Factors affecting surface tension

[Kaushik]

I read the following two points:

• Temperature is indirectly proportional to surface tension.
• Adding impurities affect the surface tension of the molecules. Adding soluble impurities increases the surface tension whereas adding sparingly soluble impurity decreases the surface tension.

Why?

Edit: Also, could you please clarify the following: The molecules on the surface experiences a net downward force and hence acts like a stretched membrane. Why does it act like a stretched membrane?

 

[hilbert2]

Usually the surface tension decreases when temperature is increased.

Soluble impurities can also decrease surface tension, for example methanol or ethanol added to water. Larger-molecule alcohols like n-pentanol that aren't completely water soluble will decrease the surf. tension more effectively because they have a polar and nonpolar end like soap molecules. I don't think there is any general rule relating the solubility of an impurity and its effect on this physical property.

 

[Kaushik]

Usually the surface tension decreases when temperature is increased.

Soluble impurities can also decrease surface tension, for example methanol or ethanol added to water. Larger-molecule alcohols like n-pentanol that aren't completely water soluble will decrease the surf. tension more effectively because they have a polar and nonpolar end like soap molecules. I don't think there is any general rule relating the solubility of an impurity and its effect on this physical property.

Why does it decrease with temperature? Please check the edit.