Selective permeability prevents protein diffusion

Steps:

• The semi-permeable membrane allows small molecules to pass but retains large proteins inside the bag.
• Biuret test detects proteins: purple indicates presence, blue or other colors indicate absence.
• After 30 minutes, protein remains in the bag, turning its biuret test purple.
• No protein diffuses to water, so its biuret test stays negative; pH shifts to 7.5 due to minor ion diffusion or buffer effects.

Why B is correct:

• Matches protein retention in bag (purple biuret) and absence in water (red biuret, indicating negative), with near-neutral pH 7.5 per diffusion principles.

Why the others are wrong:

• A: Bag biuret blue wrongly suggests no protein inside.
• C: Water biuret purple wrongly indicates protein diffusion.
• D: Identical to C, so same error in protein diffusion.

Not enough information without diagram to confirm exact setup, but B aligns best with standard results.

Final answer: B