What is Heavy water?

Normal water is made of two Hydrogen atoms and one Oxygen atom. Hydrogen atom and Oxygen atoms are covalently bonded together. Normal water is designated by H₂O.

Heavy water also has same molecular structure but Hydrogen atoms are replaced by 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, (Relative Molecular Mass = 2). Hence it is 'heavier' than normal hydrogen. This leads to the water being heavier.

What is Boyles Law?

Boyle's Law is a statement of the relationship between the pressure and volume of gasses. Specifically it states that under isothermic conditions, i.e. when temperature remains constant, the product of the pressure and volume remains constant, or

P₁ x V₁ = P₂ x V₂

where P₁ is the pressure before some change, V₁ is the volume before the change, P₂ and V₂ are the new values after the change.