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
31 exam-style questions on OCR GCSE Chemistry Controlling reactions. Each one has a worked solution and a mark scheme showing where the marks go.