Q.Which of the following statements is incorrect ?

Castner's process extracts sodium by electrolysing molten (fused) sodium hydroxide, distinct from Down's process which electrolyses fused sodium chloride.

Sodium metal is industrially extracted by electrolysis of one of its fused (molten) salts, since sodium is too reactive to be obtained by chemical reduction:

• Down's process: electrolyses fused sodium chloride (mixed with CaCl2 to lower the melting point) in a Down's cell. Sodium is liberated at the cathode and chlorine gas at the anode. This is the modern industrial method.

• Castner's process: electrolyses fused (molten) sodium hydroxide (NaOH) in an iron vessel that itself serves as the cathode, with a nickel or iron gauze anode kept separated from the cathode compartment by a wire gauze diaphragm (to prevent the sodium formed from reacting back with the oxygen/water produced at the anode). Sodium metal collects at the cathode; oxygen and water are produced at the anode.

The other two options are unrelated to sodium extraction: Ostwald's process is used industrially for the manufacture of nitric acid (catalytic oxidation of ammonia), and the cyanide process is used for extracting gold and silver from their ores (leaching with dilute NaCN/KCN solution).

Spodumene is a lithium aluminium silicate mineral, LiAlSi2O6, and is a major commercial source of the alkali metal lithium.

Among the naturally occurring minerals of the alkali metals:

• Lithium occurs in minerals such as spodumene (LiAlSi2O6), lepidolite, and petalite.
• Sodium's common minerals are rock salt (NaCl), Chile saltpetre (NaNO3), and borax.
• Potassium's common minerals are carnallite (KCl·MgCl2·6H2O) and sylvite (KCl).
• Rubidium occurs as a minor associate in lepidolite and carnallite, not as spodumene.

Spodumene is specifically and uniquely associated with lithium among these options.