Seashells compounds

The basic constituent of seashells is calcium carbonate, an insoluble compound formed from calcium ions secreted from the cells of the shellfish and carbonate ions present in seawater. But calcium carbonate is a white solid. The colors of seashells often arise from impurities and metabolic waste products captured in the solid shell as it is formed. Coloration is dictated by both diet and water habitat. For example, some cowries that live and feed on soft corals take on the hue of the coral species. Yellow and red colors often arise from carotenoid pigments such as //-carotene. Light refraction often generates the iridescent mother-of-pearl hues. 

Some of the important compounds containing the group IA and IIA metals are the carbonates, nitrates, sulfates, and phosphates. We have already mentioned the mineral trona as the source of sodium carbonate. Calcium carbonate is found in many forms that include chalk, calcite, aragonite, and marble, as well as in egg shells, coral, and seashells. In addition to its use as a building material, calcium phosphate is converted into fertilizers in enormous quantities (see Chapter 14). 

Some of the Group IA and IIA metals are found in nature in the form of carbonates, silicates, nitrates, and phosphates. For example, calcium carbonate is one of the most important naturally occurring compounds, and it is found in several forms. The most common form of calcium carbonate is limestone, which is used extensively as a building stone as well as the source of lime. Other forms include chalk, calcite, aragonite, Iceland spar, marble, and onyx. Many other materials such as egg shells, coral, pearls, and seashells are composed predominantly of calcium carbonate. Thus, it is one of the most widely occurring compounds in nature. 

As another illustration (Figure 1-9), pure calcium carbonate (a white solid present in limestone and seashells) can be broken down by heating to give another white solid (call it A) and a gas (call it B) in the mass ratio 56.0 44.0. This observation tells us that calcium carbonate is a compound. The white solid A obtained from calcium carbonate can be further broken down into a solid and a gas in a definite ratio by mass, 71.5 28.5. But neither of these can be further decomposed, so they must be elements. The gas is identical to the oxygen obtained from the electrolysis of water the solid is a metallic element called calcium. Similarly, the gas B, originally obtained from calcium carbonate, can be decomposed into two elements, carbon and oxygen, in a fixed mass ratio, 27.3 72.7. This sequence illustrates that a compound can be broken apart into simpler substances at a fixed mass ratio those simpler substances may be either elements or simpler compounds. 

Seashells, which are formed in very slow precipitation reactions, are mostly calcium carbonate (CaC03), a white compound. Traces of transition metal ions give them color. 

The distribution of isotopic masses, although nearly constant, does vary somewhat depending on the source of the element. For example, the abundance of in atmospheric CO2 is slightly different from that in seashells. The chemical history of a compound can be inferred from small differences in isotope ratios. 

Figure 2.2 The law of definite composition. Calcium carbonate is found naturally in many forms, including marble (fop), coral bottom), chalk, and seashells. The mass percents of its component elements do not change regardless of the compound s source.

The physical macroscopic notion of porosity or pores in a solid and the phenomenon of absorption of a fluid in a porous object are both quite familiar to all of us. Further, wide varieties of natural or synthetic solids, compounds, species, and materials are known to be of porous nature, for example, minerals, wood, cellulose fibers, seashells. 

Consider calcium carbonate, the major compound in seashells, marble, and coral. It is composed of three elements—calcium, carbon, and oxygen. The following results are obtained from a mass analysis of 20.0 g of calcium carbonate ... 

A variety of seashells were analyzed for tin content. Analytical results for these seashells and domestic chicken egg samples are given in Table 119. The average total tin content was between l-2ng g l and methylated tin compounds were detected in the seashells. 

The higher concentration of tin in the seashells relative to the water in which they were found would indicate the presence of a bioaccumulation process. Also, during the analyses of the sea shells, several other small unidentified signals were noted to follow the methylated tin compounds. Identification of these compounds was not done. 

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