In the fields of nuclear energy, chemical engineering, and scientific research, the terms heavy water (D₂O) and light water (H₂O) appear frequently. Although they look almost identical in appearance, their physical properties, chemical characteristics, and industrial applications differ significantly. This article provides a comprehensive comparison—covering composition, properties, uses, cost, and safety—to help readers clearly understand the key differences between heavy water and light water. 1. What Is Light Water? Light water refers to ordinary water— H₂O —composed of two hydrogen atoms (¹H) and one oxygen atom. It is one of the most common substances on Earth and is widely used for drinking, industrial cooling, cleaning, and as a general-purpose solvent. 2. What Is Heavy Water? Heavy water ( D₂O ) is a form of water in which the regular hydrogen atoms are replaced by deuterium (²H) , a stable hydrogen isotope containing one proton and one neutron. Because deuter...