Final answer:
Fructose, being a monosaccharide, diffused through the dialysis tubing into the surrounding water due to its small size, whereas lactose, a larger disaccharide, did not.
Step-by-step explanation:
After the dialysis tubing bag is placed in water and Lugol's solution for some time, the observation is that fructose is detected in the distilled water outside of the dialysis tubing bag while lactose is not. This result can be attributed to the molecular size and structure of the substances. Fructose, a monosaccharide, is smaller and can easily diffuse through the semi-permeable membrane of the dialysis tubing bag into the surrounding water. In contrast, lactose, a disaccharide, is composed of two monosaccharide molecules and is too large to pass through the pores of the membrane.
An additional reference made is to hemodialysis, where patients' blood is passed through a semipermeable tube, which permits certain waste materials like urea to diffuse out of the blood, while larger components such as red and white blood cells are retained. This underlines the selectivity of semi-permeable membranes based on molecular size in a dialysis process.