Which statement correctly describes chemical sedimentary rocks?

• A. They precipitate from solution due to changing physical or chemical conditions
• B. They are formed from lithified organic material
• C. They form exclusively from organic remains
• D. They are formed by metamorphism of sandstone

Answer: (A)

Chemical sedimentary rocks form when minerals crystallize directly from water as dissolved ions become oversaturated, often due to evaporation or changes in temperature, pressure, or chemistry of the water. This precipitation creates rocks like halite and gypsum, or silica-rich cherts and calcite deposits in caves. The statement describing them as precipitating from solution because of changing physical or chemical conditions captures that process perfectly. Other options describe rocks formed from organic material or through metamorphism, which are not how chemical sedimentary rocks form: organic remains build organic or biogenic rocks, and metamorphism alters existing rocks into metamorphic forms rather than creating chemical precipitates.