The Law of Conservation of Mass is a foundational principle in chemistry and physics, stating that matter cannot be created or destroyed. This means that during any ordinary chemical reaction or physical change, the total mass of the substances involved remains constant over time. The law asserts that the quantity of matter in a closed system will not change, regardless of the transformations it undergoes.
Defining the Conservation of Mass
The core principle of the Law of Conservation of Mass is that the total quantity of mass in an isolated system remains unchanged. This applies even when substances change their form. The mass is conserved because the individual atoms are neither created nor destroyed; they are simply rearranged into different molecular structures.
To confirm this law, scientists must use a “closed system,” meaning no matter can enter or leave the reaction environment. In a chemical reaction, the total mass of the starting materials (reactants) must equal the total mass of the resulting substances (products). This relationship is summarized as: Mass of Reactants = Mass of Products.
This conservation of mass explains why a chemical equation must be “balanced,” ensuring the same number and type of atoms exist on both sides of the reaction arrow. The law is a tool for calculating the amounts of substances involved in any chemical process.
Demonstrating the Law in Action
The conservation of mass can be observed in both physical changes and chemical reactions, where new substances are created. For example, when ice melts into liquid water, the total mass of the water molecules remains exactly the same if measured in a sealed container, despite the change in state.
A more complex example is the combustion of wood, which often appears to violate the law because the final ash weighs less than the original material. This perceived loss is a misconception because the process occurs in an open environment. When wood burns, it reacts with oxygen from the air to produce ashes, carbon dioxide gas, and water vapor. If all the products, including the invisible gases, were collected and weighed, their combined mass would exactly equal the mass of the original wood plus the mass of the oxygen consumed. This proves that the matter has only changed form and location, not been destroyed.
Historical Context and Modern Limits
The formalization of the Law of Conservation of Mass is widely credited to the French chemist Antoine Lavoisier in the late 18th century. Lavoisier performed meticulous experiments, using precise measurements to demonstrate that the mass gained by a metal when it rusted was equal to the mass lost by the surrounding air. His quantitative approach helped transform chemistry into a modern, exact science.
Lavoisier’s work effectively replaced earlier theories, such as the phlogiston theory, by proving that the total mass of a system remains unchanged during chemical transformations. Despite its foundational role in chemistry, the law has a limitation in the context of modern physics. This exception occurs in nuclear reactions, like fission and fusion, where the conservation of mass must be merged with the conservation of energy.
Under the extreme energy conditions of nuclear processes, mass and energy are interchangeable, as described by Albert Einstein’s famous equation, E=mc². This means a small amount of mass can be converted into a tremendous amount of energy, and vice-versa. Therefore, for non-nuclear processes, the Law of Conservation of Mass holds true, but scientists rely on the more encompassing Law of Conservation of Mass-Energy.