Hydrogen Heavy water
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Heavy water.
Deuterium is hydrogen where each nucleus has a neutron as well as the standard proton in it. This makes it basically twice as heavy as normal hydrogen. Heavy water is water (H2O) in which the two hydrogens are replaced by two deuterium atoms. The total molecular weight of heavy water is 20 instead of 18 for normal water, so it's really not all that much heavier.
Deuterium is not radioactive, and for most chemical purposes it is a virtually identical stand in for hydrogen, which means you could probably drink this water and suffer no ill effects. You first. (People differ on how toxic heavy water really is, but people have drunk glasses of heavy water without apparent serious ill effects. Animals have been grown on heavy water and when they are about 20-30% deuterated they do start to show serious problems.)

Click the source link for an interesting story about where this sample came from.
Source: Tryggvi Emilsson and Timothy Brumleve
Contributor: Tryggvi Emilsson and Timothy Brumleve
Acquired: 6 September, 2002
Price: Donated
Size: 1.5"
Purity: 20%