Question

why a hollow aluminium can floats on water?​

Answer 1

Answer:

Aluminum in general has a higher density than water and yes it sinks in it. But in case of hollow aluminum , the density ( mass present per unit volume) is greatly decreased even below that of water since majority of the object's volume is filled with air, so it floats.

Answer 2

When the aluminum ball had a diameter of 6.0 cm, the ball should have floated well because it had a density lower than that of water due to the air inside of the ball, just like steel ships that can float because their density has been lowered by encasing air inside the hull.