Final answer:
A hypotonic solution has a lower solute concentration than another solution, causing water to move out of it. The movement of water from the dialysis bag indicates it is hypotonic as it's moving towards a hypertonic solution until equilibrium is reached.
Step-by-step explanation:
When discussing a dialysis bag in the context of tonicity, the terms hypertonic, hypotonic, and isotonic are used to describe solutions based on solute concentration relative to another solution. A hypotonic solution has a lower solute concentration compared to another solution. If water is moving out of the dialysis bag, it suggests that the solution inside the bag has a lower solute concentration and is, in fact, hypotonic. As water moves through a selectively permeable membrane from the hypotonic solution to the hypertonic solution, it's attempting to reach equilibrium - an isotonic state - where solute concentrations are equal on both sides.
Osmosis is the movement of water across a membrane, and it occurs when there is an imbalance of solutes. If the dialysis bag appeared to be shrinking, it could be that water was initially moving out, indicating the bag was indeed hypotonic. But as the concentrations approach equilibrium, the net movement of water will decrease, and the dialysis bag will stabilize in size, rather than continuing to shrink indefinitely.