When a chemical change occurs, starting materials transform into new substances with different properties. This raises a fundamental question about matter: if substances change their form, does the total amount of matter involved also change? The structure of a balanced chemical equation provides a definitive answer by serving as a precise mathematical model for a physical phenomenon. By tallying the components on both sides of the reaction arrow, the equation confirms that matter is neither created nor destroyed during a chemical reaction.
The Law of Conservation of Mass
The principle that mass is preserved during any chemical transformation is formally known as the Law of Conservation of Mass. This fundamental physical law states that in a closed system, the mass of the reactants must exactly equal the mass of the products. This equality holds true because chemical reactions only involve the rearrangement of matter, not its creation or annihilation.
The French chemist Antoine Lavoisier confirmed this law in the late 18th century through experiments conducted in sealed vessels. By containing all the gases and solids involved, Lavoisier demonstrated that the total weight of the system remained unchanged, even when new substances were formed. His work established that mass conservation is a property of the physical universe.
Atomic Identity and the Preservation of Mass
The conservation of mass during a chemical reaction is linked to the behavior of atoms. All matter is composed of atoms, and each atom possesses a specific, inherent mass. When a chemical reaction takes place, atoms within reactant molecules break their existing bonds and form new bonds in different combinations to create product molecules.
The atoms themselves are not altered, destroyed, or transmuted into different elements during this rearrangement. For example, an oxygen atom entering a reaction will exit as an oxygen atom within a product molecule, maintaining its elemental identity and its atomic mass. Since the number and type of atoms remain the same, the collective total mass must also be preserved.
This principle explains why counting the atoms of each element on both sides of a reaction is sufficient to prove mass conservation. If the count of every type of atom is identical before and after the reaction, their combined mass must also be equal. The chemical equation must reflect this atomic-level accounting.
Balancing Equations as Mathematical Proof
The chemical equation is a symbolic shorthand for a reaction, and its structure must mathematically validate the conservation of mass. This is achieved through the careful use of subscripts and coefficients, each serving a distinct purpose. Subscripts, the small numbers written after an element’s symbol, define the fixed composition of a molecule and cannot be changed. For instance, the subscript ‘2’ in \(H_2O\) indicates that a water molecule always contains two hydrogen atoms bonded to one oxygen atom.
Changing a subscript would change the identity of the substance itself, such as turning \(H_2O\) (water) into \(H_2O_2\) (hydrogen peroxide), which is a fundamentally different compound. To balance the equation and conserve mass, chemists adjust the coefficients, which are the large whole numbers placed in front of a molecule’s formula. These coefficients represent the required quantity of molecules needed for the reaction.
Consider the reaction for the formation of water, which might initially be written as \(H_2 + O_2 \rightarrow H_2O\). This unbalanced equation violates mass conservation because there are two oxygen atoms on the reactant side but only one on the product side. To correct this imbalance, coefficients are introduced: \(2H_2 + O_2 \rightarrow 2H_2O\).
This balanced equation now shows that four hydrogen atoms (two molecules of \(H_2\)) and two oxygen atoms (one molecule of \(O_2\)) react to produce two water molecules (two molecules of \(H_2O\)). By multiplying the coefficient by the subscript for each element, the total count of atoms is checked: four hydrogen atoms and two oxygen atoms on both sides. Since the count of each type of atom is identical, the total mass on the reactant side is mathematically guaranteed to equal the total mass on the product side, proving that the reaction satisfies the Law of Conservation of Mass.