In natural (“ordinary”) water found on Earth, approximately what percentage of heavy water content (i.e., molecules containing deuterium) is present?

Introduction / Context:
Heavy water refers to water molecules where hydrogen is replaced by deuterium (D), a stable hydrogen isotope with one neutron. Natural water contains a very small fraction of deuterium-bearing molecules such as HDO and D2O. Quantifying this fraction is important for heavy-water production, isotopic analysis, and nuclear technologies that rely on deuterium as a moderator.

Given Data / Assumptions:

• We are concerned with naturally occurring water without enrichment.
• The percentage is asked as a rough, widely used benchmark for deuterium abundance.
• Small variations by geography exist but do not change the order of magnitude.

Concept / Approach:
Deuterium abundance in ocean water is roughly 0.015% (150 parts per million) by atoms, leading to about the same order for heavy water species occurrence when averaged over large bodies of water. This tiny natural concentration is the feedstock for heavy-water plants, which use chemical-exchange or distillation processes to concentrate deuterium.

Step-by-Step Solution:
1) Recognize that deuterium is rare in nature compared to protium (ordinary hydrogen).2) Recall the canonical natural abundance used in textbooks: ~0.015%.3) Match the closest choice to the standard figure.

Verification / Alternative check:
Environmental and paleoclimate studies routinely use δD measurements that pivot around this small baseline abundance; industrial heavy-water plants scale processes to enrich from roughly 0.015% up to reactor-grade concentrations (> 99% D2O).

Why Other Options Are Wrong:
7.54 and 32.97 percent are far too large, inconsistent with observed isotope distributions.0.71 percent corresponds to unrelated natural abundances (e.g., U-235 in natural uranium) and is not applicable to deuterium in water.

Common Pitfalls:
Confusing atomic percent deuterium with molecular percent D2O; overlooking that most “heavy” molecules in natural water are HDO rather than pure D2O.

Final Answer:
0.015