What is the effect of impurities on surface tension?

The presence of impurities can affect the surface tension of a liquid. Surface tension refers to the cohesive forces between the molecules at the surface of a liquid that create a "skin" or surface layer. It is the force that allows some insects, such as water striders, to walk on water.

When impurities are introduced to a liquid, they can disrupt the intermolecular forces between the liquid molecules at the surface. This disruption weakens the cohesive forces and reduces the surface tension. The extent of the effect depends on the nature and concentration of the impurities.

In general, impurities that have stronger intermolecular forces with the liquid molecules than the molecules of the liquid itself can increase the surface tension. These impurities tend to be surface-active agents or surfactants, such as soap or detergent molecules. Surfactants have a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail. When added to a liquid, they orient themselves at the liquid surface, with their hydrophilic heads in the liquid and their hydrophobic tails protruding outward. This arrangement reduces the disruption of intermolecular forces and can increase the surface tension.

On the other hand, impurities that have weaker intermolecular forces with the liquid molecules can lower the surface tension. These impurities are typically non-surfactant substances that do not readily dissolve or interact with the liquid molecules. They can disrupt the cohesive forces and reduce the surface tension.

It's important to note that the effect of impurities on surface tension can vary depending on factors such as concentration, temperature, and the specific properties of the impurity and the liquid involved.