# Surface Tension at Critical Temperature and at Boiling point [closed]

I recently did an experiment on finding the surface tension of a liquid at different temperatures, When I was going through my lab manual, I found the following statement:

As temperature decreases, surface tension increases. Conversely, as surface tension decreases strongly as molecules become more active with an increase in temperature becoming zero at its boiling point and vanishing at critical temperature.

What is the key difference between becoming zero and vanishing?

• I think it may be a result of a wrong sentence analysing and its confusing formulation. Mar 23 at 17:09
• Can you explain please? Mar 23 at 17:19
• Well, I'm pretty sure you know what b.p. is so check critical point and you should be good :/ Mar 23 at 17:50
• The first term IMHO does not relate to surface tension, as the second one, but to temperature. Mar 23 at 17:51
• critical point is set of conditions beyond which the gas cannot be liquified right? Mar 23 at 17:51

"As temperature decreases, surface tension increases. Conversely, as surface tension decreases strongly as molecules become more active with an increase in temperature becoming zero at its boiling point and vanishing at critical temperature. "

What is the key difference between becoming zero and vanishing?

The key difference between

• temperature increase becoming zero at the boiling point
• vanishing surface tension at the critical point

is this:

• The former is about the temperature of the liquid that cannot be increased above the boiling point at the given pressure, assuming equilibrium state.
• The latter is about surface tension that decreases with temperature and converges to zero at the critical point where both phases become the same and the phase boundary vanishes together with the surface tension.

Note that if you would preform a cyclical state change of the system around the critical point at the phase diagram, you would be able to achieve repeated evaporation without condensation, or vice versa, depending on the orientation of the cycle.

It is like if there was a beach and a plateau above a cliff, the converge in some direction to the same elevation so the cliff gradually disappears. If you were following repeated closed path around the point where the cliff disappears, you would be repeatedly ascend the cliff without descending it (or vice versa in the opposite direction of the path.

• The beach is like a liquid.
• the plateau is like a vapor.
• The cliff is the phase transition between them.

Chosen values from Engineeringtoolbox site:

Temperature [$$\pu{^{\circ}C}$$] Surface tension [$$\pu{e-2 N m-1}$$]
0 7.56
50 6.79
100 5.89
150 4.82
200 3.76
250 2.64
300 1.47
350 0.37
374.1 0.00
• This zero at the boiling point is confusing a boiling liquid shows bubbles that usually require surface tension to happen. also boiling point T depends on applied atmosphere so surface tension becomes zero at every possible BP? Mar 23 at 21:16
• I have rejected the idea of surface tension being zero at the boiling point at several comments and now the answer. The whole OP question is rather about grammar and stylistics than about chemistry. Mar 23 at 21:19
• So isn't it zero at Boiling point? Mar 24 at 5:08
• I am trying to say it all the time, even in comments before posting the answer. // See also engineeringtoolbox.com/water-surface-tension-d_597.html Mar 24 at 6:29

At a substance's critical point, it cannot be liquefied by pressure alone. In contrast, if a substance has reached it's boiling point it can be liquefied with an increase in pressure. The distinction between "zero" and "vanishing" is simply a distinction between pressure's ability to physically change a substance into a liquid past a certain temperature.

• what does it have to do with surface tension? Mar 23 at 17:45
• Surface tension can only exist when a substance is a liquid. Mar 23 at 18:05