The law of conservation of mass is a fundamental principle in the physical sciences asserting that mass is constant within a closed system. This means that for any chemical or physical change confined within an isolated environment, the total quantity of mass remains the same over time. The law implies that mass cannot be created or destroyed, even if the substance undergoes a transformation in its physical state or chemical form.
The Law’s Defining Principle and Origin
The concept gained formal scientific standing due to the meticulous work of French chemist Antoine Lavoisier in the late 18th century. Lavoisier is credited with popularizing the principle after conducting a series of quantitative experiments. His crucial experimental method involved carefully weighing reactants and products in sealed glass vessels, which ensured that no gases could escape or enter the system.
This careful approach allowed him to demonstrate conclusively that the total mass before a reaction was precisely equal to the total mass after the reaction. His findings effectively disproved the long-standing phlogiston theory, which incorrectly proposed that a substance lost an invisible element during combustion.
The underlying scientific explanation for this conservation is rooted in atomic theory, which posits that all substances are composed of atoms. Chemical processes involve only the rearrangement of these atoms into new combinations, rather than their creation or destruction. John Dalton’s atomic theory later reinforced this principle by asserting that atoms are indestructible during a chemical reaction, thereby fixing the total mass of the system.
Mass Conservation in Chemical Reactions
The most prominent scientific application of the conservation principle is in the study of chemical reactions, where it dictates the relationship between starting substances and resulting substances. The substances that enter the reaction are referred to as reactants, and the new substances formed by the transformation are called products. According to the law, the aggregate mass of all reactants must be exactly equal to the aggregate mass of all products.
This foundational rule forms the basis of stoichiometry, which is the calculation of quantitative relationships between substances in a chemical reaction. Because atoms are simply rearranged, the total number of atoms for each element must remain constant throughout the entire reaction process. For this reason, all written chemical equations must be balanced, ensuring that the counts of every atomic species are identical on both sides of the equation.
Balancing a chemical equation means adjusting the coefficients (the numbers placed in front of the chemical formulas) so that the total mass is conserved. For instance, in the combustion of methane, the atoms on the reactant side must exactly match the atoms in the products formed. Although the atoms transform from methane and oxygen molecules into carbon dioxide and water molecules, their collective mass does not change.
If chemists know the precise mass of one substance involved in a reaction, they can use this law to calculate the exact mass of every other substance required or produced. This predictive capability provides the framework for quantitative analysis, allowing for the accurate prediction and measurement of chemical outputs.
Everyday Demonstrations of the Principle
The principle of mass conservation is observable in many everyday phenomena, particularly those involving a change in state or form. When a log burns in a campfire, it appears to lose mass as it reduces to a small pile of ash. This apparent loss is deceptive because the system is open, allowing gases to escape.
The original wood combines with oxygen from the air, converting the total mass of the wood and consumed oxygen into ash, carbon dioxide gas, and water vapor. For example, if a 300-kilogram tree yielded 10 kilograms of ash, the missing 290 kilograms were released into the atmosphere as smoke and gases. The mass of the original tree plus the mass of the oxygen used equals the mass of the ash plus the mass of the released gases, demonstrating that the total mass is accounted for.
A simpler demonstration involves a physical change, such as the freezing of water. If a given quantity of liquid water is weighed and then allowed to freeze into a solid block of ice, the mass of the ice will be identical to the mass of the liquid water. The substance changes its physical state, but no mass is created or destroyed in the process.