That cannot be dissolved: insoluble matter. (adjective)
Difficult or impossible to solve or explain; insolvable: insoluble riddles. (adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "insoluble" in a sentence
"The insoluble form or gun-cotton is entirely _insoluble_ in nitro-glycerine."
"Those which we call insoluble generally differ from the rest only in degree."
"For transparent colored bottles, instead of sponge, the perfumers use what they call insoluble crystal salts (sulphate of potass)."