Which of the following represents ‘Heavy water’?

Concept

Isotopes are atoms of the same element with the same number of protons but a different number of neutrons, so they differ in mass while behaving the same chemically. Hydrogen has three natural isotopes — protium (¹H, no neutron), deuterium (²H or D, one neutron), and tritium (³H, two neutrons). Replacing the ordinary protium atoms of a water molecule (H₂O) with deuterium atoms gives ‘heavy water’, chemical formula D₂O.

Application

Apply this to the four formulas given:

• H₂O — two protium atoms and one oxygen atom: this is ordinary (‘light’) water, not heavy water.

• H₂O₂ — two hydrogen atoms and two oxygen atoms joined by a peroxide (O–O) bond: this is hydrogen peroxide, a different compound unrelated to isotopic substitution.

• D₂O — two deuterium atoms and one oxygen atom: this isotopic substitution is exactly what defines heavy water.

• H₂O₃ — not a recognised stable hydrogen–oxygen compound; hydrogen and oxygen form only water (one oxygen) and hydrogen peroxide (two oxygens) as stable species.

So D₂O is the formula that represents heavy water.

Cross-check

This is confirmed independently by molar mass: ordinary water (H₂O) has a molar mass of about 18 g/mol, while heavy water (D₂O) has a molar mass of about 20 g/mol, because each deuterium atom carries roughly one extra neutron mass unit compared with protium — consistent with heavy water's higher density (about 1.107 g/cm³ versus about 1.000 g/cm³ for ordinary water at room temperature).