Sulfur precipitate obscures visibility to measure reaction rate

Steps:

• Mix solutions of sodium thiosulfate and hydrochloric acid in a flask over a marked cross.
• Time how long it takes for the yellow sulfur precipitate to cloud the solution and obscure the cross.
• Repeat with varied concentrations or temperatures to compare rates.
• Calculate rate as 1/time for consistent volume and conditions.

Why C is correct:

• The reaction produces solid sulfur (S(s)), forming a colloidal suspension that reduces transparency; rate is inversely proportional to time for obscuration, per collision theory.

Why the others are wrong:

• A: No color change occurs; indicator irrelevant to this ionic reaction.
• B: Gas (SO₂) volume is small and hard to collect accurately for rate.
• D: Mass loss from gas is minimal and imprecise for this precipitation-focused reaction.

Final answer: C