The answer depends on which “gas” you mean. Gasoline, the fuel you pump into your car, is a complex blend of over 150 different hydrocarbons, molecules built from hydrogen and carbon atoms. Natural gas, the fuel that heats homes and fires stoves, is a simpler mixture dominated by methane. Both start as crude materials pulled from the earth, but their compositions are very different.
What Gasoline Is Made Of
Gasoline is not a single chemical. It is a mixture of hydrocarbons that fall into several families, each with different molecular shapes and behaviors. The largest share comes from isoalkanes (25 to 40% by volume), which are branched chains of carbon and hydrogen that burn smoothly in an engine. Aromatics, ring-shaped molecules, make up another 20 to 50%. Straight-chain alkanes account for 4 to 8%, with smaller amounts of alkenes, cycloalkanes, and cycloalkenes filling out the rest.
Benzene, one of the aromatic compounds present, is a known health concern. Its concentration in finished gasoline is kept between 0.5% and 2.5%. Refineries carefully control the balance of all these hydrocarbon families because each one affects how the fuel ignites, how much energy it delivers, and how cleanly it burns.
How Octane Ratings Work
When you choose between regular and premium at the pump, you are choosing a different hydrocarbon balance. Octane ratings measure a fuel’s resistance to “knocking,” which happens when fuel ignites too early inside the engine cylinder, creating a damaging pressure spike. The scale is built around two reference chemicals: iso-octane, a highly stable molecule assigned a rating of 100, and heptane, an extremely unstable molecule assigned a rating of 0. A fuel rated 87 octane behaves like a mixture of 87% iso-octane and 13% heptane in a test engine.
Higher-octane gasoline contains a greater proportion of branched and aromatic hydrocarbons that resist premature ignition. Most cars run perfectly well on regular 87-octane fuel. Engines with higher compression ratios or turbochargers need premium fuel because those conditions generate more heat and pressure, making knocking more likely.
Additives Beyond the Hydrocarbons
Raw refined gasoline would damage modern engines over time, so several types of additives are blended in before it reaches you. The EPA requires all gasoline sold in the United States to contain certified detergent additives. These detergents prevent carbon deposits from building up on fuel injectors and intake valves, which would reduce engine efficiency. Corrosion inhibitors protect metal fuel lines and tank surfaces from degradation. Antioxidants slow the breakdown of the fuel itself during storage, keeping it stable for weeks or months in your tank.
Summer and Winter Blends
Gasoline composition actually shifts with the seasons. The key variable is volatility, meaning how easily the fuel evaporates. In cold weather, your engine needs fuel that vaporizes readily so it can ignite on a cold start. Winter blends achieve this by increasing the percentage of butane, a lightweight hydrocarbon that evaporates easily. In summer, that same easy evaporation becomes a problem. Fuel that vaporizes too quickly contributes to smog and can cause vapor lock in the fuel system.
Federal regulations require summer gasoline (sold June 1 through September 15) to meet a maximum vapor pressure of 9.0 psi, with some areas like Beaumont-Port Arthur, Texas held to an even stricter 7.8 psi limit. During those months, butane content drops to around 2%. This seasonal reformulation is one reason gas prices typically rise in late spring, as refineries switch over to the more expensive summer blend.
What Natural Gas Is Made Of
Natural gas is far simpler than gasoline. The commercial natural gas piped to homes and businesses is 85 to 90% methane, the smallest hydrocarbon molecule (one carbon atom bonded to four hydrogen atoms). The remainder is mostly nitrogen and ethane, with trace amounts of propane and butane.
Raw natural gas straight from the wellhead, however, is dirtier. It contains significant quantities of carbon dioxide and hydrogen sulfide, both classified as acid gases. Carbon dioxide is highly corrosive when moisture is present and would quickly damage pipelines. Hydrogen sulfide is toxic even in small concentrations and smells like rotten eggs. Processing plants strip these contaminants out using chemical solvents before the gas enters the pipeline system. Heavier hydrocarbons like propane and butane are also separated and sold individually as liquefied petroleum gas (LPG), the fuel used in portable grills and some rural heating systems.
The methane-rich gas that reaches your stove is odorless on its own. Utility companies add a sulfur-containing odorant so you can detect leaks by smell.
Gasoline vs. Natural Gas at a Glance
- State: Gasoline is a liquid at room temperature. Natural gas is, as the name suggests, a gas.
- Complexity: Gasoline contains over 150 different hydrocarbon compounds. Natural gas is overwhelmingly one compound: methane.
- Source: Both originate from underground fossil deposits, but gasoline is refined from crude oil while natural gas is extracted separately, often from the same wells.
- Energy use: Gasoline powers most passenger vehicles. Natural gas heats roughly half of U.S. homes and generates a large share of the country’s electricity.