Explain why water wets glass while mercury does not
Water wets glass because of its adhesive properties, whereas mercury does not wet glass due to its cohesive properties.
Adhesion refers to the attraction between molecules of different substances, and cohesion refers to the attraction between molecules of the same substance. Both properties play a crucial role in determining whether a liquid will wet a solid surface or not.
When water comes into contact with glass, the adhesive forces between the water molecules and the glass molecules are stronger than the cohesive forces between the water molecules themselves. This makes the water molecules spread out and form a thin, continuous film on the glass surface, rather than bead up in isolated droplets. This phenomenon is known as wetting.
In contrast, mercury has stronger cohesive forces between its own molecules compared to the adhesive forces between mercury molecules and glass molecules. As a result, the cohesive forces cause mercury to bead up into spherical droplets rather than spreading out and wetting the glass surface.
The difference in wetting behavior between water and mercury is mainly due to the intermolecular forces at play. Water molecules have polar covalent bonds, resulting in a slightly positive charge on the hydrogen atom and a slightly negative charge on the oxygen atom. This polarity allows water molecules to form hydrogen bonds with other water molecules and interact strongly with the glass surface, leading to wetting.
On the other hand, mercury consists of metal atoms packed closely together, which results in metallic bonding. These bonds are strong within the mercury itself but relatively weak in their interaction with glass molecules. Therefore, mercury droplets do not spread out but tend to minimize their contact area with the glass surface, leading to non-wetting behavior.