The antonyms for the word "most dissoluble" include "most solid," "most compact," "most stable," "most permanent," and "most enduring." These words describe materials or substances that are not easily separated or broken down. Items that are solid, compact, stable, permanent, or enduring are likely to last for a long time without changing their form or structure. They are resistant to dissolving, corroding, or eroding, making them highly valuable in various contexts. Unlike "most dissoluble," which denotes how easily something can be dissolved or melted, these opposite words suggest an unchanging state of matter, which is highly desirable in many applications.