Question

how the soap can affect surface tension??

Answer 1

i understood one thing properly about surface tension aj, that it is a type of COHESIVE FORCE SO when we say that some particles like soap are suspended in a liquid it affects the cohesive forcesbetween water molecules and soap molecules so tehre is a net decrese in surface tension.
check this out:
http://www.newton.dep.anl.gov/askasci/phy00/phy00627.htm

In the solid and liquid phase, water molecules are bonded to each other by virtue of an interaction between hydrogen atoms on one molecule and the oxygen atom of another -- the process is referred to as "hydrogen bonding." The result is a kind of "skin" on liquid water's surface.

Soaps and detergents have a polar head to which water is attracted and a non-polar tail that is hydrophobic -- water repelling. When these substances are present, they weaken the strength of the skin by interfering with hydrogen bonding between water molecules because the polar end of the soap or detergent is also attracted to the water.

Answer 2

hey thnks salini