 In the double displacement reaction of an acid with a hydroxide, we will react hydrochloric acid with sodium hydroxide. The two solutions are colorless, so it's difficult to see what is happening. The acid-base indicator is bromethymol blue, which is yellow and acid and blue and base. As we add the sodium hydroxide solution, the indicator changes from yellow to blue as the solution changes from acidic to basic. Sodium chloride and water are produced in the reaction.