Thermophysical property: heavy water (D2O) exhibits its maximum density at approximately what temperature (°C)?

Introduction / Context:
Like ordinary water (H2O), heavy water (D2O) displays an anomalous density behavior with a temperature of maximum density (TMD). Understanding the shifted TMD in D2O is useful for reactor thermal-hydraulics and moderator performance analysis in heavy-water systems.

Given Data / Assumptions:

• Heavy water replaces hydrogen with deuterium, doubling hydrogen’s mass.
• We seek the temperature at which D2O reaches peak density at 1 atm.
• Comparative reference: H2O has TMD near 4 °C.

Concept / Approach:
Isotopic substitution alters vibrational modes and hydrogen bonding, shifting thermophysical properties. For D2O the TMD is higher than for H2O because heavier deuterons modify the balance of thermal expansion and hydrogen-bond network restructuring.

Step-by-Step Solution:
Recall: H2O TMD ≈ 4 °C.For D2O the TMD shifts upward; standard property tables list ≈ 11.6 °C.Select 11.6 °C from the options.

Verification / Alternative check:
Moderator handbooks for CANDU-type reactors and thermophysical databases report D2O density maximum close to 11.6 °C at 1 bar, confirming the value.

Why Other Options Are Wrong:

• 4 °C: value for H2O, not D2O.
• 0 °C: freezing point, not TMD.
• 18.6 °C and 25.0 °C: typical room-temperature vicinity; density is already decreasing with temperature.

Common Pitfalls:
Assuming D2O mirrors all properties of H2O; overlooking isotopic effects on hydrogen bonding.

Final Answer:
11.6