A student investigates the reaction between magnesium ribbon and dilute hydrochloric acid:
Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) \text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2(\text{aq}) + \text{H}_2(\text{g}) Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)They want to find out how the concentration of hydrochloric acid affects the rate of reaction. They measure the time taken for a piece of magnesium ribbon to completely dissolve.
Which two variables must the student keep constant to ensure a fair test?
time taken for magnesium to dissolve, temperature
concentration of hydrochloric acid, volume of hydrochloric acid
temperature, length of magnesium ribbon
concentration of hydrochloric acid, length of magnesium ribbon