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)

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Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 13 years and a teacher from the past 17 years. He teaches Science, Economics, Accounting and English at Teachoo