When a concentrated sodium chloride solution is electrolyzed using carbon electrodes, the primary product formed is chlorine gas at the anode (positive electrode), while hydrogen gas is produced at the cathode (negative electrode). 

• At the anode: Chloride ions (Cl-) are oxidized to form chlorine gas (Cl\(_2\)). 
• At the cathode: Water molecules are reduced to form hydrogen gas (H\(_2\)) and hydroxide ions (OH-). 
• The overall reaction produces sodium hydroxide solution alongside the chlorine and hydrogen gases.