When a red blood cell is placed in pure water, it undergoes a process known as osmosis. Osmosis is the movement of water across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration. In this case, pure water has a lower concentration of solutes (like salts) compared to the interior of the red blood cell.
As a result, water enters the red blood cell to equalize the solute concentration on both sides of the cell membrane. This influx of water causes the red blood cell to swell. If too much water enters, the cell can eventually burst, a phenomenon known as hemolysis. This is because the membrane of the red blood cell cannot withstand the pressure created by the excessive amount of water. Therefore, placing a red blood cell in pure water leads to it swelling and potentially bursting due to osmotic pressure.