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Name: Azita T.
Status: N/A
Age: 15
Location: N/A
Country: N/A
Date: Thursday, August 22, 2002


Question:
How does surface tension of liquid vary with temperature?


Replies:
To answer your question "rigorously" requires knowledge of a branch of physics/chemistry called "thermodynamics", which given your age I will assume that you do not have. You can think of surface tension as the energy that is required to stretch the surface of a liquid one incremental amount of area (Assume the surface of the liquid is in contact with air.). As you might expect this requires an input of energy, that is, the surface tension is positive. Think of blowing up a balloon. You have to "push" air into the balloon to cause it to stretch. Now, to get to your question. It is easier to stretch the surface of a liquid the warmer it gets, because the molecules at the surface are "hopping around" more, the higher the temperature is. So the surface tension always decreases with increasing temperature. The formula that results from the "rigorous" analysis of the problem is: [S.T.] = H + T*(d[S.T.]/dT) where [S.T.] is the surface tension, H is the energy required to increase the area of the surface of the liquid in contact with air, T is the absolute temperature in kelvins (which is always positive), and (d[S.T.]/dT) is the change in the surface tension per degree increase in the temperature. The energy term, H, is always positive. The term (d[S.T.]/dT) is always negative for the reason I mentioned above. The bottom line: Surface tension always decreases with increasing temperature. For many liquids H is about 50 mJ/square meter and (d[S.T.]/dT) is about -0.1 mJ/square meter per degree.

Vince Calder



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