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Name: Steve
Status: other
Grade: other
Location: IL
Country: USA
Date: Winter 2013-14


Question:
Why does a light film of oil on a hot metal surface seem to steam off when cold air passes over it?



Replies:
Steve,

I think this is about the same as why our exhaled breath clouds up as it leaves our mouth on cold, wintery days. The solubility of water in cold air is lower than in warm air, so the warm air that leaves our lungs, upon encountering the cold air, releases some water as vapor (since the colder air can no longer hold to as much water) and we see the vapor as a cloud.

Likewise, the oil is vaporizing off the hot metal. We do not see the vapor if the vaporized oil can remain essentially dissolved in the air as individual gas particles. But when cold air passes over the oil, the amount of oil that can remain in gaseous form in the air, drops suddenly, and the vaporized oil clouds up - condenses together as small oil drops, and we see it.

So it is not that the cold air causes the "steam", rather the oil had been vaporizing all along, we just get to see the cloud of oil droplets when it is suddenly chilled.

Greg (Roberto Gregorius) Canisius College


Hi Steve,

Thanks for the question. There are two effects occurring here. First, a hot liquid gives off vapor and this vapor cools and condenses to form a fog (which appears as steam to you). An example of this is Puget sound on one of the first very cold days of fall or winter. You'll see steam coming off of Puget sound with its warm water. The water vapor condenses to form "steam". The second effect operating here is that when you blow air over a surface which contains a liquid, the liquid evaporates more rapidly.

I hope this helps. Thanks Jeff Grell


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