Concentration change on evaporation:\r When an aqueous sugar solution is evaporated (water is removed and sugar is nonvolatile), the molarity of the solution will __________.

Difficulty: Easy

Correct Answer: increase

Explanation:


Introduction:
Molarity is a common concentration unit used in material balances and reactor calculations. Understanding how molarity responds to evaporation is crucial in evaporation, crystallization, and syrup concentration operations in the food and biotech industries.


Given Data / Assumptions:

  • Solute: sugar (nonvolatile under operating conditions).
  • Solvent: water (volatile and removed by evaporation).
  • Sugar does not decompose; no reactions occur.


Concept / Approach:
Molarity M is defined as moles of solute per liter of solution. During evaporation, the number of moles of sugar remains constant while the volume of solution decreases as water is removed. Therefore, the ratio moles/volume increases. This remains true for any starting concentration as long as only solvent is removed and temperature-induced volume changes are negligible compared with solvent loss.


Step-by-Step Solution:
Let initial moles of sugar = n_s (constant during evaporation).Initial volume V1 → final volume V2, where V2 < V1 after evaporating water.M1 = n_s / V1; M2 = n_s / V2.Because V2 < V1, M2 > M1 ⇒ molarity increases.


Verification / Alternative check:
Mass balance on solute confirms constant solute moles. A quick numerical example (e.g., halving the volume doubles molarity) provides an intuitive check.


Why Other Options Are Wrong:

  • Decrease/remain unchanged: Contradict the definition, since volume decreases while solute moles stay fixed.
  • “Either way” or “first decrease then increase”: No such behavior occurs when only solvent is removed and solute is nonvolatile.


Common Pitfalls:
Confusing molarity with molality. Molality increases even more directly since solvent mass decreases; molarity depends on solution volume.


Final Answer:
increase

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