When you study the fundamental 'Laws of Chemical Combination' in Class 9 and 11 Chemistry, the absolute most important rule about how atoms bond together is the Law of Definite Proportions (also known as the Law of Constant Proportions).
It was discovered by the brilliant French chemist Joseph Proust in 1799.
Alternate Name: Also universally called the 'Law of Constant Proportions'.
Discoverer: French chemist Joseph Proust (1799).
Core Concept: A compound is defined strictly by the permanent, unchangeable ratio of its elements' mass.
Limitation: Proust did not know about 'Isotopes'. The law slightly fails if a compound uses a heavier isotope of an element (like Carbon-14 instead of Carbon-12), changing the mass ratio.
Proust's law strictly states that: "In a pure chemical compound, the elements are ALWAYS combined together in exactly the same fixed ratio by mass, regardless of where the compound was found or how it was created."
This means Mother Nature follows a strict mathematical recipe. If you change the ratio of the atoms even by a tiny 1%, the magic fails, and you will accidentally create a completely different, potentially deadly chemical.
Let us aggressively prove this law using pure Water (H₂O).
Proust's Law means that if you collect a cup of pure water from a dirty river in India, melt a block of ice from Antarctica, or synthetically create water in a NASA laboratory, the mass ratio of Hydrogen to Oxygen will ALWAYS, mathematically, be exactly 1:8. It never changes.
If you look at Carbon Dioxide (CO₂):
The law states that any given pure chemical compound will always contain exactly the same elements combined together in the exact same fixed proportion by mass, irrespective of its source.
The law was boldly proposed by the French chemist Joseph Proust in the year 1799.
According to the law of definite proportions, the mass ratio of Hydrogen to Oxygen in pure water (H₂O) is always exactly 1:8.
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