Cobalt burns in nitric oxide at 150° C. giving the monoxide.
It burns with a purple flame, forming carbon dioxide and nitrogen; and may be condensed (by cooling to - 25° C.) to a colourless liquid, and further to a solid, which melts at - 34.4° C. (M.
This mixture burns with a green flame forming boron trioxide; whilst boron is deposited on passing the gas mixture through a hot tube, or on depressing a cold surface in the gas flame.
The metal is quite permanent in dry air, but in moist air it becomes coated with a superficial layer of the oxide; it burns on heating to redness, forming a brown coloured oxide; and is readily soluble in mineral acids with formation of the corresponding salts.
Burns for about two months, and yields about 200 tons of sulphur.