Solubility versus temperature: For most salts, solubility increases with temperature. Which of the following has solubility that is nearly independent of temperature?

Introduction / Context:
Understanding how solubility varies with temperature is crucial in crystallization, evaporation, and precipitation operations. While many salts show significant solubility increases with temperature (endothermic dissolution), some exhibit weak dependence, and a few show retrograde behavior (solubility decreases at higher T). Recognizing patterns helps in selecting operating temperatures to control crystal size and yield.

Given Data / Assumptions:

• Aqueous solutions under atmospheric pressure.
• Solubilities compared over common process temperatures (e.g., 0–100 °C).
• Qualitative trend recognition rather than exact numeric values.

Concept / Approach:
Sodium chloride (NaCl) is a classic example of a salt whose solubility in water changes only slightly with temperature—its dissolution enthalpy is small, so temperature has little effect. In contrast, potassium nitrate (KNO3) shows a strong positive temperature dependence (used in hot crystallization). Sodium thiosulphate (hypo) and sodium carbonate hydrates exhibit more pronounced changes. Anhydrous sodium sulfate displays complex hydration equilibria (Glauber’s salt formation) that introduce marked temperature effects.

Step-by-Step Solution:

Verification / Alternative check:
Reference solubility curves show NaCl rising from roughly 35–39 g/100 g water between 0 and 100 °C—a modest change compared with KNO3 (about 13 → 240 g/100 g water) or sodium thiosulfate with large increases and hydrates changing stability with T.

Why Other Options Are Wrong:

• Sodium carbonate monohydrate: hydration/dehydration affects solubility markedly with T.
• Anhydrous sodium sulphate: exhibits complex hydrated phases; solubility varies strongly.
• Hypo (sodium thiosulfate): shows a strong positive T dependence.
• Potassium nitrate: textbook case of strong solubility increase with T.

Common Pitfalls:
Assuming all salts follow the same trend; always check specific solubility curves and hydrate formation behavior.

Final Answer:
sodium chloride