Flux is a term used in chemistry to describe a substance that is used to promote the fusion of other substances and make them more soluble in water and acids. Fluxes can have multiple functions, such as cleaning, flowing, or purifying agents, and are used in metallurgy, extractive processes, and high-temperature metal joining processes such as welding, brazing, and soldering). Fluxes can dissolve oxides already present on the metal surface, which facilitates wetting by molten metal, and act as an oxygen barrier by coating the hot surface, preventing its oxidation). In soldering metals, flux serves a threefold purpose: it removes any oxidized metal from the surfaces to be soldered, seals out air thus preventing further oxidation, and by facilitating amalgamation, improves wetting characteristics of the liquid solder).

In electronics, flux is used to remove oxidation, aid in heat transfer, clean and prepare the joint to accept the solder, and promote even solder flow. Electronics solder usually comes with a built-in flux core, which can be made of different substances such as zinc chloride and/or ammonium chloride, or rosin. The flux core breaks up the oxide layers on the metal surfaces, but the exact chemical reaction and products depend on the type of flux used.

It is important to note that flux is different from slag, which is a byproduct of metallurgical processes that contains impurities that have been removed from the metal.

In summary, flux is a substance used in chemistry to promote the fusion of other substances and make them more soluble in water and acids. Fluxes can have multiple functions, such as cleaning, flowing, or purifying agents, and are used in metallurgy, extractive processes, and high-temperature metal joining processes such as welding, brazing, and soldering. In electronics, flux is used to remove oxidation, aid in heat transfer, clean and prepare the joint to accept the solder, and promote even solder flow.