Question about saline

What does your instructor say about that? What does your textbook say? What does your nursing drug book say? What have you done to answer your question before coming here? Are you aware that actually trying to answer your own questions with the resources you already have will (hopefully!) not only answer your question but you'll learn other stuff along the way just by looking?

I get the impression that OP believes he/she IS looking when the question was posted. However, we do not generally do homework on this site. Saline will dilute the medication. The OP needs to research why that is important.

Want to know why? It's cheaper to buy the two items in bulk separately than to buy the premixed vials. I have only seen the premixed vials on a fire department ambulance that was well funded.

The OP needs to figure out why NS and not just sterile water, too. Then she'll have a good idea of why.

And yes, OP, going to the internet and hitting "send" to get the answer is not, well, the answer. What did you find out in your books, by thinking about the problem and its components, that wasn't clear to you? That we'll be happy to help you with. But we don't answer people's homework problems for them without knowing they did more to help themselves than just go right to the "send" button. Learning to think out problems like this is a very important part of your nursing education, because you will have similar situations and questions (at a higher level as years go by) all your working life.

So, OP... tell us what (else) you did to figure out the answer, and we'll take it from there.