Question

A researcher is designing a laboratory experiment to determine whether the inorganic substance A affects the rate of a reaction between two colored liquids, X and Y. When the reaction is complete, the liquid is clear. The reaction between X and Y is similar to one used by manufacturers to produce certain chemicals in factories.
Which method would best answer his question about reaction rate and what will he find out about substance A?
First, he should measure the final temperature of the liquid when X and Y react without substance A. Then, he should measure the final temperature when he adds substance A to X and Y. He will find out if substance A is a catalyst.
First, he should measure how long it takes for the liquid to become clear if X and Y are mixed together. Then, he should measure how long it takes if he also adds substance A to X and Y. He will find out if substance A is a catalyst.
First, he should measure the final temperature of the liquid when X and Y react without substance A. Then, he should measure the final temperature when he adds substance A to X and Y. He will find out if substance A is an enzyme.
First, he should measure how long it takes for the liquid to become clear if X and Y are mixed together. Then, he should measure how long it takes if he also adds substance A to X and Y. He will find out if substance A is an enzyme.

Answer 1

Answer:
Second option.

Rationale: Reaction rate is best measured by timing how long it takes for the coloured mixture to go clear (the observable end point). Compare that time for X+Y alone and for X+Y+A (with all other conditions—concentrations, volumes, temperature, stirring—kept the same and with replicates). If A shortens the time to become clear it acts as a catalyst (speeds the reaction); if it lengthens the time it is an inhibitor; if there is no change it has no effect.

Measuring final temperature does not directly measure rate (it shows heat change), and an inorganic substance cannot be an enzyme (enzymes are biological protein catalysts).