Why does a human lung float when it is in water?

Human lungs float when placed in water due to the presence of air within them. The alveoli, tiny air sacs that make up the substance of the lungs, are filled with air during the process of breathing. When the lungs are removed from the body and placed in water, the air trapped in the alveoli continues to provide buoyancy, causing the lungs to float.

The density of an object determines whether or not it will float on water. Objects that are less dense than water will float, while those more massive than water will sink. The air inside the lungs makes them less dense than water, allowing them to remain afloat.

In contrast, once the lungs are deflated or compressed, the air within the alveoli is released, and the density of the lungs increases, causing them to sink when placed in water.