Conditions for comparing reaction rates via gas volume graphs

Steps:

• The graph shows experiment 2 produces more CO₂ at each time, indicating a faster rate.
• Rate comparison relies on volume change over time, requiring identical total possible CO₂ volume.
• Total CO₂ depends on CaCO₃ amount with excess HCl, so same CaCO₃ mass is needed.
• Acid concentration affects rate but not total volume (excess), so same [HCl] isolates other variables like particle size.

Why A is correct:

• Rate law for the reaction (rate = k [HCl]^1 [CaCO₃ surface]) requires same [HCl] to attribute volume differences to rate changes, not concentration variations.

Why the others are wrong:

• B: Incomplete phrasing; temperature likely same but not specified as "must both be of" it—focus is concentrations for volume-based rates.
• C: Temperature could vary to cause rate difference; not required to be identical for graph comparison.
• D: Larger lumps reduce surface area, slowing rate (shallower curve), but experiment 2 is faster, contradicting this.

Final answer: A