This book is a part of a series of six books written to provide complete coverage of the NCERT Science syllabus for classes 9 and L0 prescribed by the Central Board of Secondary Education (CBSE). The six books in this series are: Science for ninth Class (part- L): Physics, Science for ninth Class Part-2): Chemistry Science for ninth Class (Part-3): Biology; Science for tenth Class (Part- L): Physics, Science for tenth Class (Part-2): Chemistry and Science for tenth Class (Part-3): Biology. The most important feature of the revised editions of these books is that a large variety of solved and unsolved questions has been included to assess the learning abilities of the students. The main aim of bringing out this series is to make science more attractive, interesting and understandable to the students. The headings, subheadings, and definitions of each topic of this book have been given in different colours. This will help the students to locate the various topics easily and learn the definitions conveniently.