why does an oil drop spread on cold water, but may remain as a drop in hot water ?

Question

this is a question of topic surface tension....prefferably give an ans related to that.
but plz give another method, if u know......
thanks a lot for ur help......i will chose the most convincing answer as the best one......

Answer 1

This doesn't have anything to do with surface tension, but rather temperatures and densities. As you know, cold water is denser than hot water. Cold water will always sink whereas hot water will always rise to the surface. Now, it matters if the oil is hot or cold? If it is room temperature (which i'm going to assume from your observations in your question) then room temperature is mild- it is neither hot NOR cold. If you have a glass of HOT water and drop a mild drop of oil into the glass, the oil is more dense than the water, thus the oil will spread out quicker (trying to sink). Now, if you take a glass of COLD water and drop a drop of mild oil into the glass, the water is going to be denser and will want to stay on the bottom of the glass while the oil will try to stay as far up on the top as it can get- thus it dissuedes the oil from "spreading out". Hope this helps

Answer 2

I have been cooking for years and have never seen oil remain as a drop in hot water.
The reason oil spreads on water is that the water molecules attract other molecules as well as each other. The attraction of water molecules for each other(surface tension) is strong enough to cause it to form drops on most material.The surface tension decreases with increasing temperature so in some special circumstances this could have the effect you describe. The formation of drops on surfaces is a interaction between the two surfaces and the air, It is discribed at http://kr.cs.ait.ac.th/~radok/physics/g2.htm#G2