Answer
The combined "open-space" volumes in the ice equal the volume of the ice that extends above the liquid water level, when the ice is floating.
See diagram. Let the black ink represent the ice before melting, with the open spaces in the crystal structure. The black ink also represents the smaller volume of liquid water that is produced, after melting. The same mass is represented.
In the last chapter, we established that when ice melts, the water level doesn’t rise because the ice displaces an amount of liquid water whose weight is equal to its weight. This is shown in the diagram.
Therefore, when it melts, all the ice collapses into the volume that it occupied underwater. In other words, when the ice melts, it caves in. The volume of all the gaps is the volume of the ice that originally extended above the water level.
