Final answer:
The most common solid precipitated by water-dwelling organisms for shell and skeleton building is calcium carbonate (CaCO3), found in mollusk shells and pearls, and produced by marine algae like coccolithophores.
Step-by-step explanation:
The solids most commonly precipitated by water-dwelling organisms in order to build their shells and skeletons are calcium carbonate (CaCO3). This compound is found in the pearls and shells of most mollusks. Similarly, coccolithophores, a type of marine algae, produce distinctive looking calcium carbonate plates. Other organisms, such as diatoms, use silica (SiO2) to build their external skeletons, but it is not as common as calcium carbonate for this purpose. Although chitin and cellulose are also structural components found in various organisms, such as the exoskeletons of crustaceans (chitin) or the cell walls of plants and some algae (cellulose), they are not typically used by water-dwelling organisms for building shells or skeletons in the same way as calcium carbonate.