Why does alcohol wet glass?

The surface of any liquid consists of a layer of molecules that are more strongly attracted to the molecules below them than to the air above them. This surface tension creates a "skin" on the surface of the liquid that resists being broken.

When a drop of alcohol is placed on a glass surface, the alcohol molecules immediately begin to spread out and cover the surface of the glass. This is because the alcohol molecules are attracted to the glass molecules more strongly than they are to each other. The force of attraction between the alcohol and the glass molecules is greater than the surface tension of the alcohol, so the alcohol droplet spreads out and wets the glass.

The same principle applies to other liquids that wet glass, such as water and acetone. The molecules of these liquids are all attracted to the glass molecules more strongly than they are to each other, so they spread out and wet the surface of the glass.

Liquids that do not wet glass, such as mercury, have molecules that are not attracted to the glass molecules. In fact, the mercury molecules are repelled by the glass molecules. This repulsion is greater than the surface tension of the mercury, so the mercury droplet remains spherical and does not wet the glass.