occlusion
/ uh-KLOO-zhun /
Imagine a snowball rolled up so fast that little stones and twigs get sealed inside it, hidden where no brushing can reach. Occlusion is that same trap: a fast-growing crystal closes around bits of the surrounding solution and locks them inside.
Occlusion is a form of coprecipitation in which impurities — foreign ions or pockets of mother liquor — become physically trapped within a crystal as it grows too quickly to exclude them. Unlike surface contamination, occluded impurities are buried inside and cannot simply be washed away.
It matters because occluded material adds to the weighed mass and biases the gravimetric result. Because washing the outside does not help, the cures are different: precipitate slowly so crystals grow tidily, digest the solid so it recrystallizes cleaner, or dissolve and re-precipitate to release the trapped impurities.
When a precipitate forms too fast, droplets of the mother liquor get sealed inside the crystals; on ignition these leave a trapped residue that makes the weighed solid heavier than the pure compound alone.
Impurities buried inside crystals cannot be washed out.
Occlusion is the inside-the-crystal cousin of surface adsorption. Washing removes adsorbed impurities but not occluded ones — only reprecipitation or digestion reliably clears material that is already buried.