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Semiheavy water
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<p>Semiheavy water is the result of replacing one of the <a href="/facts/Protium/mfDCg6KH">protium</a> (normal hydrogen, 1H) in normal water with <a href="/facts/Deuterium/Bv4wfCkw">deuterium</a> (2H; or D). It exists whenever there is water with 1H and 2H in the mix. This is because hydrogen atoms (1,2H) are rapidly exchanged between water molecules. Water with 50% 1H and 50% 2H, is about 50% H2HO and 25% each of H2O and 2H2O, in <a href="/facts/Dynamic_equilibrium/jKq8wjyu">dynamic equilibrium</a>. In normal water, about 1 molecule in 3,200 is HDO (1H2HO) (one hydrogen in 6,400 is 2H). By comparison, <a href="/facts/Heavy_water/3dbSrTw5">heavy water</a> D2O or 2H2O occurs at a proportion of about 1 molecule in 41 million (i.e., 1 in 6,4002). This makes semiheavy water far more common than "normal" heavy water. </p><p>The freezing point of semiheavy water is close to the freezing point of heavy water at 3.81°C compared to the 3.82°C of heavy water. </p>