer and against the glass. Slowly drip the next color onto the first. This will take a lot of patience. Go slow. The colors will begin to mix at first and then your original color will start to show. Repeat with the next dense color and the least dense color until you have stacked all of the colors.

What's going on? Density is mass (how many molecules are in an object) divided by volume (how much space an object takes up). As you add sugar to the water, more and more sugar molecules will take over the space, making the water more dense. The cup containing the 8 tablespoons of sugar will be the most dense, the cup with 2 tablespoons will be least dense. With this sugar water experiment, we put the most dense solution on the bottom. Why do you think that is? What will happen if you try it with the least dense solution on the bottom?

TIPS
Supersaturated Solution 
If you attempt to dissolve sugar in water, you reach a point where you cannot dissolve any more sugar. This is called a saturated solution. However, if you heat this solution, more sugar will dissolve. When the solution is cooled, the sugar will remain in solution. This is called a supersaturated solution, which is very unstable and will crystallize easily.

Density Column Mixup 
What happens if you shake or mix up your sugar density column? The colors will not separate and go back to the rainbow, like a water-oil density tower. The sugar will mix evenly with the water.

Patience 
This experiment takes a lot of patience. You won't see the color you are adding right away. Keep carefully dripping the sugar water solution and you will see it begin to stack up.

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References:
Steve Spangler Science: http://www.stevespanglerscience.com/lab/experiments/colorful-sugar-density-tower
Anne Marie Helmenstine: http://chemistry.about.com/od/chemistrydemonstrations/ht/rainbowinaglass.htm