why are the sulphides of the alkaline earth metals generally more covalent in their properties than are the corresponding oxides?

 
A hydrated colourless solid(A) is water - soluble and finds use in medicine as purgative. When a solution of (A) is treated with ammonium phosphate, a white ppt.is formed.(A) gives a pink mass of cobalt nitrate test. what is A?

please explain how to reach the answer