simply as CaC0 3 (s) + H 2 SOiaq)----+ Ca 2 +(aq) + SO/-(aq) + H 2 0(l) + CO 2 (g) In this equation the calcium sulfate is represented by sep-arate hydrated ions because calcium sulfate is quite water soluble and dissolves in rainwater. Thus, in areas bathed by rainwater, the marble slowly dissolves away. In areas of the building protected from the rain, the cal-cium sulfate can form the mineral gypsum (CaS0 4 . 2H 2 0). The· 2H 20 in the formula of gypsum indicates the presence of two water molecules (called waters of hydration) for each CaS0 4 formula unit in the solid. The smooth surface of the marble is thus replaced by a thin layer of gypsum, a more porous material that binds soot and dust. What can be done to protect limestone and marble structures from this kind of damage? Of course, one ap-proach is to lower sulfur dioxide emissions from power plants (see Fig. 5.33). In addition, scientists are experi-menting with coatings to protect marble from the acidic atmosphere. However,
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