Explainer

# How Gasoline Becomes CO2

## A gallon turns into 19 pounds?

Last week, Slate published the first installment of the “Green Challenge,” a program that helps participants reduce the amount of carbon dioxide they put into the atmosphere. We started by asking people to think about the effects their cars have on the environment: “For each gallon of gas your car burns, it releases about 19 pounds of carbon dioxide.” Explainer readers wondered about this statistic: If a gallon of gasoline weighs about 6 pounds, how can it produce three times that much greenhouse gas?

The carbon from the gasoline mixes with oxygen from the air. Gasoline consists mostly of hydrocarbons—chains of carbon encircled by atoms of hydrogen. When the hydrocarbons burn, they break apart and recombine with the air. This reaction produces heat, as well as two chemical byproducts: water and carbon dioxide.

For example, consider a single molecule of octane—a typical hydrocarbon that you’d find in gasoline. Octane consists of eight atoms of carbon and 18 atoms of hydrogen, written as C8H18. If you break down the octane and mix it with enough oxygen (O2), you’ve got the ingredients—i.e., the atoms of carbon, hydrogen, and oxygen—to make eight molecules of carbon dioxide (CO2) and nine molecules of water (H2O). The eight molecules of CO2 weigh about three times more than the one molecule of octane you started with. That doesn’t mean you’ve violated the law of conservation of mass; instead, you’ve added the weight of the oxygen from the air to the weight of the carbon from the gasoline. (For a more in-depth discussion of this reaction, click here.)

This reaction gives only a general sense of what happens when you burn a gallon of gas. First, the combustion that occurs in a car engine doesn’t work perfectly, which means not every hydrocarbon gets converted into carbon dioxide and water vapor. Sometimes there’s not enough oxygen available to complete the reaction, in which case hydrocarbons can be converted into poisonous carbon monoxide (CO). Burning gasoline can also release nitrous oxide and other gases.