normal saline or 0.9% saline contains Na, Cl and water molecules which when a RBC is placed into this solution (isotonic) this means there is equal amounts of water distribution going into and out of the cells concurrently indicating no change in size or shape of the RBC.
However when a RBC is placed into water is bursts because the RBC inside has less water then the outside and water wants to follow its concentration gradient and thus moves from an area of high concentration to an area of low concentration. This means that water will flow into the RBC (more ions like NA, CL, K etc) then remain outside the cell causing it to swell up and eventually burst.
