Final answer:
When lettuce sits in salad dressing for a while, it wilts because of osmosis. The salad dressing has a higher salt content than the lettuce, creating a hypertonic solution that draws water out of the lettuce cells and causes them to lose turgor pressure.
Step-by-step explanation:
When lettuce sits in salad dressing for a while, it wilts because of a process called osmosis. Osmosis is the movement of water from an area of higher water concentration to an area of lower water concentration through a selectively permeable membrane, such as the cell membranes in lettuce leaves.
The salad dressing has a higher salt content than the lettuce, which creates a hypertonic solution. This means that the concentration of solutes (such as salt) is higher outside the lettuce cells than inside. As a result, water is drawn out of the lettuce cells, causing them to lose turgor pressure and wilt.