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Science Investigations that all students should do Before High School, #8: Density Explorations

#8: Density Explorations

Rainbow Density Column

Make a rainbow with different densities of sugar water.
Using six heat-safe containers, add 1 tablespoon of sugar to the first cup and add an increasing amount of sugar to each cup, adding an additional tablespoon each time. 
Add food coloring to each cup, according to the colors of the rainbow.
Dissolve the sugar and food coloring  in one cup of boiling water for each container.
Move from least dense to most dense, releasing each new layer on the bottom of a test tube, with the straw, instead of trying to pour it onto the top.

Density Column That Holds Objects

Here is another one we did...maple syrup, rubbing alcohol, dishwashing liquid (purple colored), water dyed blue and vegetable oil.

Density Layers in Soil

You can also apply the concept of density while making a soil profile test of the soil in the backyard. To make a soil profile, fill a vial (we actually used an clean, empty spice bottle) with about one inch of dirt. 
Add a pinch or so of alum. This acts as a dispersing agent, helping the soil  particle to break into smaller part and settle out into layers by density. Fill to the top with water. Cover and shake vigorously and then let stand. The hard part is getting them not to touch it again at this point, to give it a chance to layer. While they are waiting, I asked them to make predictions of what they would see. 
Once it layered, we talked about what was in each layer and that some layers were larger than others. The floating layer is organic matter. The top layer is clay (usually mixed with the water), the middle layer is silt and the bottom layer is sand. It is fun to compare two sample to see the difference in the ratios. If you let the soil profile sit on a shelf for several days, the layers become even more pronounced.
You can compare two types of soil to compare and contrast the differences.

You can also make a treat to have when you are studying soil layers.The top layer, representing the organic material, is a mixture of nuts and chips on a layer of  top soil pudding. A gummy worm added to represent the living things in the layers of soil. The next layer down, the topsoil (pudding) is mixed with gravel (cookie crumbs). The next layer, the subsoil, is made up of some larger pieces of cookie to represent weathered rock and then at the very bottom, (cookie) bedrock.

Archimedes' Density Discovery

The most widely known anecdote about Archimedes tells of how he invented a method for determining the volume of an object with an irregular shape. A new crown had been made for King Hiero II, and Archimedes was asked to determine whether it was of solid gold, or whether silver had been added by a dishonest goldsmith. Archimedes had to solve the problem without damaging the crown, so he could not melt it down into a regularly shaped body in order to calculate its density. While taking a bath, he noticed that the level of the water in the tub rose as he got in, and realized that this effect could be used to determine the volume of the crown. The submerged crown would displace an amount of water equal to its own volume. By dividing the weight of the crown by the volume of water displaced, the density of the crown could be obtained. This density would be lower than that of gold if cheaper and less dense metals had been added. Archimedes then took to the streets naked, so excited by his discovery that he had forgotten to dress, crying "Eureka!" meaning in Greek "I have found it!"
With this story in mind, we set up a simple demonstration to show this concept. Don't worry no bathtub involved. We took a bowl and set it in a baking dish. We then filled the bowl to the brim with water.

We then took two objects of identical weight.We determined that the rubber ball symbolized the lump of gold, and placed it in the bowl of water. It displaced a certain amount of water.We then placed our candle, which represented the crown, and we saw that it displaced some more water, showing that although its weight was similar, its density was not.
I have found out that both 4 quarters and 5 Hershey's Chocolate Kisses weigh 1 ounce, so these can be used. You can use a postal scale if you have one, to prove this as part of your experiment. Carry out the experiment the same as above and you should see that the candies have more mass and displace more water.

Salinity and Temperature Densities

Sometimes you will see little currents in the straw as one type/color of liquid passes another.
This is a bit more challenging. The task is to create four distinct layers in straw cylinders using only colored water and salt. The ocean is made up of layers of water of different densities. Cold water is denser than warm water, water with salt is denser than fresh water. The more closely packed the molecules in a substance, the denser the substance.
Please excuse the messiness of our school table.
 I usually clean it off after every activity, but has been one of those crazy weeks for us.

For this experiment you will need: - 4 Styrofoam cups or insulated containers, kosher salt, plastic straws, medium raw potato, medicine droppers, a package of food coloring

Have students write in their journals a key to the colors:
red-hot and salty
blue-cold and salty
yellow-hot and fresh
green-cold and fresh

 Have four containers to hold water and label them as above, adding about 15 drops of the appropriate food coloring to each container. Fill the containers with water, two with hot water and two with cold. Add 1/3 cup of kosher salt to both the red and blue water. Stir. Add ice to the blue and green containers.

Cut potato into 1'' thick slices. Cut two straws in half for each student. Insert straw into the potato at a 45 degree angle.

Sometimes the water will leak out of the bottom. You can ignore it if it is a small amount, or you can take the straw out of the potato over a sink, rinse out the straw and start over again at a new place in the potato, trying to push the straw in deeper.
Have your students decide what order they think the liquids will layer according to density. Have them try out their predictions by adding a tiny bit of the liquids to the straws so that they fill the straw about 1/2 inch, starting with the most dense and adding them one at a time.

 Your students should have layers, starting from the bottom, or the most dense, blue (cold and salty), red (hot and salty), green (cold and fresh), and yellow (hot and fresh).

We got a different result however, in ours. That happens sometimes. In that case, we explore why we got discrepant results. Our layers were, starting with the bottom, or most dense, red (hot and salty), blue (cold and salty), green (cold and fresh), and yellow (hot and fresh). The best explanation we could come up with is that our hot water may have allowed more of the salt to dissolve into the water than the cold. Do you have any other possible explanations?

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  1. Another wonderful collection of ideas for us to do in the New Year - thank you so much. We did a fun hot and cold water density demo recently, I'm going to post it soon. Enjoy the holidays!

  2. I need to try some of these when we reach Archimedes!

