A glass can be filled slightly above the rim with water. Explain why the water does not overflow

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A glass can be filled slightly above the rim with water....

Key Concepts

Surface Tension

Surface tension is the property of a liquid’s surface that arises due to cohesive forces between the molecules. In the context of the glass filled with water, the molecules at the surface are more strongly attracted to one another than to the surrounding air, creating a sort of 'skin' on the water. This skin can hold the liquid slightly above the edge because it resists external forces, such as gravity, that would otherwise cause the water to spill.

Cohesion

Cohesion refers to the attractive forces between like molecules. In water, these cohesive forces, predominantly due to hydrogen bonding, keep the molecules together. This attraction reinforces the effect of surface tension, allowing the water to form a convex meniscus that can balance the weight of the water held above the rim of the glass.

Meniscus Formation

A meniscus is the curve found at the surface of a liquid in contact with a solid boundary, and its shape is determined by the interplay of cohesive and adhesive forces. In a glass, the water forms a convex meniscus at the top due to the strong cohesive forces, and this curved surface effectively contains the water just above the rim without it spilling over.

Laplace Pressure

Laplace pressure is the pressure difference across the curved surface of a liquid, generated by surface tension. This pressure difference plays a crucial role in supporting the additional weight of the water above the rim. The curvature of the meniscus results in a pressure that is higher on the concave side and lower on the convex side, effectively balancing gravitational forces and preventing the water from overflowing.

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