Examples of insoluble substances are sand, plastic, wood, metal, glass and cloth. These substances never dissolve in water or any other solvent at room temperature and pressure. Sugars and inorganic salts are also examples of insoluble substances. Insoluble substances cannot be extracted from a solution.
Certain liquids such as oil, petrol and diesel are also insoluble in water. Such liquids are called immiscible liquids. Most ionic compounds are soluble in water, while covalent compounds are insoluble in water. Solids like glass and plastic melt when heated to a very high temperature, but they never dissolve in any liquid. The chemical properties of soluble substances are different from those of insoluble substances.