Chemical substances transform according to the laws of physics. A chemical equation uses symbols and formulas to describe this transformation, showing what is consumed and what is created. The rearrangement of atoms in a chemical reaction must adhere to conservation principles, meaning matter is neither lost nor gained.
The Foundation: The Law of Conservation of Mass
The basic principle governing chemical change is the Law of Conservation of Mass (LCoM), established by Antoine Lavoisier in the late 18th century. This law states that in any closed system, the total mass of substances involved in a chemical reaction remains constant. Matter is neither created nor destroyed during the process.
Chemical reactions involve breaking existing bonds and forming new ones, which simply rearranges the atoms into different molecular structures. The individual atoms remain intact and unchanged, regardless of the structures they form. Therefore, the total number and type of atoms present before the reaction must equal the total number and type after the reaction. The mass of the reactants will always be mathematically identical to the mass of the products.
Unbalanced Chemical Representation
A chemical equation uses formulas to represent the reactants (starting materials on the left side of an arrow) and the products (new substances formed on the right side). Simply writing the raw formulas often results in an unbalanced equation, even though the formulas accurately describe the molecules involved. For example, when hydrogen gas (\(H_2\)) and oxygen gas (\(O_2\)) combine to form water (\(H_2O\)), the initial equation is \(H_2 + O_2 \rightarrow H_2O\).
This initial representation does not reflect atomic conservation. Counting the atoms shows two oxygen atoms on the reactant side (\(O_2\)) but only one oxygen atom on the product side (\(H_2O\)). An equation written this way appears to violate the LCoM because the count of atoms for at least one element is unequal across the reaction arrow. Chemists must correct this mathematical inconsistency to ensure the equation accurately models the physical event.
Using Coefficients to Validate Mass Conservation
Balancing an equation is the mathematical action required to align the chemical representation with the physical law of mass conservation. This is achieved by placing whole-number coefficients in front of the chemical formulas. A coefficient indicates the precise number of molecules required for that substance to participate in the reaction. These coefficients establish the specific ratio of molecules needed to ensure the total count of every type of atom is exactly the same on both the reactant and product sides.
Using the water formation example, \(H_2 + O_2 \rightarrow H_2O\), the oxygen imbalance is corrected by placing a coefficient of 2 in front of the water molecule, yielding \(H_2 + O_2 \rightarrow 2H_2O\). This balances the oxygen atoms (two on each side) but creates an imbalance for hydrogen, as the product side now has four hydrogen atoms. To resolve this, a coefficient of 2 is placed in front of the reactant hydrogen molecule, resulting in the final balanced equation: \(2H_2 + O_2 \rightarrow 2H_2O\).
The final, balanced equation directly illustrates the Law of Conservation of Mass because it mathematically confirms that no atoms were lost or gained. The equation now shows four hydrogen atoms and two oxygen atoms on both the reactant and product sides. By equating the count of atoms for every element, the balanced equation verifies that the total mass before the reaction equals the total mass after the reaction.