Question

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

Short answer

The water doesn't overflow immediately when a glass is overfilled due to surface tension, a property of water that causes its surface to shrink to the smallest possible area. This results in a dome-like bulge of water above the rim of the glass instead of immediate overflowing.

Step-by-step solution

Identifying the phenomenon

This is a phenomenon that can be observed in the kitchen when filling a glass with water slightly above the rim; the water does not overflow immediately. Instead, it forms a sort of 'bulge' or 'dome' above the rim of the glass.

Explaining Surface Tension

The reason this happens is due to a property of liquids called surface tension. Surface tension can be thought of as the tendency of liquid surfaces to shrink into the minimum surface area possible. This is caused by cohesive forces between similar liquid molecules. In water, these forces are especially strong due to hydrogen bonding between water molecules.

Surface tension and the glass rim

When a glass of water is filled to the rim, the cohesive forces between the water molecules prevent them from spilling over the edge immediately. Instead, these forces pull the water molecules together, creating a dome-like shape above the rim. This dome or bulge is simply the water molecules attempting to shrink into the minimum surface area, as is their tendency due to surface tension.

Reiterating the main point

The water doesn't overflow immediately due to the surface tension among the water molecules. By sticking together, they form a bulge above the rim of the glass instead of spilling over.