Question

It is postulated that huge deposits of NaCl (rock salt) and CaCO3 (chalk and marble) are sites of erstwhile oceans, where the salts had been concentrated through weathering by rain and wind and leaching by rivers. Select the correct explanatory statement in this context.

• A. Both NaCl and CaCO3 are highly soluble in water.
• B. NaCl is soluble in water but CaCO3 is not. Hence, concentration of CaCO3 in the oceans through weathering is an untenable hypothesis.
• 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.
• D. NaCl and CaCO3 are igneous rocks and have crystallized as such during the slow cooling process when the earth was born. Hence, the ocean postulates is baseless.

Hint:

Remember that the solubility of certain salts, like CaCO3, is affected by the pH of the surrounding environment. Acidic conditions increase the solubility of basic salts like CaCO3.

Answer 1

The Correct Option is C

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: \( \text{CaCO}_3(s) + \text{H}_2\text{CO}_3(aq) \rightleftharpoons \text{Ca(HCO}_3)_2(aq) \).
   • 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).

Answer 2

Explanation: 

During weathering, rocks break down due to rain, wind, and rivers. Minerals and salts are often dissolved and carried away into oceans.

• NaCl (rock salt) is highly soluble in water, so it's easily leached and transported.
• CaCO₃ (calcium carbonate) is sparingly soluble in water, but its solubility increases in the presence of acidic gases like $CO_2$.

Atmospheric $CO_2$ dissolves in rainwater to form carbonic acid, which reacts with $CaCO_3$ to form calcium bicarbonate, which is soluble in water. 

$\text{CaCO}_3 + \text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{Ca(HCO}_3)_2$ 

Thus, both NaCl and CaCO₃ can be transported to oceans and deposited there over time. 

Correct answer: (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.