Diffusion of alkali in agar gel

Steps:

• Prepare acidic agar with colorless indicator; add alkali to central well, turning it pink where pH rises.
• Pink color diffuses outward as OH⁻ ions spread, creating a circular colored zone.
• Use card's 1 cm concentric circles under petri dish to measure diameter of pink ring over time.
• Plot diameter vs. time to calculate diffusion rate from slope.

Why C is correct:

• C describes measuring color front diameter against marked circles to quantify diffusion, aligning with Fick's first law of diffusion (rate proportional to concentration gradient).

Why the others are wrong:

• A ignores diffusion measurement, focusing only on initial color change.
• B assumes linear spread without circular geometry or timed observation.
• D misinterprets setup as pH testing, not diffusion tracking.

Not enough information on exact choices A/B/D.

Final answer: C