Y9 science students were given the task of finding out the four-digit combination number to a locked safe. In order to break the code, students had to investigate chemical solutions for clues. Four mysterious pink solutions of different alkaline concentrations, were available. A different volume of hydrochloric acid was needed to neutralise each alkaline solution, turning its colour from pink to clear. The higher the concentration of the alkaline solution, the more hydrochloric acid was needed to neutralise it. Each numerical volume of neutralising acid revealed one number to the safe combination. Once all four volumes were known, the four-digit code was cracked.