Evaporation: a physical change
The molecules stay the same; they just move apart and go from liquid to gas. Steam from boiling water is still water. Cool gasoline vapour and it becomes gasoline again.
Where does the gasoline burned in a car come from, and where does it go?
In short
Gasoline doesn't disappear when it burns, nor does it just “turn to vapour”: its atoms form new molecules. The carbon atoms become carbon dioxide, the hydrogen atoms become water vapour, and both leave through the exhaust into the air. Millions of years ago those atoms were inside tiny sea creatures; long before that, they were in stars.
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For 1 litre of gasoline
- ≈ 2.3 kgof carbon dioxide from burning 1 litre of gasoline
- ≈ 0.9 kgof water (as vapour) from the same litre
- ≈ 2.5 kgof oxygen taken from the air for that combustion
- 20–35%of the energy that becomes motion in the engine
Combustion: atoms change places
Take octane (C₈H₁₈), one of the molecules in gasoline. Press “Burn”: the same atoms find new partners. Their numbers don't change; no atom disappears and none is created.
Before: 2 octane + 25 oxygen molecules
| Atom count | Before | After |
|---|---|---|
| Carbon (C) | 16 | 16 |
| Hydrogen (H) | 36 | 36 |
| Oxygen (O) | 50 | 50 |
Real gasoline is a mixture of hundreds of hydrocarbons; octane is used here as an example. Engines also produce small amounts of carbon monoxide and nitrogen oxides.
Did it evaporate, or did it burn?
Both happen, but they are very different things. First the gasoline evaporates, then it burns.
Combustion: a chemical change
The molecules break apart and the atoms build new molecules with oxygen. The resulting carbon dioxide and water are no longer gasoline; cooling them won't turn them back.
Why does it get heavier when it burns?
A litre of gasoline weighs about 740 grams, yet burning it produces more than 3 kilograms of gas. That's because about 2.5 kilograms of oxygen from the air join in. Mass is conserved: the atoms before and after add up to the same total.
A long journey: from star to exhaust, from exhaust to tree
Billions of years ago
The atoms are born
Gasoline's hydrogen atoms formed in the Big Bang and its carbon atoms inside stars. When the Solar System formed, these atoms became part of Earth.
Tens to hundreds of millions of years ago
Sunlight and tiny sea creatures
Countless microscopic sea creatures (plankton and algae) photosynthesised: they took carbon from carbon dioxide in the air and hydrogen from water and bound them into sugars and fats. The Sun's energy was stored in those chemical bonds.
When they died
Buried on the sea floor
When they died they sank to the sea floor. In oxygen-poor mud they could not fully rot, and were buried under layer upon layer of sediment.
Over millions of years
Slow cooking underground
As the layers thickened, the buried remains sank several kilometres down. At about 60–120 °C, over millions of years, they turned into chains of carbon and hydrogen: oil and natural gas.
In the last 160 years
From well to refinery
Oil is pumped from wells and taken to a refinery. There it is heated, and the fractions that separate by boiling point are collected at different heights: gas, gasoline, kerosene, diesel, heavy oils and asphalt. This is called distillation.
Today
In the tank: liquid gasoline
Gasoline is not one substance but a mixture of hundreds of hydrocarbons with 4 to 12 carbon atoms. A litre weighs about 740 grams and contains about 640 grams of carbon and 100 grams of hydrogen.
In the engine, in an instant
First, evaporation
Gasoline sprayed into the engine breaks into tiny droplets, evaporates and mixes with air. This is a change of state: the gasoline molecules are still the same; they have only gone from liquid to gas.
At the spark
Then combustion: atoms swap partners
When the spark plug fires, gasoline molecules react with oxygen from the air. The molecules break apart: carbon atoms join oxygen to make carbon dioxide, and hydrogen atoms join oxygen to make water. The heat released makes the gases expand, pushes the pistons and moves the car. Only about 20–35% of that energy becomes motion; the rest is lost as heat.
Within seconds
Out of the exhaust
Most of the exhaust gas is nitrogen from the air that passes through the engine unchanged; most of the rest is carbon dioxide and water vapour. Small amounts of carbon monoxide and nitrogen oxides also come out; the catalytic converter turns most of them into less harmful gases.
Within a few days
The water vapour becomes rain
The water vapour behaves like any other water in the air: it condenses as it cools, joins clouds and returns to the ground as rain or snow within about 9–10 days on average.
From years to millennia
The carbon dioxide stays in the air a long time
Carbon dioxide is colourless and invisible. Plants take up some of it by photosynthesis and the oceans absorb some, but a large share stays in the air for centuries, even millennia. Because we are returning to the air, within a few centuries, carbon that was buried over millions of years, the amount of carbon dioxide in the air is rising and the climate is warming.
And then
The cycle goes on
A carbon atom from an exhaust pipe may one day end up in a tree's leaf, then in an apple, and then perhaps in you. Atoms don't vanish; they just join new molecules.
Frequently asked questions
Where does gasoline go when it burns?
It doesn't vanish. Its carbon atoms combine with oxygen from the air to make carbon dioxide, its hydrogen atoms become water vapour, and both go out through the exhaust into the air. Burning 1 litre of gasoline produces about 2.3 kg of carbon dioxide and 0.9 kg of water; the total weight increases because oxygen from the air is added.
What is the white smoke from an exhaust pipe?
The white plume you see in cold weather is mostly water vapour from combustion cooling into tiny droplets, just like your breath on a cold day. Carbon dioxide is colourless and invisible.
Is oil made from dinosaurs?
No. Oil formed mostly from microscopic sea life (plankton and algae) buried on the sea floor and heated over millions of years.
Do atoms die?
Almost all the atoms in your body are stable and can last unchanged for billions of years. When a living thing dies, its atoms do not disappear; they pass into soil, air and other living things and join new arrangements.