Chemical in (s) mixed with chemical in (aq) vs. chemical in (aq) mixed with chemical in (aq) Hopefully the title is comprehensive enough. 

But I am wondering, does it make a difference if you take a powder and mix it with an aqueous solution or take a solution with the very same chemical formula as the powder and mix it with the same aqueous solution?
Short answer is yes. A lot of times you are trying to control the exothermicity of a reaction by doing a slow addition. This is kind of hard with a powder, and if you dissolve it in a lot of solvent, you can add it slower than you would be able to with a powder.

Without the slow addition, the temperature may increase and decrease your yield or selectivity.