Dialysis is a common laboratory technique that operates on the same principle as medical dialysis. Typically a solution of several types of molecules is placed into a semipermeable dialysis bag, such as a cellulose membrane with pores, and the bag is sealed. The sealed dialysis bag is placed in a container of a different solution, or pure water. Molecules small enough to pass through the tubing (often water, salts and other small molecules) tend to move into or out of the dialysis bag, in the direction of decreasing concentration. Larger molecules (often proteins, DNA, or polysaccharides) that have dimensions significantly greater than the pore diameter are retained inside the dialysis bag.  One common reason for using this technique would be to remove the salt from a protein solution. The technique will not distinguish between proteins effectively.