Types Of Chemical Reactions

1. Analyze the Hypothesis: The core idea is that large deposits of NaCl and CaCO₃ originated from ancient oceans, concentrated through processes involving water (weathering by rain, leaching by rivers). This implies these substances were transported by water.
2. Evaluate Solubility:
   • NaCl (Sodium Chloride): Rock salt is highly soluble in water. Its presence in oceans and formation of large deposits upon evaporation is well-understood.
   • CaCO₃ (Calcium Carbonate): Chalk, limestone, and marble are primarily CaCO₃. Calcium carbonate has very low solubility in pure, neutral water.
3. Evaluate Option (a): "Both NaCl and CaCO3 are highly soluble in water." This is false because CaCO₃ has low solubility in pure water.
4. Evaluate Option (b): "NaCl is soluble in water but CaCO3 is not. Hence, concentration of CaCO3 in the oceans through weathering is an untenable hypothesis." The first part about solubility difference is roughly correct (though "not soluble" is an oversimplification for CaCO₃). However, concluding the hypothesis is "untenable" solely based on low solubility in pure water might be premature, as solubility can be affected by environmental factors.
5. Evaluate Option (c): "The solubility of CaCO3 in water is pH dependent and is enhanced by acidic atmospheric gases. Hence, water, especially under certain conditions (like slightly acidic water), and marine organisms build shells from dissolved components, contributing to CaCO₃ deposits.
6. Conclusion: This statement is misleading and likely FALSE in its conclusion.
7. Evaluate Statement (c): The solubility of CaCO₃ in water is pH dependent and is enhanced by acidic atmospheric gases. Hence, CaCO₃ may be leached into water during weathering.
   • Atmospheric CO₂ dissolves in rainwater, forming carbonic acid (H₂CO₃), making rain naturally slightly acidic.
   • Carbonic acid reacts with CaCO₃ to form calcium bicarbonate (Ca(HCO₃)₂), which is much more soluble in water: .
   • This process explains how CaCO₃ is chemically weathered and transported by slightly acidic surface waters.
   • Conclusion: This statement accurately describes the mechanism allowing CaCO₃ transport and deposition, making it TRUE and supporting the postulate.
8. Evaluate Statement (d): NaCl and CaCO₃ are igneous rocks... Hence, the ocean postulates is baseless.
   • Igneous rocks form from cooled magma/lava (e.g., granite).
   • NaCl forms rock salt (halite), an evaporite sedimentary rock. CaCO₃ forms limestone (sedimentary) or marble (metamorphic). They are not typically igneous.
   • Conclusion: The premise about the rock type is incorrect, making the statement FALSE.
9. Final Choice: Statement (c) provides the correct explanation for how CaCO₃, despite low solubility in pure water, can participate in the geological cycle described in the postulate.

The correct option is (c).