In the Exploring Density experiment, we looked at how different liquids have different densities, and how the different densities helped the liquids stay in separate layers, even when combined in the same container. In this experiment, we will explore how the density of an object can be altered without changing the object’s mass.
Before we begin, let’s review some key terms. Volume is how much space an object takes up. Mass is how much an object weighs. Density is the comparison of mass to volume. Something that doesn’t weigh that much but takes up a lot of space has low density. Something that doesn’t take up a lot space but weighs a lot has high density. In the previous comparison, we looked at a 1-pound bag of marshmallows vs. a baseball. Both have the same mass (or weight), but the baseball contains that mass in a much smaller volume (or space) than the marshmallows, so the baseball has a higher density.
If you can increase the volume of an object without increasing its mass, you will change its density. Imagine a balloon. If you weigh a balloon before you blow it up and weigh it again after you blow it up, you will see that the weight of the balloon increased only a little bit (due to the weight of the air inside the balloon), but the volume of the balloon increased dramatically. The uninflated balloon has a much greater density than the inflated balloon because the weight is confined to a much smaller space. Let’s try increasing the volume of something without increasing the mass at all!
The Experiment
Supplies: Two clear drinking glasses, eight raisins, tap water, a clear soda drink (club soda, 7-Up, etc.)
What to do: Fill one glass with water. Fill the other glass with the soda drink. Drop four raisins in each glass. Observe the raisins. What did the raisins in each glass do? How long did it take for the raisins to stop?
What is happening: When you first drop the raisins into each glass, they sink to the bottom of the glass because they have a greater density than the liquids they are in. The raisins in the glass of water do nothing, but the raisins in the soda begin to move after a short time. Raisins have a rough, dented surface. If you inspect the raisins in the water glass, you should see some air bubbles attached to each raisin. There are not enough bubbles on the raisins in the water glass to affect the raisins’ density, and there is no other source of bubbles in the water. Soda is carbonated, meaning it has carbon dioxide gas as one of its ingredients. Carbon dioxide gas is what makes soda “bubbly”. The soda releases carbon dioxide bubbles, and these bubbles attach to the rough surface of the raisins. The carbon dioxide bubbles increase the volume of each raisin, but the mass of each raisin stays relatively the same. When the volume increases but the mass does not, the overall density of the raisin is lowered. The raisins are now less dense than the soda, so they rise to the surface.
Once the raisins get to the surface of the soda, the carbon dioxide bubbles pop, causing the raisins’ density to change again, and they sink as a result. Once the raisins reach the bottom of the glass again, the process repeats itself, sending the raisins back toward the surface of the soda. The raisins will bob up and down for several minutes, until all of the carbon dioxide has escaped and the soda is flat. This experiment demonstrates how an increase in volume can lead to a decrease in density in an object, as long as the mass of that object is not significantly affected.
Take It Further
Try putting the raisins in a jar with a lid or directly into a bottle of soda. What happens to the raisins when you put the lid or cap back on? What happens when you take it back off? When put the raisins into a container and seal the container, the raisins will eventually stop the rising and sinking cycle. When it is in a glass, the carbon dioxide released by the soda can escape into the atmosphere. In a closed container, some carbon dioxide gets released, but it cannot leave the bottle, so pressure builds up in the space between the surface of the soda and the bottle cap. When you open the bottle, the hissing noise you hear is the built-up carbon dioxide escaping the enclosed space. As long as the cap is on the soda bottle, the contents of the bottle are under pressure. That pressure prevents carbon dioxide bubbles from forming, and any bubbles that do form cannot grow as large as they would in an open container or glass. Once the container is opened and the pressure released, then the bubbles are free to form again, and the raisins will resume their floating and sinking cycle!
Try this experiment with other dried fruits, like cranberries, or other small fruits or nuts, like grapes, blueberries, almonds, or peanuts. Are the results the same?
Links
To learn more about density, head to Kiddle Encyclopedia!