what is flux? in chem

Flux is the substance added during the extraction of metals.On heating it reacts with the earthly impurities(gangue) to form fusible (molten) mass known as slag. They may be acidic or basic in nature.

Acidic flux  are is used to remove the  basic  impurities

For example silica.

Basic fluxes are used to remove acidic impurities

For example -Lime.