Heavy Water and Hydrogen Isotopes
Hydrogen unusual three-isotope family (protium, deuterium, tritium) and heavy water (D₂O) behave subtly differently from ordinary hydrogen and water in ways that show up repeatedly in exam questions: which isotope is unstable, how D₂O reacts in place of H₂O, and how it affects solubility.
Identifying the Radioactive Isotope of Hydrogen (Tritium) is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Tritium is the radioactive isotope of hydrogen
Hydrogen has three isotopes: protium (1H₁, the ordinary, overwhelmingly abundant form, stable), deuterium (1H₂, also called heavy hydrogen, stable, found in trace natural abundance), and tritium (1H₃, radioactive, β-emitting, with a half-life of about 12.3 years, present only in trace amounts naturally but producible artificially). Only tritium is radioactive — protium and deuterium are both stable isotopes, so a question asking which hydrogen isotope is radioactive should point specifically to 1H₃, not to deuterium or to both 2 and 3 together.
Heavy water reacts analogously to water, with D replacing H
Heavy water (D₂O) undergoes the same types of reactions as ordinary water, just with deuterium appearing in the products where hydrogen normally would. Calcium carbide reacting with D₂O follows the identical pattern to its reaction with ordinary water (CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂), just with every hydrogen replaced by deuterium: CaC₂ + 2D₂O → C₂D₂ (deuterated acetylene) + Ca(OD)₂. This kind of isotope-labelling question is really just testing whether you can write the ordinary reaction correctly and then substitute D for H throughout, not a fundamentally different reaction.
Ionic compounds are generally slightly less soluble in heavy water
D₂O has subtly different physical properties from H₂O (including weaker or differently-structured hydrogen bonding and a marginally different dielectric behaviour), which generally makes it a slightly poorer solvent for ionic compounds than ordinary water — so an ionic compound dissolved simultaneously in both tends to show LOWER solubility in heavy water than in ordinary water. This is a genuine, measurable isotope effect (not identical solubility in both), and it is one of the reasons D₂O is mildly toxic to biological systems in significant concentration — many biological processes rely on the specific hydrogen-bonding behaviour of ordinary water, which heavy water does not replicate exactly.
OlympIQ exam lens
- ✓Only tritium (1H₃) is radioactive; protium (1H₁) and deuterium (1H₂) are both stable
- ✓Heavy water reacts analogously to ordinary water, with D appearing wherever H normally would in the products
- ✓CaC₂ + 2D₂O gives C₂D₂ (deuterated acetylene) + Ca(OD)₂, the direct D-analogue of the CaC₂ + H₂O reaction
- ✓Ionic compounds are generally LESS soluble in heavy water than in ordinary water — a genuine isotope effect, not identical solubility
Do not assume deuterium is radioactive just because it sounds like an unusual, heavy isotope. Deuterium is completely stable — only tritium, the third and rarest hydrogen isotope, is radioactive.
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