insoluble

adj

1. (physical)That cannot be dissolved.

   “Petroleum is largely insoluble in water.”

2. That cannot be solved.

   “The reason Fisher concluded that the problem is likely insoluble is that the ability of real-world agents to act on new perceived opportunities of arbitrage – including those that turn out to be incorrect – makes stability impossible to demonstrate without additional strong (and unrealistic) assumptions.”

3. That cannot be explained.

   “Near-synonyms: unexplained, mysterious; see also Thesaurus:incomprehensible, Thesaurus:mysterious”

4. That cannot be broken down or dispersed.

noun

1. Any substance that cannot be dissolved.

   “As there is a partial vacuum inside the drum, the liquid is sucked inside the drum and the insolubles are deposited on the outer surface of the membrane filter.”