Dear Mr. Bakr,

My first reaction is taken from a practical point of view. Depending on the crystal growth rate (dependent on initial concentration) you might have unintentional contaminants within the crystal (enclosures) that affect crystal structure and light absorbance/scatter.

Regards,

Marius

Normally one does not expect any major difference in the solid material obtained from a 0.25 M or a 0.5 M solution of a subtance X (in this case CuSO4.5H2O) because the same substance is expected to be formed from the solution when the solvent evaporates.  The differing spectra  can possibly be attributed to the way the spectra were recorded.  Was any reference material used for measuring absorbance? Normally for optical spectral studies done (especially in solution) in a double beam UV-Vis instrument one takes the solvent (water) as the reference and the solution as the (solvent + sample) sample. Did you use a double beam UV-Vis instrument? Did you use a colourless crystal like BaSO4 as reference?   What is the reason for a kink in the spectrum at ~ 350 nm in the o.5M crystal? 

Thanks for your reply @Dr. Srinivasan. We have used double beam spectrometer with air as a reference (empty reference). I have no idea about the kink at 350 nm but maybe it is related to the change in light source between the UV and the visible ranges.

If you use air as reference you are running into serious problems (see link). Also, have you measured identical orientations of the crystal (which is triclinic...)?

Article Employing Theories Far beyond Their Limits-The Case of the (...

Thanks for your reply Dr. Thomas Mayerhöfer ... What kind of reference should I use and why? .. Identical orientations were used in the measurements ...

 An aqueous solution of copper sulfate which contains hydrated Cu(II) ions in solution is expected to absorb  weakly in the red end of the visible spectrum which can be assigned for the d-d transition of the d9 copper(II) ion.  Comapred to this, the spectrum of the solid is quite different. 

There is no proper kind of reference. The only way to calculate absorbance in this case from transmittance data is presented e.g. in the reference I gave.