NOVELTY - Apparatus comprises: a hydroxy-producing cathode; an acid-producing anode; a direct current (DC) electricity (712) source connected to cathode and anode; a vessel or container into which water solution (710) is added and anode and cathode are submerged; metal carbonate mass containing manufactured, waste or natural form of metal carbonate, e.g. calcium carbonate or magnesium carbonate in form of aggregate, powder or particles; hydroxy ions and acid; metal ions; and carbonic acid or carbon dioxide and water.    USE - The apparatus is useful for forming metal hydroxide, removing acid gas which is carbon dioxide from gas mixture, to produce hydrogen and oxidative gases e.g. oxygen and chlorine which are useful as chemical reactants or feedstocks or used to produce oxidized forms of hydrogen e.g. water and hydrochloric acid and produce electrical, mechanical and/or thermal energy and to produce metal salt e.g. metal carbonate and/or metal bicarbonate for industrial, chemical, manufacturing, agricultural, aquacultural, environmental or other purposes (all claimed), where the metal hydroxide is useful as chemical feed stock, for neutralizing acidity and in elevation of solution pH and production of cement.    ADVANTAGE - The apparatus: provides hydrogen production process which is carbon consumptive or carbon-negative; reduces concentration of other gases dissolved in water and reacting with metal hydroxide; provides metal salt, hydrogen and oxidative gases that can be stored, removed or disposed; and reduces gas emission from electrolyzed water solution.    DETAILED DESCRIPTION - Apparatus comprises: a hydroxy-producing cathode; an acid-producing anode; a direct current electricity (712) source connected to the cathode and anode, where the DC electricity source is capable of producing a DC electric current path between cathode and anode; a vessel or container into which a water solution (710) is added and anode and cathode are at least partially submerged, where the water solution has a concentration to allow the DC electric current to pass in the electric current path between the anode and cathode; a metal carbonate mass that is positioned near or around the anode, where the metal carbonate mass has a manufactured, waste or natural form of metal carbonate, e.g. calcium carbonate or magnesium carbonate as contained limestone or dolomite in form of an aggregate, powder or particles that are fractured, sintered, ground, perforated, drilled out or made porous by other means to the water solution; and hydroxy ions and acid, where the hydroxy ions and the acid are produced by splitting of water by DC electric current in water solution and the hydroxy ions are generated at cathode and acid is generated at anode; metal ions which are derived from acid dissolving at least some of the metal carbonate mass into metal ions and carbonate ions or metal ions in the water solution, and the metal ions travel to the cathode and forms the metal hydroxide; and carbonic acid or carbon dioxide and water or alternatively carbonic acid and carbon dioxide and water, which is produced by the carbonate ions traveling to the anode and reacts with the acid and forms the carbonic acid or the carbon dioxide and water or alternatively forming the carbonic acid and the carbon dioxide and the water. INDEPENDENT CLAIMS are also included for:    (1) forming metal hydroxide comprising: (a) providing a water electrolysis cell that has the anode, cathode, and the source of electricity; (b) immersing the cathode and anode in the water solution whose ion concentration can provide the current path; (c) placing the metal carbonate mass near or around the anode; (d) using the electricity to impress a DC voltage across the anode and cathode to generate acid at the anode and hydroxy ions at cathode and the acid has a strength to dissolve at least some of the metal carbonate mass and consume at least some of the acid; (e) dissolving the metal carbonate into metal and carbonate ions; and (f) allowing metal ions, as derived from the dissolution of the metal carbonate mass or as otherwise contained in the water solution, to travel toward the cathode and form the metal hydroxide, and allowing the carbonate ions to travel toward the anode to form carbonic acid or carbon dioxide and water; and    (2) removing acid gas from a gas mixture (702) comprising: steps (a)-(f) as above per se and contacting and reacting the metal hydroxide with the acid gas contained in the gas mixture, and producing metal salt and reducing the concentration of the acid gas in the gas mixture.    DESCRIPTION OF DRAWING(S) - The figure shows a flow chart illustrating a system.    System (700)    Gas mixture (702)    Hydrogen (704)    Water solution (710)    DC electricity (712)