The law states that in a chemical compound

Two or more elements are always present in same ratio by mass

It does not matter where the compound came from or who prepared it.

Law of constant proportion-Teachoo-01.jpg


Water (H 2 O) is a chemical compound. 

It contains 2 elements; Hydrogen and Oxygen


These 2 elements are present in constant proportion by mass which is of 1 :8  

(Since Mass of H 2 = 2g and O=16g, therefore H 2 :O = 2:16 = 1:8)

It means that in 9 grams of Water

There will always be 1 gram of Hydrogen and 8 Grams of Oxygen

It does not matter what the source of water is (rain water/tap water/sea water) or how the water was prepared (melting of ice/cooling of water vapour)

Law of constant proportion 2 image-Teachoo-01.jpg



Example -

In text Question - Page 33 Q2

NCERT Question - Q2

  1. Class 9
  2. Chapter 3 Class 9 - Atoms And Molecules (Term 2)

About the Author

CA Maninder Singh's photo - Founder at Teachoo
CA Maninder Singh
CA Maninder Singh is a Chartered Accountant for the past 11 years and a teacher from the past 11 years. He teaches Science, Accounts and English at Teachoo