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10 grams pure deuterium oxide (D2O, heavy water) in glass vial
Deuterium is a rare isotope of hydrogen (making up 0.015% of all hydrogen) that, unlike regular hydrogen, has a neutron. This neutron doubles its mass and gives it slightly different chemical properties. Full D2O molecules (as opposed to DHO molecules) make up about 0.0000023% of all water molecules.
Deuterium oxide, or heavy water, is about 11% heavier than regular water. It also has a higher melting point and higher boiling point. It is nontoxic, though it cannot be taken as a replacement for regular water. Studies have shown that a body water mass of about 50% deuterium oxide is fatal. Deuterium oxide is used in some nuclear reactors as a neutron moderator. Regular water and some other materials can also be used for this purpose.