Cobalt(II) chloride
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Cobalt chloride refers to inorganic compounds with the formula CoCl2·n. These compounds include anhydrous CoCl2 as well as several hydrates. The anhydrous form is a blue solid; the dihydrate is purple
Cobaltous chloride
Cobaltous chloride anhydrous
The deep blue colour of this moisture indicating silica gel is due to cobalt chloride. When hydrated the colour changes to a light pink/purple.
Desiccant
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A desiccant is a hygroscopic substance that is used to induce or sustain a state of dryness in its vicinity; it is the opposite of a humectant. Commonly encountered pre-packaged desiccants are solids
Cobalt(II) chloride
…on the substrate and concentration. It is impregnated into paper to make test strips for detecting moisture in solutions, or more slowly, in air/gas. Desiccants such as silica gel can incorporate cobalt chloride to indicate when it is "spent" (i.e. hydrated). Cobalt is essential for most higher forms of life…
Canisters are commonly filled with silica gel and other molecular sieves used as desiccant in drug containers to keep contents dry.
Silica gel in a sachet or porous packet
Toluene is heated under reflux with sodium and benzophenone to produce dry, oxygen-free toluene. The toluene is dry and oxygen free when the intense blue coloration from the benzophenone ketyl radical is observed.