Question: In a drop of water (H2O), do all water molecules have the same mass?
Most water molecules have one oxygen atom, with an atomic mass of 16 atomic mass units (u), covalently bonded to two hydrogen atoms with atomic masses of 1 u each. These water molecules have a mass of 16 + 1 + 1 = 18 u.
However, some molecules have isotopes of a slightly different mass. A hydrogen atom, for example, can have a mass of 2 u if it is a 2H (deuterium) isotope. If a water molecule has one 2H isotope, it would have a mass of 19 u. With two 2H isotopes or one heavy oxygen isotope (18O), its mass would be 20 u.
Not all molecules of water have exactly the same mass.
Question: Why does the isotopic composition of ice in Antarctica change due to temperature changes elsewhere? What will happen to the proportion of heavy isotopes in the ice in Antarctica if global temperatures rise?
Hint: review the IRMS learning tool if needed.