In nuclear and physical chemistry, heavy water used as a moderator in some reactors is chemically known as which of the following?

Introduction / Context:
Heavy water is an important substance in nuclear technology and physical chemistry. It is used as a moderator in certain types of nuclear reactors to slow down neutrons. Although it looks like ordinary water, its molecules contain heavier isotopes of hydrogen. This question asks for the correct chemical name of heavy water.

Given Data / Assumptions:

• The term heavy water is given without formula.

• The options mention neutral water, rainwater, deuterium oxide, tritium oxide and distilled water.

• The learner is expected to know the difference between ordinary hydrogen and its isotopes.

• Basic nuclear chemistry concepts are assumed.

Concept / Approach:
Ordinary water has the chemical formula H₂O, where H represents the common isotope of hydrogen called protium. Heavy water, however, contains deuterium atoms in place of ordinary hydrogen. Deuterium is a hydrogen isotope with one proton and one neutron in its nucleus. When each hydrogen in water is replaced by deuterium, the resulting compound is deuterium oxide, written as D₂O. This substance has slightly different physical properties and is used in nuclear reactors as a moderator.

Step-by-Step Solution:
Step 1: Recall that heavy water is not simply pure or neutral water, but water containing a heavier isotope of hydrogen. Step 2: Identify deuterium as the hydrogen isotope that makes water heavier when it replaces normal hydrogen. Step 3: Understand that heavy water therefore has the formula D₂O, which is called deuterium oxide. Step 4: Examine the options and locate deuterium oxide among them. Step 5: Choose deuterium oxide as the correct chemical name for heavy water.

Verification / Alternative check:
Standard chemistry texts on atomic structure and isotopes explicitly define heavy water as deuterium oxide, D₂O. Nuclear technology references also refer to heavy water reactors, describing how D₂O is used as a moderator. These sources clearly differentiate heavy water from distilled water or rainwater, which are still ordinary H₂O with very low impurities but not isotopically different. This repeated emphasis confirms that deuterium oxide is the correct answer.

Why Other Options Are Wrong:
Neutral water with pH 7 describes ordinary pure water in terms of acidity, not isotopic composition. Rainwater is natural water that may contain dissolved gases and impurities and is still mostly H₂O. Tritium oxide would involve the radioactive isotope tritium and is not the standard industrial heavy water used as a moderator. Distilled water is water that has been boiled and condensed to remove impurities; it remains H₂O. None of these terms corresponds to the special isotopic composition that defines heavy water.

Common Pitfalls:
Learners may confuse heavy water with distilled or very pure water because the word heavy sounds like it might mean strong or pure. Another pitfall is to think that any uncommon name such as tritium oxide might refer to heavy water. To avoid these errors, it is crucial to link heavy water specifically with deuterium and to remember the formula D₂O and the name deuterium oxide.

Final Answer:
Heavy water is chemically known as deuterium oxide.