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Water sometimes drips from the exhaust of a running car...

  • #1

Homework Statement



You may have noticed that water sometimes drips from the exhaust of a car as it is running. Is this evidence that there is at least a small amount of water originally present in the gasoline?

Homework Equations



not sure if there is one.

The Attempt at a Solution



I'm not sure of the exact chemistry behind this, but I think just because there's water as a product doesn't mean there was water as a reactant — it could well have just been created out of hydrogens from some molecule and oxygens from other molecules. However, this is pretty vague, and I'd like some help coming up with a more chemistry—appropriate answer.
 

Answers and Replies

  • #2
Borek
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No, amount of water being a product of combustion is orders of magnitude larger than amount of water that can be dissolved in gasoline.
 
  • #3
@Borek — could you clarify what orders of magnitude is and how I know it is larger than amount of water that can be dissolved in gasoline...? I'm not sure I understand.
 
  • #4
Borek
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Basically when we say something is order of magnitude greater we mean it is abut ten times greater, two orders of magnitude is about 100 times, three orders of magnitude is about 1000 times and so one (these are just ballpark figures, not exact, it is a bit handwavy, but often very convenient).

Gasoline can contain some very small amount of dissolved water (something like 0.1% w/w if memory serves me well, can't find any data quickly now). So, say you have a pound of gasoline - it will contain no more than about 0.001 pound of water at best (I am assuming there is no water puddle below the gasoline). Burning one pound of gasoline produces around 1.4 pound of water - so the amount of water produced is about 1400 times higher. That's about three orders of magnitude more water produced than originally present, so no matter whether the gasoline was initially "wet" or not you can see water dripping.
 
  • #5
jim mcnamara
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@Borek gave you a great answer.

But some aspects are a bit less obvious. The usual cause of dripping water is that the exhaust is a hot gas made of carbon dioxide and water (and other molecules, too, we'll ignore them). The water vapor in the hot exhaust hits the cold metal of the exhaust pipe/manifold. Then it condenses. Like the water that forms on the outside if a glass of a cold drink on a warm, humid day.

After the exhaust systems get warm, condensation stops. But of course water vapor (it is in a gaseous phase, like oxygen) is still the exhaust, anyway.

Water in gasoline comes from two sources: condensation in the tank, poor practices by the guy selling you the gasoline. And there is very little or no mixing of the gas and water, because water does not dissolve at all well in gasoline, it sinks to the bottom of the gas tank, and causes the engine to stop running if there is more than a tiny quantity of water down there. So any visible drip out of the tailpipe has to have come come from condensation.


Popular science explanation:
https://www.quora.com/Is-gasoline-soluble-in-water-Why-or-why-not
https://en.wikipedia.org/wiki/Water_vapor - we live in an atmosphere that has lots of it.
 
  • #6
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"... sinks to the bottom of the gas tank, and causes the engine to stop running if there is more than a tiny quantity of water down there..."

It's a long time ago but, when we were on holiday in Wales, soon after we refilled our old car's tank at a tiny village's pump, the engine began misfiring and spluttering. Local garage guy diagnosed damp petrol, prescribed a pint of 'Methylated Spirits', aka ethanol cut with methanol, astringent and dye. With that added, we rocked the car to and fro to mix the tank. The mechanic then drained our carburettor and fuel line. After a few more minutes of unhappiness, the engine ran sweetly again.

Do NOT try this with a modern car-- The Engine Management system would be very, very unhappy...
 

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