You can make colorful columns that demonstrate the concept of liquid density at your own kitchen table with just water, sugar and food coloring. An eyedropper, siphoning bulb, syringe (minus a needle,) or anything else that allows you to slowly drip liquid from one cup to another are useful for the layering step. If you have a tall, thin glass, like a cordial glass, or a test tube, it’s easy to see the layers in your gradient!
Start with two cups of hot tap water and measure half a cup into each of four cups. To the first cup, add 2 Tbs. sugar, to the second add 4 Tbs. sugar, to the third, 6 Tbs. sugar and to the fourth, 8 Tbs. sugar. Stir until the sugar dissolves. If the sugar won’t dissolve, an adult may microwave the cup for 30 seconds and stir again. Always use caution with hot liquids. If the sugar still won’t dissolve, try adding a Tbs. warm water.
Now, add 2 drops food coloring to each cup. We added red to the cup with 2Tbs, yellow to the one with 4Tbs, green to the to the one with 6Tbs, and blue to the cup with 8Tbs.
Density is mass (how many atoms are in an object) divided by volume (how much space an object takes up.) Sugar molecules are made up of lots of atoms stuck together. The more sugar you add to a half cup of water, the more atoms it will contain and the denser it will be. Less dense liquids float to the top of more dense liquids. Which of your sugar solutions is the most dense? The one with the most sugar in it (8 Tbs.)
Put the most dense sugar solution(blue in this case) in the bottom of a tall, thin glass or test tube. Now, use your dropper to gently drip the next densest liquid (green) on top of the blue layer. It works best to drip the sugar solution against the side of the cup just above the surface of the liquid. You can also drip it onto the back of a spoon, like in the photo above. Add the yellow layer, and finally the red layer, which only contains 2Tbs sugar per half cup and is the least dense.
What happens if you mix the layers up? They won’t separate back out like oil and water would, because the sugar will disperse (spread) equally through the mixture.
Researchers sometimes use density gradients to isolate different parts of cells by breaking the cells up, putting the cell debris on top of a density gradient and spinning it in a centrifuge. Cellular fragments of different shapes and molecular weights move through the gradient at different rates.