In nuclear engineering terminology, “heavy water” (D2O) is called “heavy” because it is the oxide of deuterium, the heavy isotope of hydrogen, which makes the molecule heavier than ordinary water (H2O). What is the most accurate reason for the term “heavy”?

Introduction / Context:
Heavy water, written as D2O, appears in many nuclear engineering applications, especially in reactors that use it as a moderator. Learners often mix up why it is called “heavy.” This explanation clarifies the nuclear-chemistry basis for the name and distinguishes it from merely being “denser.”

Given Data / Assumptions:

• D symbolizes deuterium, the hydrogen isotope with one proton and one neutron.
• Heavy water formula: D2O; ordinary water formula: H2O.
• Deuterium roughly doubles the mass contribution of hydrogen in the molecule.

Concept / Approach:
The adjective “heavy” is rooted in isotopic composition, not thickness or viscosity. Because deuterium has an extra neutron relative to protium (normal hydrogen), D2O molecules are more massive than H2O molecules. This isotopic substitution yields a higher molecular weight and slightly higher density, but density is a consequence, not the defining reason for the term.

Step-by-Step Solution:
Identify isotopes: protium (H, mass ≈1) vs. deuterium (D, mass ≈2).Construct molecules: H2O vs. D2O.Compare molecular masses: D2O has greater mass because each hydrogen site is replaced by heavier deuterium.Conclude naming: “heavy” refers to the oxide of heavy hydrogen (deuterium), not to oxygen isotopes or viscosity.

Verification / Alternative check:
Heavy water’s physical properties (e.g., density about 10% higher than H2O at room temperature) are consistent with the isotopic mass increase. However, textbooks define “heavy water” specifically as deuterium oxide, confirming the naming basis is isotopic composition.

Why Other Options Are Wrong:
It is denser: true as a property, but not the naming reason.Heavy, viscous liquid: viscosity change is minor; not the naming origin.Heavier oxygen isotope: heavy water uses normal oxygen-16 unless specified; the “heavy” refers to hydrogen.Dissolved heavy salts: irrelevant to isotopic definition.

Common Pitfalls:
Confusing cause (isotopic substitution) with effect (density increase).Assuming “heavy” implies impurities or high viscosity.

Final Answer:
It is an oxide of deuterium (heavy hydrogen)