Question

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

Short answer

Surface tension, along with cohesion and adhesion, prevents the water from overflowing.

Step-by-step solution

Understand Surface Tension

Surface tension is a property of the surface of a liquid that allows it to resist external force. Water molecules at the surface are attracted to one another, creating a 'skin-like' surface that can hold a small amount of liquid above the rim.

Analyze Cohesion of Water Molecules

Cohesion is the attraction between water molecules due to hydrogen bonding. This cohesive force creates enough tension on the surface of the water to support a slight dome shape above the rim of the glass without spilling.

Observe the Role of Adhesion

Adhesion occurs when water molecules are attracted to different substances, such as the glass. This interaction helps water cling to the edges of the glass, further preventing it from spilling over.

Consider Balance of Forces

The balance between gravity pulling the water down and surface tension along with cohesive and adhesive forces keeping the water in place is why the water doesn't spill. As long as these balancing forces aren't disrupted, the water can stay slightly above the rim.

Key concepts

Cohesion

Cohesion refers to the attraction between molecules of the same substance. For water molecules, this occurs due to hydrogen bonds that form between them. These bonds are relatively weak individually, but when combined, they create a significant cohesive force.
This is why water can form droplets and maintain a dome-like shape when slightly overfilled in a glass. The cohesive forces pull the water molecules towards each other, creating surface tension that resists the downward pull of gravity.
Your water behaves almost like a stretched elastic membrane. This is thanks to cohesion, which holds the molecules tightly together.

Adhesion

Adhesion is the property that makes water molecules stick to different substances. When you pour water into a glass, the water molecules are not only attracted to each other but also to the glass surface.
This attraction helps the water spread across and cling to the glass, utilizing the adhesive force. It's this adhesive property that prevents water from spilling out easily, as the water molecules stick to the surface of the glass, creating a thicker edge of water along the rim.
This effect, coupled with cohesion among the water molecules, allows for a slight overfill without the water pouring over the edge.

Hydrogen Bonding

Hydrogen bonding is responsible for many of water's unique properties, including cohesion and adhesion. A hydrogen bond forms when a hydrogen atom, covalently bonded to a highly electronegative atom like oxygen, experiences attraction to another electronegative atom nearby.
In a water molecule, the hydrogen atoms are positively charged, while the oxygen atom is negatively charged. This polarization creates a scenario where water molecules are naturally drawn to each other, establishing hydrogen bonds.
These bonds are the reason why water forms a 'skin' at its surface, allowing it to resist external forces. So, when water is slightly above the rim of a glass, hydrogen bonding plays a crucial role in supporting this dome of water by interlinking cohesive and adhesive forces.