Chemical reactions transform substances, often leading to dramatic changes in appearance, color, or state. Despite these profound transformations, a fundamental principle of chemistry dictates that mass is neither gained nor lost during the process. This rule governs all chemical transformations, establishing that matter is conserved despite its change in form.
What the Law of Conservation of Mass Means
The Law of Conservation of Mass (LOM) states that within any system closed to the transfer of matter, the total mass of the materials undergoing a chemical change remains constant over time. This means the combined mass of the reactants must precisely equal the combined mass of the products. The principle can be concisely expressed mathematically as \(M_{reactants} = M_{products}\).
The system must be closed for the law to hold true. If a reaction occurs in a sealed container, the mass measurement taken before the reaction will be identical to the mass measurement taken afterward. This framework establishes a reliable foundation for quantitative analysis in chemistry.
The Atomic Level Mechanism
The underlying reason for mass conservation rests entirely on the behavior of atoms during a chemical change. Chemical reactions involve the rearrangement of atoms, not their creation, destruction, or transformation into different elements. The transformation from reactants to products occurs when existing chemical bonds holding the atoms together in the initial molecules are broken, and new bonds are subsequently formed to create different molecular structures.
Although the molecules themselves are fundamentally changed, the individual atoms—such as carbon, hydrogen, or oxygen—remain intact throughout the process. Since mass is concentrated almost entirely within the atomic nucleus, the mass of the individual atoms is unaffected by the breaking and forming of chemical bonds.
The slight changes in energy that occur during bond formation and breaking are theoretically associated with an equally small change in mass, according to Einstein’s mass-energy equivalence (\(E=mc^2\)). However, the energy changes involved in chemical reactions are so minuscule that the corresponding mass change is undetectable by even the most precise laboratory balances. Therefore, for all practical purposes in chemistry, the conservation of mass holds absolutely because the number and type of atoms remain identical before and after the reaction.
Experimental Proofs and Demonstrations
Empirical evidence for the Law of Conservation of Mass was established through the rigorous, quantitative experiments of Antoine Lavoisier in the late 18th century. Lavoisier is recognized for changing chemistry from a qualitative discipline to a quantitative science by carefully weighing reactants and products. He utilized sealed glass vessels to conduct reactions, allowing him to precisely measure the mass of both the starting materials and the resulting products without losing any gaseous components.
One of Lavoisier’s most known experiments involved heating mercury metal in a sealed vessel with air. The mercury reacted with oxygen from the air to form a red powder, mercuric oxide. When he weighed the entire sealed apparatus, he found the total mass remained exactly the same before and after the metal had transformed.
This quantitative approach conclusively overturned the previously accepted phlogiston theory, which incorrectly assumed mass could be lost during processes like combustion. Common misconceptions about the law arise when reactions are conducted in open systems, such as burning paper. The apparent loss of mass when the paper turns to ash is simply the escape of gaseous products, like carbon dioxide and water vapor, into the atmosphere. When all these gaseous products are captured and weighed, the conservation of mass is fully confirmed.
Representing Conservation Through Balanced Equations
Chemists use balanced chemical equations as the symbolic and mathematical confirmation of the Law of Conservation of Mass. An equation is balanced when it ensures that the number of atoms of each element on the reactant side exactly matches the number of atoms of that same element on the product side. The mass of the reactants must equal the mass of the products because the total number of atoms, each possessing an unchanging mass, is conserved.
In the notation of a chemical equation, small numbers called subscripts define the fixed composition of the molecule and cannot be altered. Instead, whole numbers known as coefficients are placed in front of the chemical formulas, indicating the number of molecules required for the reaction. For instance, in the combustion of methane (\(mathrm{CH}_4\)), balancing ensures there is one carbon atom, four hydrogen atoms, and four oxygen atoms on both sides of the reaction arrow. Adjusting these coefficients is the mathematical mechanism used to confirm that no atoms were created or destroyed, thereby proving that the total mass of the reactants equals the total mass of the products.