a) Give reasons for the following: i)Rusting of iron is quicker in saline water then in ordinary water. ii)Aluminum metal can't be produced by the electrolysis of aqueous solution of aluminum salt.

(i) In order that rust should be formed iron must go into solution and hydrogen must be given off in the presence of oxygen or certain oxidizing agents. This presumes electrolytic action, as every iron ion that appears at a certain spot demands the disappearance of a hydrogen ion at another, with a consequent formation of gaseous hydrogen. The gaseous hydrogen is rarely visible in the process of rusting, owing to the rather high solubility and great diffusive power of this element. Substances which increase the concentration of hydrogen ions, such as acids and acid salts, stimulate corrosion, while substances which increase the concentration of hydroxyl ions inhibit it.

 

(ii) Alluminium oxides aren't soluble in water to any useful extent.