Cerous hydroxide, Ce(OH)₃

Cerous hydroxide, Ce(OH)₃, is obtained as a white precipitate by the addition of ammonia or alkali hydroxide to a solution of a cerous salt, or by the action of water on cerium carbide, CeC₂. It may be dried without losing its white colour if air is completely excluded. It cannot, however, be further dehydrated to cerous oxide, for the latter, as it is produced, attacks the water present and oxidises.

Cerous hydroxide, like the other hydroxides of the rare earth elements, is a strong base. It readily oxidises in the air, becoming first violet and finally yellow, owing to its conversion into ceric hydroxide. Oxidising agents effect the change more rapidly. Owing to the ease with which it is oxidised, cerous hydroxide is a strong reducing agent, reducing cupric salts to cuprous, mercuric salts to mercurous, etc.