Final answer:
Detecting lipase activity in tube 5 is difficult because the pH of 2.0 is highly acidic and can denature the lipase, making it inactive. Since lipase operates best in a neutral to slightly alkaline environment, the extreme acidity prevents proper enzyme functioning and makes additional pH changes difficult to detect.
Step-by-step explanation:
The difficulty in detecting whether lipase is active in tube 5 (lipase, vegetable oil, bile salts, pH 2.0 buffer) lies in the very low pH value of the medium. Lipase is an enzyme that catalyzes the breakdown of lipids and normally works in the neutral to slightly alkaline pH found in the small intestine, where the conditions are optimal for its activity. Given that bile has an alkaline pH (7.6 to 8.6) and lipase requires this pH range to function effectively, a pH of 2.0 is highly acidic and can denature the lipase, rendering it inactive. Additionally, at such a low pH, any further decrease in pH would be minuscule and hard to detect, making monitoring lipase activity difficult. Since the substrate (vegetable oil), enzyme (lipase), and bile salts are present in tube 5, the most plausible explanation for the difficulty in detecting lipase activity is because the pH is already very low.