Explain why mercury does not wet glass while water does?

Question: Explain why mercury does not wet glass while water does?

The ability of a liquid to wet a solid surface is determined by the balance between cohesive forces within the liquid and adhesive forces between the liquid and the solid. Cohesive forces are the attractive forces between molecules within a liquid, while adhesive forces are the attractive forces between the liquid and the solid surface.

In the case of water wetting glass:

- Water molecules have strong cohesive forces between them, causing them to stick together and form a droplet.

- Water molecules also have relatively strong adhesive forces with the molecules on the glass surface.

- The adhesive forces between water and glass are stronger than the cohesive forces within the water droplet, causing the water to spread out and wet the glass surface.

On the other hand, mercury does not wet glass because:

- Mercury molecules have weaker cohesive forces compared to water molecules, so mercury forms compact, spherical droplets rather than spreading out.

- Mercury has weak adhesive forces with glass. The adhesive forces between mercury and glass are not strong enough to overcome the cohesive forces within the mercury droplet, preventing it from wetting the glass surface.

The contact angle formed between the liquid droplet and the solid surface is a useful way to visualize this phenomenon. For water, the contact angle is small (usually less than 90 degrees), indicating good wetting. In the case of mercury on glass, the contact angle is large (usually more than 90 degrees), indicating poor wetting.

In summary, the differences in the cohesive and adhesive forces between water and mercury play a significant role in determining their wetting behavior on a glass surface. Water wets glass due to stronger adhesive forces, while mercury does not wet glass due to weaker adhesive forces.

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