insoluble
🇺🇸
/ɪnˈsɑːljəbl/
🇬🇧
/ɪnˈsɒljəbl/
Unable to be solved or understood.
The mystery of the missing necklace was insoluble for a long time.
Not able to be dissolved in a liquid.
The oil and water mixture was insoluble, so they remained separate.