Final answer:
The independent variable is the type of metal used to react with hydrochloric acid. The controlled variables are the concentration of the acid and the contact time. Reactivity is assessed by measuring the rate of hydrogen gas formation and other reaction products.
Step-by-step explanation:
The question focuses on making a fair comparison of the reactivity of metals with hydrochloric acid by keeping two variables constant: the concentration of hydrochloric acid and the amount of time the metals are in contact with the acid. These are known as controlled variables. However, the independent variable is the factor that the experimenter changes to test its effect on the reaction rate, which in this case, could be the type of metal being reacted with hydrochloric acid.
The mass of the reactants and the concentration of the acid are closely monitored to ensure consistency in experimental conditions. Observing reactions of metals like zinc with hydrochloric acid, where products such as zinc chloride and hydrogen gas are formed, helps to understand the reactivity series of metals. Variables like the size of the metal pieces and the temperature of the solution also play significant roles in reaction rates, affecting the surface area and kinetic energy, respectively.