Retrograde (inverted) solubility:\nWhich sodium salt listed typically exhibits an inverted solubility curve, i.e., solubility increases as temperature decreases (over a certain range)?

Introduction / Context:
Most salts become more soluble as temperature increases. However, some hydrates exhibit retrograde (inverted) solubility over a temperature interval, where solubility increases as temperature decreases. Recognizing such exceptions is important in crystallization process design and in troubleshooting unexpected precipitation on cooling or heating.

Given Data / Assumptions:

• Hydrated salts can change stable hydrate form across temperatures.
• We focus on qualitative trend over a practical temperature range.
• Industrial relevance: solubility-driven separation and scaling control.

Concept / Approach:
Retrograde solubility often arises from hydrate stability changes and dissolution enthalpy effects. Sodium carbonate monohydrate can display an inverted solubility region due to phase equilibria among hydrates (e.g., monohydrate versus decahydrate), influencing how much solid dissolves when the temperature changes.

Step-by-Step Solution:
Identify candidate salts: NaCl has near-normal slight increase; Na2S2O3 shows strong normal increase with T.Hydrated carbonates/bisulfites may show complex behavior due to hydrate equilibria.Sodium carbonate (monohydrate) is known to show retrograde behavior in certain ranges, matching the description.

Verification / Alternative check:
Solubility charts used in crystallization references depict non-monotonic/retrograde regions for certain hydrates, including sodium carbonate forms.

Why Other Options Are Wrong:
NaCl does not exhibit inverted behavior; Na2S2O3 increases markedly with T; sodium bisulfite is unstable and not a standard example for retrograde solubility in design texts; acetate shows normal trends.

Common Pitfalls:
Assuming all salts behave monotonically; ignoring hydrate transitions can mislead crystallizer operation.

Final Answer:
Sodium carbonate (monohydrate)