Question

An ice cube made up of normal water floats in liquid water, while that made up of heavy water sinks in the normal liquid water. This is due to the fact that in heavy water, each molecule of water has two deuterium atoms instead of the two hydrogen atoms. Further, a deuterium atom differs from a hydrogen atom by the only fact that deuterium has one neutron in its nucleus along with one proton, while the hydrogen atom has only one proton in its nucleus.

Considering the fact that an extra neutron in the deuterium nucleus does not increase the volume ofthe heavy water molecule, estimate the percentage difference in densities of normal water and heavy water.

Answer 1

A normal water molecule is H_2​O, while a heavy water molecule is D_2​O.

Mass of H_2​O = 2(1) + 16 = 18 u

Mass of D₂​O = 2(2) + 16 = 20 u

Since the volume remains approximately the same, density is proportional to mass.

`(ρ"heavy")/(ρ"normal​") = 20/18` ​​

Percentage increase in density:

`(20 - 18)/18 xx 100`

= `2/18 xx 100`

= 11.1%

Heavy water is approximately 11.1% denser than normal water.​