Solutions, Acidity, and Reaction Rates

Chemical Equations and the Law of Conservation of Mass

A chemical equation is a written description of how matter changes during a reaction. It represents the transformation of starting substances into completely new substances. In every chemical equation, the starting materials are called reactants and are placed on the left side. The newly created substances are called products and appear on the right side. An arrow points from the reactants toward the products, indicating the direction in which the reaction proceeds.

Under the law of conservation of mass, matter cannot be created or destroyed during any chemical reaction. Because every single atom that enters a reaction must be present at the end, a chemical equation must be balanced. An equation is balanced when the total number of atoms of each element is exactly equal on both the reactant side and the product side.

To balance an equation properly, you write whole numbers called coefficients directly in front of the chemical formulas. These coefficients indicate the relative ratios of reacting and produced particles, or the ratios of moles of those substances. You must never change the subscripts, which are the smaller numbers written inside or after chemical formulas to show how many atoms or groups of atoms make up that compound. Changing a subscript alters the actual chemical identity of the compound, whereas changing a coefficient simply adjusts the quantity of that substance without altering what it is.

Counting atoms correctly is essential when checking whether an equation is balanced. To count atoms of an element, multiply its subscript by any applicable parentheses subscript and the formula's coefficient; if the element appears in more than one part of the formula, add those counts. When a formula contains parentheses, subscripts written outside those parentheses multiply all atoms of the elements located inside them.