Q.What are the factors affecting the surface tension of a liquid?

The surface tension of a given liquid is not a fixed constant -- it changes measurably with several external factors. (1) CONTAMINATION: any contamination or impurity present on a liquid's surface affects the magnitude of its surface tension, with the size of the effect depending on how contaminated the surface is. (2) DISSOLVED SUBSTANCES: dissolving a substance in a liquid can raise or lower its surface tension depending on which substance is dissolved -- a highly soluble substance such as common salt (sodium chloride, NaCl) dissolved in water INCREASES water's surface tension, while a sparingly soluble substance such as phenol, or a soap solution, mixed into water DECREASES its surface tension (this is exactly why soap and detergent solutions have noticeably lower surface tension than plain water, which is central to how they clean). (3) ELECTRIFICATION: when a liquid is electrically charged, its surface tension DECREASES, because the external electric force acting on the charged surface tends to expand its 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 t degC is $T_t=T_0( …