Sometimes, during the winter, some very special structures can be found on frozen lakes: frozen methane bubbles. Watch this video to see some of them in action:
How is methane gas created?
Shallow lakes contain a lot of mud. The mud itself contains a lot of organic material, which is decomposed by bacteria. The bacteria convert biomass in order to get energy and substances for their metabolism and for their reproduction. To do this, they need to oxidize the organic compounds. This does not happen through simple electron transfer. Rather, they split off hydrogen with the help of enzymes (dehydrogenases) and coenzymes such as NAD.
C6H12O6 -> 2 C3H4O3 + 4 [H] + Energy
This is an example reaction that could occur. Glucose reacts to form pyruvic acid (C3H4O3) and hydrogen. The [H] stands for convertible hydrogen, for example in the form of NADH/H+.
Aerobic organisms transfer the hydrogen’s electrons to oxygen. Anaerobic organisms, therefore, have to reduce other substances – this process is called fermentation.
During the fermentation process, we need a molecule that acts as an oxidizing agent. For this, either organic or inorganic compounds can be used. Organic compounds are for example acetaldehyde (for the formation of ethanol in alcoholic fermentation) or pyruvic acid (for the formation of lactic acid in lactic acid fermentation). Inorganic compounds could be sulfates, phosphates, CO2, or hydrogen carbonates.
Using methanogenesis, the chemical reaction equation should be written out in full. You can choose between three spellings.
As you can see, mainly gases are formed. The mixture is also called swamp gas. In addition to methane, it also contains carbon dioxide, hydrogen, hydrogen phosphide (phosphanes) and hydrogen sulfide. The phosphines are responsible for the fact that swamp gas sometimes ignites by itself when it escapes from the water and when it comes into contact with atmospheric oxygen.
The parallel formation of iron (II) ions and hydrogen sulfide is also remarkable. Under certain conditions, both can react to form the precursors of iron (II) disulfide FeS2. This forms the well-known minerals marcasite and pyrite.
How the methane ice bubbles form
Swamp gas rises like a string of pearls from certain soil pores. The gas bubbles penetrate the water surface. That reminds of sparkling champagne.
When the lake freezes, the gas that is bubbling from the bottom cannot escape. It collects under the ice. This creates a gas bubble. This becomes larger and larger and spreads out to form a flat lens. At the same time, however, the lake’s ice cover is also growing. If the ice formation occurs faster than the bubble enlargement, the flat gas bubble will be enclosed by the ice all around.
As the gas continues to bubble out of the same spot on the ground, a new bubble begins to form under the first “ice bubble” – and so it continues until a row of ice bubbles is stacked on top of each other.
Investigation of swamp gas
The swamp gas can be collected by placing a glass cylinder filled with water over the escaping gas. You can do some experiments with this gas.
Start by checking the smell. In general, this will be used to determine the presence of hydrogen sulfide.
Then one can examine the flammability of the gas mixture.
Flammability of swamp gas
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The glass cylinder is closed with a glass pane. In a darkened room the glass pane is pushed aside and with a burning stick, the gas is ignited.
The flammability is sometimes not particularly convincing. This is particularly noticeable when there is a lot of CO2 in the gas mixture. In this case, a few ml of sodium hydroxide solution (c = 1 mol / l) (C) are placed in the cylinder filled with gas (in order to bind the CO2), and the closed vessel is shaken for one or two minutes. Then proceed – without pouring out the caustic soda – as mentioned above.
If you want to specifically detect methane, you have to work with a gas chromatograph.