hi. i'm having a hard time answering the following questions for my lab report. someone please help?
Well, without the whole experiment, it's hard to give a definitive answer, but we can give you some hints. That said, we don't really need more info, because the real useful answer, has to come from you, and what you know, and observed.
1. why must zinc dust be added gradually Copper Sulfate solution?
Try and think about what happed in the experiment, and what could have gone wrong with too quick an addition. This should be pretty intuitive -- why do we ever gradually add anything, instead of quickly dumping some in. I'm talking about cooking here, or making a bowl of cereal, or sugaring your tea.
2. what is the purpose of testing copper that was previously submerged in zinc sulfate supernate with ammonia solution?
Try and look up some reactions that ammonia undergoes. Look for copper and zinc, metal and ionic salts, specifically.
3. why is it important to add HCl to the precipitate (Cu) which was previously submerged in zinc sulfate supernate?
Really depends on the particular experiment. But like all these questions, what's in your mind is more important than the absolutely "correct" answer.
4. why is it not advisable to dry copper directly over the bunsen flame?
Again, why do we avoid, in situations other than beginning chemistry labs, just putting things over open flame?