A biotechnology laboratory prepares a lactose-rich growth medium for yeast. The following sequence occurs in a single flask over 24 hours:
I. Warm water and powdered lactose are mixed, forming an apparently homogeneous solution.
II. A freeze-dried yeast culture is added. The fluid becomes increasingly turbid, and small gas bubbles appear at the surface.
III. After 12 hours, the pH of the medium has dropped from 7.0 to 4.5, and a distinct sour odor is detected.
IV. The mixture is cooled to 4 °C, causing suspended solids to settle without further gas evolution.
Use the information above to answer the questions that follow.
(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.