Final answer:
Red blood cells swell and potentially burst in a hypotonic solution and shrink in a hypertonic solution. In an isotonic solution, they retain their normal shape as the net water movement is zero.
Step-by-step explanation:
When red blood cells (RBCs) are placed in solutions of varying osmolarity, they experience changes in volume due to osmosis, which is the movement of water across the cell membrane. In a hypotonic solution, water enters the RBCs, causing them to swell and potentially burst (hemolysis). Conversely, in a hypertonic solution, water leaves the RBCs, leading to cell shrinkage or crenation. An isosmotic or isotonic solution, on the other hand, has the same concentration of solutes as the RBCs, so the cells retain their normal shape without net water movement.