Final answer:
You can't fully test lipase activity in tube 5 because the acidic pH of 2.0 is not conducive for the function of lipase or for the emulsification of fats by bile salts, both of which require a more neutral to slightly alkaline environment.
Step-by-step explanation:
Tube 5 containing lipase, vegetable oil, bilesalts, and a pH 2.0 buffer cannot fully test lipase activity due to the highly acidic environment which is not optimal for lipase function. Lipases, which break down lipids, have an optimal pH closer to that of the small intestine, where they usually function. The small intestine has a more alkaline pH, where enzymes like trypsin also operate optimally with a pH around 8.
Moreover, bile, which emulsifies fats into micelles, functions best in a pH close to neutral or slightly alkaline. The highly acidic pH in tube 5 would therefore prohibit the bile salts from effectively emulsifying the vegetable oil into micelles, limiting the surface area available for the lipase to act on and consequently limiting the digestion of lipids.