化學問題, 多多指教

1. Because when iodine is in very low concentration, it is hard to distinguish the colour since iodide has pale colour. Startch can give a strong colour with thiosulphate which can indicate all iodine are reacted.
2. Because when it is near the end point, the solution change to pink. However, when you shake well, the colour disappeare, which mean it is not the end point. The end point is reached when the permanent pink colour appear, so the report should write as that.

1. It is because the iodine is only pale yellow in color, sometimes it is very diffcult to be detected the color change from yellow to colourless. Thus, strach is added as the visual aid in order to determine the end point better.

2. It is only a marker's opinion. In order to get high marks in the public examination, some key words on the color changes may be required to follow, and hence a teacher will insist the student to remember them. In fact, the color change is a visual change and it is subjective. Your "observerable "purple" and "pink" may not be the same as mine.