Sulfur hexafluoride is transparent, so if you fill a fish tank with it you can’t really see that that tank’s filled with anything other than air. However, since sulfur hexafluoride is denser than air, you can float a light boat on the invisible gas for a cool demonstration of density.
Note: Air is about 80% nitrogen gas, which has the formula N2, and a molecular mass of 28 atomic mass units: the molecular mass is the sum of the atomic masses of all the atoms in a molecule. Sulfur hexaflouride has the formula SF6 and a molecular mass of 146 amu, making it about 5 times denser than air.
Which is denser? Which is least dense? Water, butter or honey? This might be a trick question.
When we bake bread we usually put all the wet ingredients –honey, water and butter– into a mason jar. If you do it carefully, the substances stratify: the honey forms a nice layer at the bottom as the water floats above it; and the butter, which has the lowest density, floats on top. You need to be careful about, since the honey can dissolve into the water if it is mixed, however, with a little careful pouring, this is an easy way to demonstrate density differences.
The butter, however, can be most interesting. If you put the butter in last, it will float on top of the water as it should. However, if you put it in first and then pour the honey on top of it, or even if you put it in second, after the honey is already in the jar, the butter will stick in the viscous honey and not float to the top.
What’s really neat, is what happens when you microwave the mixture with the butter stuck in the honey. The solid butter melts, and, because it’s less dense than the water above it, as well as because water and oils (like butter) don’t mix, little bubbles of butter will form and float upwards to the top. It’s like a lava-lamp only faster. And, in the end, the butter forms a liquid layer floating on the water.
Oil slick in the Gulf of Mexico on April 25th, 2010. Image from NASA.
The scale of the disaster caused by the oil leaking into the Gulf of Mexico from the damaged oil rig is increasing day by day. We are preparing to go on the end-of-year adventure trip soon, but I’m wondering if students might be interested in heading down to the Gulf coast to volunteer in the clean-up.
NASA’s Earth Observatory has some amazing imagery on its page on the oil leak. Many of the images also show the mouth of the Mississippi and its delta, which tie directly into our observations in the sandbox. The impact of the oil spill also brings up the topic of density differences in fluids, something we’ve seen in the making bread jars, but applied to a much larger scale.
