Introduction:
In Chemistry optional answers, it is indeed necessary to provide explanations for the principles or mechanisms involved in chemical reactions. This is because understanding the principles and mechanisms behind chemical reactions is crucial for a comprehensive understanding of the subject. By explaining these principles and mechanisms, candidates can demonstrate their knowledge and comprehension of the concepts, and showcase their analytical and problem-solving skills.

Importance of Explaining Principles:
1. Conceptual Clarity: Explaining the principles involved in chemical reactions helps to establish a strong foundation of knowledge. It allows the examiner to assess the candidate's understanding of the fundamental concepts underlying chemical reactions.

2. Application of Knowledge: Understanding the principles enables candidates to apply their knowledge to a wide range of chemical reactions. It helps them identify the factors that influence the reaction, predict the products, and understand the conditions required for a successful reaction.

3. Problem Solving: Principles of chemical reactions serve as guidelines for solving complex problems. By explaining these principles, candidates can showcase their ability to analyze and solve problems related to chemical reactions.

Importance of Explaining Mechanisms:
1. Comprehensive Understanding: Explaining the mechanism of a chemical reaction provides insights into the step-by-step process by which reactants are transformed into products. It helps candidates understand the intermediates, transition states, and rate-determining steps involved in the reaction.

2. Predicting Reaction Pathways: Mechanistic explanations allow candidates to predict the most favorable reaction pathways and understand the factors that control the reaction outcome. This knowledge is invaluable in designing efficient synthetic routes in organic chemistry.

3. Reactivity and Selectivity: Mechanistic explanations help candidates understand the factors influencing reactivity and selectivity in chemical reactions. They can explain why certain reactions occur preferentially over others and the role of catalysts and reaction conditions in controlling the reaction outcome.

Conclusion:
In Chemistry optional answers, providing explanations for the principles and mechanisms involved in chemical reactions is essential. It demonstrates a candidate's comprehension of the subject, problem-solving abilities, and application of knowledge. By explaining these principles and mechanisms, candidates can present a well-rounded answer that showcases their understanding of the underlying concepts and their ability to analyze and predict chemical reactions.