Which statement best describes the water molecules in a hydrate? they slide around the ions throughout the crystal lattice. they vibrate but are held in fixed positions in the crystal lattice. they have no motion because they are held in fixed positions in the crystal lattice. they vibrate randomly along the outer edges of the crystal lattice.

Which statement best describes the water molecules in a hydrate?

They slide around the ions throughout the crystal lattice. 
They vibrate but are held in fixed positions in the crystal lattice. 
They have no motion because they are held in fixed positions in the crystal lattice.
They vibrate randomly along the outer edges of the crystal lattice.

The best answer seems to be (1). (2) is invalid because the water does not bond with the ions. (3) violates laws of chemistry, while (4) does not hold true for all substances as sometimes the water of hydration can be "inside" the crystal lattice.