Heavy water is so called because it

Heavy water, chemically known as deuterium oxide (D2O), is so called because it contains the heavier isotope of hydrogen, known as deuterium [t1][t2]. While ordinary hydrogen (protium) consists of only one proton, deuterium contains both a proton and a neutron in its nucleus, making it approximately twice as heavy as ordinary hydrogen [t3]. Consequently, a molecule of D2O is roughly 10% heavier than a standard H2O molecule [t3]. This increased mass results in higher density and different physical properties compared to normal water [t3]. Although seawater contains dissolved salts [c1][c3] and mercury contamination is a known environmental pollutant [t4][t7], these factors do not define 'heavy water'. Similarly, saline water used in medical contexts is simply a solution of sodium chloride in water and is not related to isotopic composition [c3].

Sources

1. [1] https://hwb.gov.in/frequently-asked-questions-faqs
2. [2] https://pubs.usgs.gov/gip/7000057/report.pdf
3. [3] FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.) > Chapter 12: Water (Oceans) > SALINITY OF OCEAN WATERS > p. 104
4. [4] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 12: The Oceans > Salinity of the Ocean > p. 106
5. [5] https://www.epa.gov/sites/default/files/2019-03/documents/ambient-wqc-mercury-1980.pdf
6. [6] https://pmc.ncbi.nlm.nih.gov/articles/PMC7731724/