Step 1: Raisins contain a concentrated solution.
A raisin contains a concentrated solution of sugars and other dissolved substances inside its cells. When placed in pure water, a concentration gradient is established across the cell membrane.
Step 2: The cell membrane acts as a semipermeable membrane.
The raisin's cell membrane allows solvent (water) molecules to pass through but restricts the passage of larger solute molecules. This is a semipermeable membrane.
Step 3: Direction of water movement.
Water moves from the region of lower solute concentration (pure water outside) through the membrane into the region of higher solute concentration (inside the raisin).
Step 4: Result and name of phenomenon.
The entry of water increases internal pressure, causing the raisin to swell. The movement of solvent inward through the semipermeable membrane is called endosmosis (a type of osmosis). The phenomenon responsible is endosmosis (osmosis).