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Building upon the foundations laid by Lavoisier, another crucial principle in chemistry is the Law of Constant Proportions, also known as the Law of Definite Proportions. This law, established by Joseph L. Proust, states: "In a chemical substance the elements are always present in definite proportions by mass."

This law essentially means that a pure chemical compound, no matter its source or how it was prepared, will always contain the same elements combined in the exact same fixed ratio by mass. This provides a clear distinction between compounds and mixtures, as mixtures can have variable compositions.

Let's consider some classic examples:

• Water (H?O): No matter where you get your water from – whether it's from a river, a well, or a laboratory synthesis – it will always be composed of hydrogen and oxygen. Crucially, the ratio of the mass of hydrogen to the mass of oxygen in water is always 1:8. This means that if you decompose 9 grams of water, you will consistently obtain 1 gram of hydrogen and 8 grams of oxygen. This fixed ratio is inherent to the compound 'water'.
• Ammonia (NH?): Similarly, in ammonia, nitrogen and hydrogen are always present in a fixed ratio of 14:3 by mass. This holds true regardless of the method used to produce the ammonia or its original source.

The consistency described by the Law of Constant Proportions was a major challenge for scientists to explain in the late 18th century. It hinted at a deeper, underlying structure of matter that governed how elements combined. This problem paved the way for the development of a groundbreaking theory by British chemist John Dalton. Dalton's atomic theory, which was built upon the philosophical idea of indivisible atoms, provided a comprehensive explanation for both the Law of Conservation of Mass and the Law of Definite Proportions. His theory proposed that elements combine in simple whole-number ratios, thus accounting for the fixed mass proportions observed in compounds. This elegant explanation solidified the Law of Constant Proportions as a fundamental truth in chemistry, illustrating the precise and ordered nature of chemical combination.

Fun Fact: The Law of Definite Proportions is why chemists can predict exactly how much of each element is needed to create a specific compound, and how much product will be formed, making chemical synthesis predictable and efficient.

Question for You: Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?