A biotechnology laboratory prepares a lactose-rich growth medium for yeast. The following sequence occurs in a single flask over 24 hours:
(a) Classify each stage (I–IV) as involving predominantly physical change, predominantly chemical change, or a combination. Provide a brief justification for each classification.
(b) For stage II, propose a plausible biochemical reaction responsible for the observed gas evolution, identifying reactant(s) and product(s). Explain how the reaction supports your classification in part (a).
(c) Compare the microscopic (particulate) changes occurring during stages I and IV. Discuss how temperature influences these changes and why no new substances are formed.
(d) A student claims that the drop in pH during stage III is sufficient evidence of a chemical change even if no gas were produced. Evaluate this claim and support your reasoning with at least two pieces of chemical evidence.