Explorers

The Explorers spent a session in early December 2011 proving that water and oil indeed do not mix, but that air bubbles can be convinced to travel back and forth between the two.

The reason WHY water and oil don’t mix is that the two liquids have different densities. “Density” is a measure of how closely packed together the molecules are that make up a substance—substances whose molecules are packed more tightly are more dense than those whose molecules are spread apart in a looser fashion. One way to measure this quality, at least with liquids, is to pour one into another. If the two liquids share the same density, they will mix together—but if they have different densities, then the one with the lower density (more loosely packed molecules) will float in a layer atop the one with the higher density.

The Explorers began by making a prediction and then performing exactly this experiment with oil and water. Starting with small clear cups with an inch of oil inside, they poured in some water. The water sank right through the oil and formed a bottom layer, leaving the yellowish oil sitting on top. This proved that the density of water is greater than that of oil.

Next, they considered the question of what would happen if drops of food coloring were added. There were several possibilities: the food coloring could float on top (if its density was less than both the water and oil); it could mix with the oil (if they were the same density); it could be trapped between the two existing layers (if its density was more than the oil but less than the water); it could mix with the water (if the two densities were the same); or it could sink all the way to the bottom (if its density was greater than both the oil and the water).

Actually dropping in the food coloring was an interesting experience. It formed blobs that sank right through the oil (thereby having a greater density), but then tended to get “hung up” at the water’s top boundary for a while before breaking through into it and then mixing. Mr. Ramsey explained that this was due to air bubbles getting trapped below the blobs of food coloring—these bubbles helped to hold up the blobs until they popped or escaped, at which point the blobs dropped down into the water layer. There they mixed with the water, demonstrating that the food coloring had the same density as the water.

For the final activity of the day, the Explorers repeated this cycle—oil, water, and food coloring—in small half-liter water bottles. Then they created their own versions of “lava lamps.” The traditional lava lamp is made from colored wax inside an oil or water-filled container. This wax normally has a greater density than the liquid and therefore sits at the bottom. When the lava lamp is turned on, though, a light and heater are activated at the bottom. When the wax heats up it expands and becomes less dense than the liquid, causing it to rise up to the top. There it is away from the heat source and cools off, therefore contracting and becoming again more dense than the liquid. This makes it sink back to the bottom, where it again heats up and starts the whole cycle again, continuing as long as the lava lamp heater is turned on.

•  
• 1 of 3
• ››

For our version, we had no heat source and no wax. So, we needed to find a way to force our colored water to rise up through the oil layer. We accomplished this with alka-seltzer tablets. These “effervescent” medicine tablets are normally dissolved in water and then drunk to soothe upset tummies—but for our purposes we were only interested in the dissolving process itself. The tablets sink to the bottom of water and then furiously produce bubbles of carbon dioxide gas as they dissolve.

What we discovered is that these bubbles can form the basis of a low-budget lava lamp. We dropped pieces of tablets (whole tablets are too big to fit into the opening of the water bottles) into the bottles, where they sank through the oil and to the bottom of the water. There they began dissolving, and the resulting bubbles not only rose up through both the water and the oil, but they also dragged some of the colored water with them. It was pretty cool to watch this happen, as colored bubbles streamed up through the oil, only to pop at the top and allow the trapped colored water to fall back to the bottom. It took a minute or so for each tablet piece to dissolve completely, and then we could start the whole process again by tossing in another piece.

The students took home their new lava lamps (TIGHTLY sealed with bottle tops—Mr. Ramsey emphasized that spilling the oil and colored water at home would create a mess that no one would be happy about) along with additional tablet pieces, giving them the chance to show their families the whole process.

Check out the photo gallery to see some pictures of our density activity. For a video demonstration of the process, click on the link below.

Lava Lamp Video