Final answer:
The law of conservation of mass, indicating that matter is neither created nor destroyed in a chemical reaction, was proposed by Antoine Lavoisier.
Step-by-step explanation:
The law of conservation of mass was proposed by the French chemist Antoine Lavoisier in 1789. This fundamental chemical law states that matter is neither created nor destroyed during a chemical reaction. For instance, when burning wood, the mass of the result—soot, ashes, and gases—is equivalent to the original mass of the charcoal and oxygen before the reaction occurred. Lavoisier's experiments, such as heating metallic tin in a closed vessel, provided crucial evidence for this principle and significantly contributed to the development of modern chemistry.