Relating to cobalt in its +2 oxidation state (meaning it has lost two electrons), as opposed to cobaltic which is +3.
From cobalt + the chemical suffix -ous (indicating a lower oxidation state). The -ous ending comes from Latin and traditionally marked the lesser of two oxidation states.
Cobaltous compounds are often pink or red in solution, which is why cobalt chloride is used in those 'magic' color-changing hygiene indicators that turn blue when wet!
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