Final answer:
Pumice is the aphanitic igneous rock with the highest silica content and the most dissolved water vapor. It is a frothy-textured volcanic glass that floats on water due to its low density.
Step-by-step explanation:
The aphanitic igneous rock with the highest silica content and the most dissolved water vapor is pumice. Pumice is a frothy-textured volcanic glass with a low density, causing it to float on water.