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Q & A: Oil and evaporation of water

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Most recent answer: 10/22/2007
Does oil effect the evaporation of water? I need a lot of information.
- katie (age 12)
Hi Katie,

A thin layer of oil on top of water will mostly prevent the water from evaporating, because the oil will float on top of the water and keep the water molecules from escaping from the water's surface. Water dissolves very little in ordinary oil, so hardly any water molecules diffuse through the oil.

Some oils evaporate themselves, though, and if you wait long enough, the oil may evaporate away, leaving a hole for the water underneath to evaporate. If the oil layer is too thin, it may leave a hole somewhere for the water molecules to escape by evaporation. Emulsifying agents, like soap, stick to oil molecules and to water molecules, allowing the two to mix. Adding soap to water with oil on top of it may allow the water to evaporate.

Really dirty oil, like the stuff you get out of car engines if the oil isn't changed often enough, may contain so many small metal particles in it that it will sink in water. This kind of oil will just sit on the bottom and not affect the evaporation rate.


(published on 10/22/2007)

Follow-Up #1: categories?

What category does 'Oil and Evaporation of Water' fall under? Is it like chemistry or physics or what?
- sally (age 12)
New York, New York; USA

Hi Sally- Schools often try to cut things up into neat categories, but that doesn't make it a good way to think. We try to make our answers make sense but our categories don't really mean much. Nature doesn't divide up neatly that way, and certainly not along the lines of our site's categories. That's why many of our answers show up in more than one category.

The way oil and water don't mix is usually thought of as being a chemical fact, although it follows from the physical properties of their molecules. That oil sits on top also involves gravity, since it's what makes one side 'top' and the other 'bottom'.

This answer, for instance, is going in (at least)

Category #1:
Everything Else
Category #2:
Everything Else
Category #3:
Making Stuff Move
Category #4:
States of Matter and Energy
Boiling, Evaporating & Condensing
Category #5:
Underwater and in the Air
It Floats!

Mike W.


(published on 10/22/2007)

Follow-up on this answer.