Iodine diffusion causes starch-iodine color reaction

Steps:

• In the experiment, starch suspension is inside a semi-permeable membrane, with iodine solution outside.
• Iodine molecules are small and diffuse across the membrane from high to low concentration.
• Starch molecules are large and cannot pass through the membrane pores.
• Inside the membrane, iodine reacts with starch to form a blue-black complex, changing the suspension's color.

Why A is correct:

• Diffusion is the net movement of particles from high to low concentration through a semi-permeable membrane; iodine's small size allows it to enter the starch suspension and trigger the color change via complex formation.

Why the others are wrong:

• B: Iodine starts outside, so it diffuses inward, not outward.
• C: Starch molecules are too large to diffuse through the membrane into the iodine solution.
• D: Starch cannot diffuse out due to its size, and the color change occurs inside the suspension.

Final answer: A