Hobbies And Interests
Chemical Composition
Rock salt is sodium chloride (NaCl); limestone is calcium carbonate (CaCO3). The two minerals have no chemical similarity. The two elements contained in rock salt, sodium and chlorine in the form of chloride ions, form an ionic bond. In limestone, calcium forms an ionic bond with a carbonate group (CO3); the carbonate group contains covalent bonds.
Solubility and Reactivity
Both limestone and rock salt dissolve in water. However, salt dissolves readily and rapidly; limestone dissolves only over time. Cave formations of limestone build up over millennia as calcium carbonate precipitates out of ground water. Salt's dissolution in water takes seconds rather than years or centuries. As readily soluble in water as salt is, it does not react to acids. Limestone reacts strongly to acids, which is why limestone building materials deteriorate more rapidly in an acidic environment.
Formation
Both limestone and rock salt come from mines, but these deposits' origins are quite different. Rock salt deposits typically result from the slow evaporation of brackish inland seas. The Dead Sea's high salinity marks it as a present-day version of a future salt deposit. Limestone can come from ancient seas as well, but it comes from calcium-rich reefs, not from the seawater itself. Crushed calcium carbonate shells from sea creatures slowly compact to form another type of limestone that geologists call coquina; fossilized shells still remain visible in this friable stone.
Commercial Applications
Limestone makes an attractive and versatile building material, whereas rock salt is far too fragile and water-soluble to use for construction. Salt's neutrality to acids suits it to industrial applications that require a nonreactive substance. Porous varieties of limestone filter liquids of impurities, something that highly soluble and nonporous rock salt could not do. Rock salt melts snow and ice on roads in northern climates, while limestone gravel improves traction on those roads.