Effect of solute on aqueous specific heat:\nAs the concentration of a solute increases in an aqueous solution, how does the specific heat usually change?

Introduction / Context:
Specific heat capacity directly affects energy required for heating and cooling process streams. Water has a very high specific heat, and dissolving solutes typically lowers the mixture’s heat capacity—knowledge that is crucial for evaporators, crystallizers, and heat exchangers.

Given Data / Assumptions:

• Aqueous solutions at moderate concentrations.
• No phase change; temperature range where properties vary smoothly.

Concept / Approach:
Most inorganic salts and many organics reduce the ability of water to store thermal energy per unit mass, leading to a decrease in specific heat with increasing solute concentration. The strong hydrogen-bond network of water contributes to its large C_p; solutes disrupt this structure and replace water with lower-C_p material.

Step-by-Step Solution:
Consider a mass-weighted mixture: C_p,mixture ≈ Σ (mass fraction_i * C_p,i).Since C_p,solute < C_p,water for many common solutes, increasing solute fraction lowers the mixture’s C_p.Therefore, the general trend is a decrease.

Verification / Alternative check:
Handbook data for salt solutions (NaCl, CaCl2, KNO3) show monotonic decreases of C_p with concentration at constant temperature.

Why Other Options Are Wrong:
“Increases” contradicts common data; “unchanged” ignores strong composition dependence; while some unusual systems may deviate slightly, the engineering rule-of-thumb is a decrease.

Common Pitfalls:
Assuming linearity across all ranges; at very high concentrations or near phase changes, behavior can be nonlinear.

Final Answer:
decreases