Introduction:
Choosing an optional subject for the UPSC exam requires careful consideration of one's strengths and weaknesses. While a weak understanding of International Relations may make it challenging to choose it as an optional subject, it is still possible to choose Chemistry as an alternative.

Considerations:
When deciding on an optional subject, several factors need to be taken into account. Some of these considerations include:

1. Interest and Aptitude: Assess your interest and aptitude in Chemistry. If you have a genuine curiosity and enthusiasm for the subject, it can motivate you to overcome any initial difficulties.

2. Background and Preparation: Evaluate your educational background and prior knowledge in Chemistry. If you have a basic understanding or have studied Chemistry in school, it may provide a foundation to build upon during your preparation.

3. Availability of Study Material: Ensure that adequate study material is available for Chemistry. Search for reliable sources, such as textbooks, reference books, and online platforms like EduRev, that offer comprehensive study material for the subject.

4. Scoring Potential: Research the scoring potential of Chemistry as an optional subject. Analyze the previous year's question papers and marking trends to understand the pattern and distribution of marks. This will help you gauge whether Chemistry can be a scoring subject for you.

5. Overlap with General Studies: Examine the overlap between Chemistry and the General Studies (GS) papers of the UPSC exam. If Chemistry covers certain topics or concepts that are part of the GS syllabus, it can provide an advantage in terms of integrated preparation.

Preparation Strategy:
To overcome a weak understanding of International Relations and succeed in Chemistry as an optional subject, follow these strategies:

1. Thorough Study: Begin by studying the basic concepts of Chemistry and gradually progress to advanced topics. Focus on building a strong foundation to ensure a better understanding of the subject.

2. Practice and Problem Solving: Solve a variety of practice questions and numerical problems to strengthen your problem-solving skills. This will enhance your ability to apply theoretical concepts to practical situations.

3. Seek Guidance: Enlist the help of experienced teachers, mentors, or subject experts who can guide you through the preparation process. They can provide clarity, suggest effective study techniques, and recommend additional resources.

4. Regular Revision: Allocate sufficient time for revision of previously studied topics. Regular revision will help reinforce your understanding and prevent forgetting important concepts.

5. Mock Tests and Previous Year Papers: Practice mock tests and solve previous year's question papers to familiarize yourself with the pattern, time management, and difficulty level of the exam. This will also help you identify areas that require further improvement.

Conclusion:
While a weak understanding of International Relations might make it challenging to choose it as an optional subject, Chemistry can be a viable alternative. By considering your interest, aptitude, preparation resources, scoring potential, and overlap with GS, and implementing a focused preparation strategy, you can overcome initial difficulties and excel in Chemistry as your optional subject for the UPSC exam.