When celery is placed in salt water, the portion of the celery stalk placed in the salt water becomes soft, limp and mushy. The celery loses its firmness.
Celery becomes soft and mushy when soaked in salt water because salt water is a hypertonic solution. In a hypertonic solution, the concentration of certain solutes is higher than in a living cell. Since the concentration of salt is higher outside the celery, it causes a net movement of water out of the celery's cells. When the water moves out of the celery, its cells shrink, which causes the celery to become limp and soft. Conversely, if celery is placed in fresh water, a hypotonic solution, water moves into the celery's cells and causes them to expand. Soaking celery in fresh water results in the celery becoming stiff.