Question

I didn't understood about law of constant proportions
for class 9

Answer 1

HI SIR HERE IS YOUR ANSWER,

also known as the principle of constant proportions states that although the salinity of various samples of seawater may vary, the ratio of major salts is constant. No matter how much the salinity increases or decreases in the ocean, the proportions of ions relative to each other remains constant.

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Answer 2

In chemistry, the law of definite proportion, sometimes called Proust's law or the law of definite composition, or law of constant composition states that a given chemical compound always contains its component elements in fixed ratio and does not depend on its source and method of preparation. 
Examples of the Law of Definite Proportions. Let us take, for example, the compound, water. Whatever the source of water, its composition is that of two atoms of hydrogen and one atom of oxygen. This figure shows that water, from any source, is always made up of two atoms of hydrogen and one atom of oxygen.