What is an insoluble substance?

Sand (quartz) in water is an example of an insoluble substance; therefore, sand can be separated from the water by filtration or sedimentation. On the other hand, soluble substances can be silicon fluoride; when sand (SiO2) reacts with hydrofloric (HF) acid, the following chemical reaction occurs: SiO2 + HF = Si(F)2 + H2O. Based on this chemical reaction, the silicon fluoride product, Si(F)2, is soluble in the hydrofloric acid media. In other words, soluble substances form one phase with the dissolving media (chemical), and therefore cannot be separated from the liquid solution. Many thanks.

A substance is said to be "insoluble" if it is not soluble in water.

Sand, fats, wood, metals, and plastic are a few examples.

They refuse to dissolve when we try to mix them in water.

An insoluble substance is a substance that does not dissolve in a particular solvent, typically water, to a significant extent at a given temperature and pressure. This means that the substance remains as solid particles suspended in the solvent rather than forming a homogeneous mixture.