Water in a glass tube that contains grease or oil residue...

Name: Problem 51, Chapter 11: Chemistry: Structure and Properties
Uploaded: 2019-11-08T16:36:52-08:00
Duration: 41 s
Channel: Lizabeth Tumminello
Description: Water in a glass tube that contains grease or oil residue displays a flat meniscus (the tube on the left in the accompanying photo), whereas water in a clean glass tube displays a concave meniscus (the tube on the right). Explain this observation.

Water in a glass tube that contains grease or oil residue displays a flat meniscus (the tube on the left in the accompanying photo), whereas water in a clean glass tube displays a concave meniscus (the tube on the right). Explain this observation.

Key Concepts

Wetting

Wetting describes how a liquid spreads across or adheres to a surface. A clean, hydrophilic surface, like clean glass, attracts water molecules due to strong adhesive forces, causing the liquid to climb slightly along the walls and form the characteristic concave meniscus. This behavior contrasts with surfaces that have been altered by contaminants, which may reduce wetting.

Adhesion and Cohesion

Adhesion is the attraction between different substances, such as water and glass, while cohesion refers to the attraction between like molecules within the water. In a clean glass tube, strong adhesive forces between water molecules and the glass overcome the cohesive forces within the water, pulling the water upward along the glass and creating a curved, concave meniscus. When the glass surface is contaminated, the reduced adhesive interaction limits this effect.

Surface Tension and Contact Angle

Surface tension, a result of cohesive forces among liquid molecules, causes the liquid surface to behave like an elastic membrane. The contact angle, defined as the angle at which a liquid interface meets a solid surface, is influenced by the surface’s cleanliness. A hydrophilic surface yields a low contact angle and a concave meniscus, while contamination increases the contact angle, potentially flattening the meniscus.

Surface Contamination

Surface contamination, such as residual grease or oil, alters the intrinsic properties of the glass by reducing its hydrophilic nature. This change diminishes the adhesive forces between the water and the glass, preventing the water from rising along the walls and resulting in a flat or less curved meniscus. The contaminated surface exhibits modified wetting behavior compared to a clean surface.

Transcript

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