Density stratification
1 2016-05-17T12:17:31+00:00 Mariek Schmidt 3b678a5bd42eb8bf9a55fb761e5f17b11ce872c1 10 1 Caption: A graduated cylinder filled with various liquids to illustrate density. It's also really colorful and pretty. From the bottom to the top 0–11 ml Maple syrup orange 11–30 ml Gradient from Dish soap (green) to water (dyed blue) 30–34 ml Water (dyed blue) a 34–39 ml Wine (dyed red) a 39–49 ml Vegetable oil b 49 ml–top Olive oil b a b They sorta blend into each other. Instructions for creation (what i did; may not be the most efficient) 1 Add maple syrup to an empty graduated cylinder; take care to avoid spilling it on the sides (I used a pipette for this). 2 Add one drop of blue food coloring to about 100 ml of water. 3 Pour in some of the blue water using a pipette if wanted (if you get it on the sides, let the cylinder dry after) 4 Pipette or pour in some dish soap, it should settle below the water without many bubbles, though it may form a gradient (which is pretty too.) 5 Use a pipette to suction off any soap bubbles that may have formed on the water surface. 6 Pour in vegetable oil 7 Pour in olive oil. The olive oil should float above the vegetable oil, but it may form a gradient. There will be bubbles in the oil; these should dissipate in a few hours. 8 Pipette in some more blue water—it should sink, and the oil will slow it down enough so it won't mix with the now turquoise soap-water gradient. 9 Add some red food coloring to some cooking alcohol. 10 Pipette in the alcohol; it should sink through the oil in a similar way and settle above the pure water layer. There may be a short purple transition zone. 11 Let the cylinder settle for two or three hours. I didn't include any milk, but if you do, it should be somewhere between the water and vegetable oil. Be prepared to wait a few days for it to settle into a nice defined layer. Image credit: Kevin Song plain 2016-05-17T12:17:31+00:00 Mariek Schmidt 3b678a5bd42eb8bf9a55fb761e5f17b11ce872c1This page is referenced by:
-
1
2016-05-17T11:47:01+00:00
Density
28
image_header
2019-06-11T15:42:53+00:00
Density is the measure a substance’s mass per unit volume. It is most often represented by the Greek letter rho, ρ. The equation for density is:
ρ = m /V
where m is the mass and V is the volume. Typical units are grams per cubic centimeter (g/cm3), or kilograms per cubic meter (kg/m3).
Relative differences in density will cause a denser, less buoyant substance to sink, while a lighter, more buoyant one will rise. This causes layering (also called density stratification) to occur; the densest stuff will be at the bottom, or closest to the center of gravity, while the lightest will be at the top, or farthest from the center of gravity.
For the case of a differentiated rocky body, its center is also its center of gravity. Solid iron metal is the densest and is thus found at the center of gravity. Crusts of terrestrial planets are commonly rich in a low-density silicate mineral called plagioclase feldspar, and are found farther away from the center of gravity.
One of the lines of evidence that the Earth has a dense core is that the mass of the Earth is much greater than can be attributed to rocks seen at the surface. Basalt is a typical crust rock for terrestrial planets. The density of basalt (3.2 g/cm3) is significantly less than the mean density of the Earth (5.52 g/cm3). Therefore, there must be a very dense core.
Relationship between density and temperature
The volume of a substance correlates with temperature; when a substance heats up, its volume expands, and when the substance cools down, the volume contracts. An example of this phenomena is your car tires. In the winter, it is common for the air in your car tires to contract from the cold, causing your tire pressure to go down and the warning light on your dash to go on. But once you drive around for awhile, your tires warm up, and the tire pressure light will go away. (Unless you really do have a flat tire!)
So now let’s think about how the changing volume with temperature affects density. Density is mass per unit volume. With increasing temperature, the mass stays constant, but it is spread out over a larger volume, causing density to decrease. Conversely, with decreasing temperature, mass stays constant, but it is squeezed into a smaller volume, and density increases. Density is therefore inversely proportional to temperature (when temperature goes up, density goes down).
This relationship between temperature, density and buoyancy is really well illustrated by a lava lamp. Be groovy and impress your friends by teaching them how a lava lamp works:
For the planets, changes in density related to changes in temperature drive important geologic processes, including- the rise of hot magma (molten rock) relative to surrounding colder rock,
- the rise of hot fluids and eruptions of geysers on Earth, or Saturn’s moon Enceledes,
- mantle convection and plate tectonics, which in turn governs the distribution of volcanoes and earthquakes on the Earth. (This is a really, really important process for the Earth, so stay tuned for the next module.)
