Introduction:
Physical chemistry is a branch of chemistry that focuses on the study of the physical and chemical properties of matter, as well as the changes it undergoes during chemical reactions. It combines principles of physics and chemistry to understand the behavior of atoms, molecules, and systems at the molecular and atomic levels.

Number of Questions:
The number of questions one can expect from Physical Chemistry varies depending on the specific examination and its weightage. However, here are some key factors that can help you estimate the number of questions:

1. Syllabus Coverage:
The syllabus of Physical Chemistry for UPSC exams usually includes various topics such as thermodynamics, chemical kinetics, quantum mechanics, spectroscopy, and electrochemistry. The number of questions can be determined based on the weightage given to each topic in the syllabus.

2. Previous Year's Papers:
Analyzing previous year's papers can give you an idea of the number of questions asked from Physical Chemistry. It allows you to identify trends and patterns in terms of the distribution of questions across different topics.

3. Exam Pattern:
Understanding the exam pattern is crucial as it provides insights into the number of questions asked in each section. Physical Chemistry questions can be part of the Chemistry section, which is generally divided into multiple parts, such as Organic Chemistry, Inorganic Chemistry, and Physical Chemistry. The number of questions from each part can vary, and it is essential to be familiar with the exam pattern to estimate the number of questions from Physical Chemistry.

4. Weightage of Physical Chemistry:
The weightage assigned to Physical Chemistry in the overall Chemistry section can also give an indication of the number of questions. If Physical Chemistry has a higher weightage, it is likely that more questions will be asked from this subject.

Conclusion:
In conclusion, the number of questions one can expect from Physical Chemistry in UPSC exams depends on various factors such as the syllabus coverage, previous year's papers, exam pattern, and weightage assigned to Physical Chemistry. By analyzing these factors, you can estimate the approximate number of questions and prepare accordingly. It is crucial to study the entire syllabus of Physical Chemistry to perform well in the examination.