The correct answer is (A).
Getting to the Answer: The experiment measured the electrical potential (the charge with respect to ice crystals) for solutions each containing either F-, Cl-, Br-, or I- at different concentrations. To determine which ion produced the greatest electrical potential, other variables need to be held constant. Evaluate each answer choice to determine which setup varies only the ion type.
(A) Correct: the effect of ion type on electrical potential could be determined if the other independent variable in the Experiment, ion concentration, was held constant
(B) Incorrect: comparing electrical potential of a single sodium salt solution at different concentrations would give the effects of varying concentration for that particular ion, not the effects of varying ion type
(C) Incorrect: comparing electrical potential of a single sodium salt solution at different concentrations would give the effects of varying concentration for that particular ion, not the effects of varying temperature
(D) Incorrect: The Experiment did not study collision rates; if the collision rates were different instead of constant, the effect of ion type on electrical potential could not be determined
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