Normal water is made of two hydrogen atoms and an oxygen atom covalently bonded
together. This gives the famous H20 formula.
Heavy water has exactly the same structure, except the hydrogen atoms are isotopes
of hydrogen called 'Deuterium'. Standard hydrogen has one proton in its nucleus
(Relative Molecular Mass = 1), deuterium has one proton AND one neutron in it
nucleus, (RMM = 2) hence it is 'heavier' than normal hydrogen. This leads to the
water being heavier. Mass of water = 18, mass of heavy water = 20.
Answered by: Simon Hooks, Physics A-Level Student, Gosport, UK
'The strength and weakness of physicists is that we believe in what we can measure. And if we can't measure it, then we say it probably doesn't exist. And that closes us off to an enormous amount of phenomena that we may not be able to measure because they only happened once. For example, the Big Bang. ... That's one reason why they scoffed at higher dimensions for so many years. Now we realize that there's no alternative... '