These two laws, established by pioneers like Antoine Lavoisier and Joseph Proust, form the foundation of modern Chemistry. They explain how matter behaves and combines during chemical reactions.
1. Law of Conservation of Mass
Established by Antoine Lavoisier in 1789, this law focuses on the quantity of matter.
Statement: Mass can neither be created nor destroyed in a chemical reaction.
Explanation: In any chemical reaction, the total mass of the reactants (starting materials) is always equal to the total mass of the products (resulting materials).
Mathematical Expression:
$$\text{Mass of Reactants} = \text{Mass of Products}$$Example: If you burn 12g of Carbon in 32g of Oxygen, you will get exactly 44g of Carbon Dioxide. No atoms are lost; they are simply rearranged into a new form.
2. Law of Constant Proportions
Established by Joseph Proust, this law is also known as the Law of Definite Proportions. It focuses on the composition of compounds.
Statement: In a chemical substance, the elements are always present in definite proportions by mass.
Explanation: No matter where a compound comes from or how it is prepared, it will always contain the same elements combined in the same fixed ratio by weight.
Example: Pure water ($H_2O$) always contains Hydrogen and Oxygen in a mass ratio of 1:8.
If you decompose 9g of water, you get 1g of Hydrogen and 8g of Oxygen.
If you decompose 18g of water, you get 2g of Hydrogen and 16g of Oxygen.
This ratio remains 1:8 whether the water is from a river, a lab, or a rain cloud.
[Image showing the fixed mass ratio of elements in a water molecule]
Summary Table
| Law | Key Scientist | Focus | Main Idea |
| Conservation of Mass | Antoine Lavoisier | Total Weight | Mass before = Mass after. |
| Constant Proportions | Joseph Proust | Composition | Elements combine in fixed ratios. |
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