A topic for the extended essay in chemistry or biology must be one that is based on an existing theory or come up with a new one and support it with relevant data. Students tend to overcomplicate their research by picking difficult topics that consist of various aspects, assuming that the examiner will be impressed and provide them with a high grade. This is a false misconception. Examiners want to see students pick the simplest of topics and see how in-depth the student can conduct research. I would recommend picking a topic that genuinely interests you, because the entire 4000 words will have to be based on it. The bottom line is that there are no good or bad topics, it solely depends on how well the student can show interest and critically analyse, evaluate and draw conclusions from their research.