Question

SEP Construct an Explanation Look at Figure 2. Why does surface tension decrease with increasing temperature?

Answer 1

Surface tension is the energy cost (per unit area) to make a liquid surface. Two complementary ways to see why it falls as temperature rises:

1) Molecular picture
Surface tension comes from cohesive forces: molecules at the surface have fewer neighbors and form stronger net inward attractions than molecules in the bulk. Increasing temperature raises molecular kinetic energy and thermal agitation, which disrupts the ordered, cohesive arrangement at the interface and weakens the net attractive forces holding the surface together. With weaker cohesive interactions, less energy is required to create new surface area, so the surface tension decreases.

2) Thermodynamic picture (compact)
The surface free energy per unit area γ depends on temperature. From thermodynamics,
dγ/dT = −S_s,
where S_s is the surface excess entropy per unit area, which is positive. Hence dγ/dT is negative and γ decreases with increasing T. At the critical temperature the difference between liquid and gas disappears and γ → 0.

Both views explain the typical downward slope of the curve in Figure 2 and why different liquids (different cohesive forces and surface structure) show different temperature dependences.