Insoluble describes something that cannot be dissolved or solved. The word works in two different ways:

When talking about substances, insoluble means it won't dissolve in a liquid. Sugar is soluble in water: stir it in and it disappears. But sand is insoluble: no matter how long you stir, those tiny grains stay solid. Oil is insoluble in water, which is why oil floats on top instead of mixing in. Scientists need to know which substances are soluble and which are insoluble when they're working with chemicals or designing experiments.

When talking about problems, insoluble means impossible to solve or figure out. An insoluble mystery has no clear answer no matter how hard detectives investigate. An insoluble conflict between two friends might seem impossible to resolve. Sometimes math problems that stumped people for centuries were called insoluble until someone finally cracked them.

The opposite of insoluble is soluble: something that can dissolve or can be solved.