Definition
A chemical salt (CoCl₂) used as a humidity indicator because it changes color depending on the amount of moisture it has absorbed. When dry, cobalt chloride is deep blue; as it absorbs moisture, it turns progressively lighter blue, then lavender, and finally pink when fully saturated. It is impregnated into the silica gel desiccant used in aircraft instrument and equipment dehydrator plugs.
Plain English
A blue chemical that turns pink as it soaks up moisture. Mechanics use it to tell at a glance whether the drying agent inside sealed equipment is still working or needs to be replaced.
Context Anchor
Seen in aircraft maintenance and storage, especially on moisture indicator cards, drying plugs, and packaging used to keep parts or instruments dry.
Derivation
Cobalt is the metal in the compound, and chloride means it is combined with chlorine. The useful point for aviation is not the metal itself, but that this cobalt compound visibly reacts to moisture by changing color.
Why Pilots Care
Excess moisture causes corrosion and electrical failures; the color change gives an immediate visual warning before damage occurs.
Grounding Statement
If the indicator has turned pink, treat it as a sign that moisture has reached an area that was meant to stay dry.
Intuition Check
Do not think of cobalt chloride as a cleaner, coating, or fuel additive. In this context, its job is to indicate moisture by changing color.
Example Sentence 1
The dehydrator plug on the stored magneto had turned pink, so the mechanic replaced it with a fresh blue cobalt chloride unit.
Example Sentence 2
After the aircraft sat on the ramp in high humidity, the cobalt chloride indicator had turned pink, prompting a full inspection.