If I put a red blood cell into pure water it will swell and burst, why is this?

Answer 1

Because of osmotic pressure. The cell has electrolytes dissolved in it, fluid (H2O) travels from outside the cell, through the membrane of the cell and into the cell. This continues and the water is TRYING to dilute the solution (technically, its the osmotic pressure doing the pushing or pulling depending upon how you want to look at it) and finally there is enough fluid inside the membrane that the cell wall can't hold it any longer and the cell bursts.
http://en.wikipedia.org/wiki/Osmotic_pressure