Tag: chemistry

Iron Stained Walls

The limestone walls of the quarry were stained red with iron precipitate.

The cliffs of the quarry were stained red. Blood, seeping out from between the bedding planes between layers of rocks, might have left similar traces down the sides of the near-vertical cliffs’ faces. But these stains are actually made of iron.

Rain falling on the land above the quarry, seeps into the ground. There it moves downward through the soil, leaching out some of the minerals there, but going ever downward. Downward until it meets a layer of soil or rock that it can’t get through. Clay layers are pretty impermeable, though in this case it’s a layer of coal. The water can’t move through the near-horizontal coal seam very fast, so instead it moves sideways across, and eventually seeps out onto the cliff face.

The red on the walls of the cliffs are an oxidised iron precipitate (rust). The iron most likely was dissolved out of the pyrite in the coal seams.

The seeping water still has those minerals it dissolved in the soil. It also has more dissolved minerals from the coal it encountered too. Coal forms in swamps when trees and other plants fall into the waters and are buried before they can completely decompose. Decomposition is slow in stagnant swampy waters because most of the insects and microorganisms that do the decomposing usually need oxygen to help them with their work. Stagnant water does not circulate air very well and what little oxygen gets to the bottom of the swamp-water is used up pretty fast. You could say that conditions at the bottom of the swamp are anoxic (without oxygen), or reducing.

Coal formation. Image from the National Energy Education Development Project.
A shiny pyrite crystal in a lump of coal (happy holidays). Image via USGS.

Iron in air will rust as it reacts with water and oxygen — rust is the red mineral hematite (Fe2O3) that you see on the walls of the quarry. Iron in a reducing environment, on the other hand, will form minerals like pyrite (FeS2). According to our guide, the thin coal seam in the quarry has a fair bit of pyrite. In fact, because of the pyrite, the coal has too much sulfur for it to be economical to burn. Like the landfill gas, hydrogen sulfide, burned sulfur turns into sulfur dioxide, which reacts with water droplets in the air to create acid rain so sulfur emissions are regulated.

The water that seeps along and through the coal seam will dissolve some of the pyrite, putting iron into solution. However, the iron will only stay dissolved as long as the water remains anoxic. As soon as the high-iron water is exposed to air, the iron will react with oxygen to create rust. Thus the long stains of rust on the cliff walls show where the water emerges from underground and drips down the cliff face.

Diagram showing the coal seam, and the seeping water that creates the iron (rust) staining.

Iron precipitate in other environments

On our Natchez Trace hike we found it quite easy to stick fingers into the red precipitate at the bottom of the stream.

We’ve seen the precipitation of iron (rust) as a result of changes in redox (oxidizing vs reducing) conditions before: on the sandbar on Deer Island in the Gulf of Mexico; in the slow streams along the Natchez Trace Park‘s hiking trails in Tennessee. Iron precipitation is an extremely common process in natural environments, and it’s easily noticeable. Just look for the red.

The rich black of decaying organic matter, sits just beneath the rusty-orange surface sediment. The red is from hematite (rust) and shows that the surface is oxidizing, while the black shows that just a few centimeters beneath the surface, there is no oxygen to decompose the organic matter (a reducing environment). This image was taken on Deer Island on the Gulf Coast.

Landfills: Dealing with the Smell (H2S)

Hydrogen Sulfide:
H2S

Diagram of the hydrogen sulfide system in a landfill.

Although it makes up less than 1% of the gases produced by landfills, hydrogen sulfide (H2S) is the major reason landfills smell as bad as they do. H2S is produced by decomposition in the landfill, and if it’s not captured it not only produces a terrible, rotten-egg smell, but also produces acid rain, and, in high enough concentrations, it can be harmful to your health (OSHA, 2005; Ohio Dept. Health, 2010).

Decomposition

A wall partially covered with drywall. Image via FEMA via Wikimedia Commons (Nauman, 2007).

Some hydrogen sulfide is produced when organic matter decays, but for big landfills like the one we visited, construction materials, especially gypsum wallboard (drywall), are probably the biggest source.

Gypsum is a calcium sulphate mineral, that’s made into sheets of drywall that are used cover the walls in most houses because it’s easy to work with and retards fire. The U.S. used 17 million tons of gypsum for drywall in 2010 according to the USGS’s Mineral Commodity Summary (USGS, 2011 (pdf)).

Gypsum:

CaSO4•2(H2O)

As you can see from the chemical formula, each gypsum molecule has two water molecules attached. In a fire, the heat required to evaporate the water keeps the temperature of the walls down to only 100 degrees Celcius until the water has evaporated out of the gypsum board.

A number of landfills have banned drywall because it produces so much hydrogen sulfide, but the one we visited still takes it. It’s big enough that they capture the landfill gasses, including the hydrogen sulfide, and then separate it from the other, more useful gasses, like methane, which can be burned to produce heat energy. H2S can also be burned, but they you end up contributing to acid rain.

H2S and Acid Rain

When hydrogen sulfide reacts with oxygen in the atmosphere it produces sulfur dioxide.

2 H2S (g) + 3 O2 (g) —-> 2 SO2 (g) + 2 H2O (g)

Sulfur dioxide, in turn, reacts with water droplets in clouds to create sulfuric acid.

SO2 (g) + H2O (g) —-> H2SO4 (aq)

Acid rain accelerates the dissolution of statues. (Image by Daniele Muscetta)

When those droplets eventually coalesce into raindrops, they will be what we call acid rain.

Acid rain damages ecosystems and dissolves statues. It used to be a major problem in the midwestern and eastern United States, but in 1995 the EPA started a cap and trade program for sulfur dioxide emissions (remember sulfur dioxide is produced by burning hydrogen sulfide) that has made a huge difference.

The head (top) of a well (vertical metal pipe) that captures the gas from inside the landfill.

Capturing H2S

Probably because of the EPA’s restrictions, the landfill company pipes all the gases it collects through scrubbers to extract the hydrogen sulfide. There are a few ways to capture H2S, they all involve running the gas through a tank of some sort of scavenging system that holds a chemical that will react with hydrogen sulfide and not the other landfill gases. At the landfill we visited the remaining landfill gas, which consisted of mostly methane, was used for its energy.

The History of the Periodic Table

Fitted to a cylinder, the elements on this periodic table would form a spiral. Image via Wikipedia.

Spurred by Philip Stewart‘s comment that, “The first ever image of the periodic system was a helix, wound round a cylinder by a Frenchman, Chancourtois, in 1862,” I was looking up de Chancourtois and came across David Black’s Periodic Table Videos. They put things into a useful historical context as they explore how the patterns of periodicity were discovered, in fits and starts, until Mendeleev came up with his version, which is pretty much the basis of the one we know today.

The cylindrical version is pretty neat. I think I’ll suggest it as a possible small project if any of my students is looking for one. You can, however, find another interesting 3d periodic table (the Alexander Arrangement) online.

Why Gold is Precious

The precious metals are those few that are not gasses, and not reactive. Of these, only gold (Au) and silver (Ag) are not extremely rare and hard to extract and work.

A while back, I posted a radio article by Planet Money on why gold is so valuable, and has been used for money for so long (God, Glory and GOLD: but why gold?). They’ve now created a nice video explaining the same thing. Though there’s less detail, the dramatic visuals (of the reactivity of sodium for example) make it quite interesting.

Making a Non-Stick Frying Pan the Old Fashioned Way: Creating Polymers at Home

"Seasoning" a cast iron frying pan creates a non-stick coating. (Image by Evan-Amos via Wikipedia).

Back in the day, if you wanted a non-stick cooking skillet, your best option was to do it yourself by seasoning a cast metal pan. Sheryl Canter has an excellent post describing the science behind the “seasoning” process. The key is to bake on a little bit of oil to create a strong cross-linked polymer surface. This is a nice tie into our discussion of polymers and polymerization in the middle school science class; although I’m not sure how many of my students have actually seen a cast iron pan, or even know what cast iron is.

Normal polymers are long molecules made up of smaller molecules linked together, much like a paperclip chain.
Cross-linked polymers are created when the long chained polymers are linked together by cross-links. It makes for a much sturdier molecule.

To season, you coat the pan with a thin layer of oil and bake it for a while (without anything in it). Baking releases free radicals from the metal that react with the oil to create a cross-linked polymer that’s really hard to break down or wear out, and prevent food from sticking to the pan. Different, cross linked polymers are used in car tires for their durability, but probably not for their lack of stickiness.

Apparently, linseed oil is the best seasoning agent, but it might be a bit hard to find.

Most non-stick, artificial surfaces, are also made of polymers of hydrocarbons, silicon oxides and other interesting chemicals.

Making a cross-linked polymer with borax and polyvinyl alcohol.

In the lab, you can make your own cross-linked polymer “slime” by adding a solution of borax (sodium tetraborate) to a solution of polyvinyl alcohol (1:1 ratio of concentrations) (Practical Chemistry, 2008).

The result is a satisfying goo.

Cross-linked polymer "slime".

Crystals, Non-Crystals and Quasicrystals

Quasicrystalline ordering of a aluminum-palladium-manganese alloy. Image by J.W.Evans via Wikipedia.
Regular ordering of a halite crystal. The atoms that make up salt crystals are arranged in a cubic shape. The smaller grey atoms are sodium (Na), and the larger green ones are chlorine (Cl).

Daniel Shechtman was just awarded the Nobel Prize in Chemistry (2011). He discovered that matter can exist not only as crystals, which have a regular geometric arrangement of atoms, and amorphous non-crystals that do not, but also as quasicrystals which have a different type of atomic ordering.

The Guardian has an interesting article on Schechtman, whose discovery was roundly disbelieved by other scientists and he was ridiculed for years.

In an interview this year with the Israeli newspaper, Haaretz, Shechtman said: “People just laughed at me.” He recalled how Linus Pauling, a colossus of science and a double Nobel laureate, mounted a frightening “crusade” against him. After telling Shechtman to go back and read a crystallography textbook, the head of his research group asked him to leave for “bringing disgrace” on the team. “I felt rejected,” Shachtman said.

— Sample (2011): Nobel Prize in Chemistry for dogged work on ‘impossible’ quasicrystals in The Guardian

Hat tip to M. Eisenberg for this link.

The Story of Stuff and the Life Cycle of a Cell Phone

The life cycle of a cell phone. (Produced by the EPA. Link goes to a pdf).

The EPA’s student resource page has a few interesting publications on the life cycles of a few common products: CD/DVD’s, cell phones, and soccer balls.

They’re a bit noisy, and would probably benefit from being reproduced in a more interactive format (Flash maybe), but they’re still a useful resource for talking about life cycles.

They’re a less dramatic presentation which can supplement the advocacy of the Story of Stuff video.

Phosphorus: What is it good for?

So other than digging in Morocco, where do we get more phosphorus? Here’s a hint: the symbol for phosphorus on the periodic table… is “P.”

— Horwich (2011): The end of phosphorus on APM’s Marketplace.

Marketplace’s Jeff Horwich has an excellent article on the uses of the element phosphorus, where it comes from, why it’s getting scarce, and where we might get more.

The answers to these questions are:

  • It’s a key element in DNA, so the major use is fertilizer,
  • most of it comes from Morocco these days,
  • since Morocco supplies about 85% of the world supply, they’re developing a bit of a monopoly and the price is going up,
  • the main alternative sources are manure and urine that have lots of phosphorous. In fact, burning sewage leaves behind a phosphorous rich ash.

Marketplace tells the story in much more detail.