Final answer:
A testable question about the effect of reactant concentration on the rate of a reaction can involve measuring how different concentrations of hydrochloric acid react with zinc to release hydrogen gas and thus determine the reaction rate.
Step-by-step explanation:
A unique testable question surrounding one of the variables that affect the rate of chemical reactions could be: How does the concentration of reactants influence the rate of a reaction between acid and metal? As we know from collision theory, the rate of a reaction is dependent upon the frequency and energy of collisions between reactant molecules. In this context, you can test how varying the concentration of, for example, hydrochloric acid affects the rate at which it reacts with a metal such as zinc to produce hydrogen gas. This kind of experiment not only teaches about the impact of reactant concentration but also can illustrate the effect of surface area by comparing reactions with zinc in the form of a large lump versus zinc filings.
When planning this experiment, it would be important to control other variables such as temperature, to strictly evaluate the impact of concentration changes. By measuring the volume of hydrogen gas produced over time, students can quantify the reaction rate and understand the underlying principles that govern chemical reactions in different conditions.