Combustion is reverse photosynthesis

This is an excerpt from WaitButWhy, a super interesting and mind-boggling well-done blog about things that matter and others that are just plain… interesting. Please check out the site and particularly, this series about Elon Musk, Tesla, and Space Exploration!

Anyway, it may sound silly but I have always wondered: what is fire really? I know it’s just super-heated gas, but this is the best description I’ve ever heard. It also serves the double purpose of explaining carbon emissions in dead simple terms:

“When a plant grows, it makes its own food through photosynthesis. At its most oversimplified, during photosynthesis, the plant takes CO2 from the air [Interesting that we all think plants grow up from the ground, when in fact the stuff of the plant—carbon—actually comes from the air] and absorbs light energy from the sun to split the CO2 into carbon (C) and oxygen (O2). The plant keeps the carbon and emits the oxygen as a waste product. The sun’s light energy stays in the plant as chemical energy the plant can use.

“So wood is essentially a block of carbon and stored chemical energy.

“When you burn a log, all you’re doing is reversing the photosynthesis. Normally, oxygen in the air just bounces off carbon molecules in wood—that’s why trees aren’t constantly on fire. But when an oxygen molecule gets moving fast enough and smashes into a log’s carbon molecule, they snap together and the oxygen and carbon are reunited again as CO2. This snapping releases chemical energy, which knocks into other nearby oxygen molecules, causing them to get going fast—and if they get going fast enough, they’ll snap together with another of the log’s carbon molecules, which releases more chemical energy. This causes a chain reaction, and the log is now on fire. So a log burning is the process of the carbon in the log combining with oxygen in the air and floating off as CO2.”

By Tim Urban