What color indicates the endpoint in the described precipitation/dye separation method?

• A. Purple
• B. Blue
• C. Green
• D. Red

Answer: (A)

In this precipitation-dye separation, the endpoint is identified by the color change of the indicator dye used in the procedure. The dye is chosen so that when the precipitation is complete, the dye in the solution adopts its endpoint color, signaling that you should stop. Here, the endpoint color is purple, so once the solution turns purple you’ve reached the moment to stop adding reagents. The other colors would correspond to different dyes or conditions and wouldn’t reliably indicate completion in this specific protocol.