What will happen to a red blood cell placed in a solution of 99.3% water and 0.7% salt?

Because red blood cells are isotonic in a solution of 0.9% salt, if you reduce the salt levels, the water will be greater and the cell will swell, causing it to become hypotonic.

Since the concentration of water is higher than that of the cell, water will move into the cell by osmosis, and so the cell will swell up with water, and maybe even burst.

This is known as a hypotonic solution, where the concentration of solute in the cell is higher than that of the solution.

When a red blood cell is placed in a solution of 99.3% water and 0.7% salt, water will move from the solution into the red blood cell through the process of osmosis. This movement of water occurs because the red blood cell has a higher concentration of solutes (such as ions and proteins) compared to the surrounding solution. As water moves into the red blood cell, it will cause the cell to swell and potentially burst if the influx of water is too rapid or excessive. This process is known as hemolysis. Therefore, placing a red blood cell in such a solution may lead to swelling and potential rupture of the cell.