Placing a dialysis tube into a clear container of salt water and letting it sit for 4 hours will the circumference increase or decrease?Why did the circumference of the bag change?
I am wondering if your bag was placed into the salt water empty, or with water or a water-based solution in it. Different initial conditions would change the answer.
Dialysis tubing is a selectively permeable membrane that lets water through but does not let other molecules pass. If you put an empty bag into salt water, a very small amount of water will find its way into the bag, but that will be all. As soon as the water goes into the bag, it will be drawn back out into the salt solution in the container.
If your bag contained plain water, or water with less solute in it than the salt water has, water will leave the bag by osmosis, and the bag will become smaller around. This is because osmosis pushes water molecules to move from where there is a higher water concentration to where the water concentration is lower.
Conversely, if the bag contained a saltier solution than the container, water will move into the bag and it will get larger, again because of osmosis.