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Surface Tension and Temperature
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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