Question

A saturated solution of copper (II) sulfate was prepared by adding an excess of
small crystals to a flask of water. The flask which contained the saturated
solution and many small crystals of CuSO4 was sealed and placed in a cupboard
for several months. When it was removed, most of the small crystals were gone
and several large crystals had appeared. However, the concentration of CuSO4
in the solution was unchanged. Explain, describing the events that occur at the
molecular level, how it is possible for the crystals to change without a change in
the concentration of the solution.

Answer 1

The small crystals have just combined and formed a large crystals

Answer 2

@Brair is incorrect. Since the flask was sealed and left alone, there can't be any large-scale motion of crystals without stirring, which doesn't take place. In a saturated solution, you've got both aqueous solute and solid solute sitting at the bottom of the flask. The equilibrium established between the two of them looks like:
\[CuSO_4(s) \rightleftharpoons CuSO_4(aq)\]
Since the solution is saturated, you can't force more solute into solution without pushing some dissolved solute out somewhere else inside the flask. The rate of dissolving must equal the rate of crystallization. That's what equilibrium really means. Not that a reaction has stopped, but the rates of 2 competing reactions are equal. Over time, as small crystals of CuSO4 dissolve, other crystals must be forced out and crystallize onto other crystals inside the flask, making them larger.

Answer 3

Addition of the small crystals disrupt the uniform distribution of the saturated ions in the solution therefore acting as a nucleation site for the dissolved ions to bind to and continuously attract and connect building up to a larger crystal matrix.