Factors affecting the surface tension of a liquid

Surface tension is not a fixed, unchanging property of a liquid -- it varies with several factors. (1) CONTAMINATION: the presence of any contamination or impurity on a liquid's surface affects its surface tension, with the size of the effect depending on the degree of contamination. (2) DISSOLVED SUBSTANCES: dissolving a substance in a liquid can raise or lower its surface tension depending on the substance -- a highly soluble substance like common salt (sodium chloride, NaCl) dissolved in water INCREASES water's surface tension, whereas a sparingly soluble substance like phenol, or a soap solution, mixed into water DECREASES its surface tension. (3) ELECTRIFICATION: when a liquid is electrified, its surface tension DECREASES, because the external electric force on the charged surface acts to expand the liquid's surface area, working directly against the natural contraction that surface tension itself produces. (4) TEMPERATURE: surface tension DECREASES, roughly LINEARLY, as temperature rises; over a small temperature range, the surface tension $T_t$ at a temperature t degC is $T_t=T_0(1-\alpha t)$, where $T_0$ is the surface tension at 0 degC and $\alpha$ is the temperature coefficient of surface tension for that liquid. A more accurate relation, due to van der Waals, is $T_t=T_0\left(1-\dfrac{t}{t_c}\right)^n$, where $t_c$ is the liquid's CRITICAL TEMPERATURE (in the same absolute or Celsius scale as t) and the exponent n varies from liquid to …