A chemical reaction is a process in which reactants, the starting substances, are transformed into products through the breaking and forming of chemical bonds. The substances that enter the reaction are reorganized at the atomic level to produce new substances with different properties. A balanced chemical equation represents this transformation by using coefficients to ensure that the same number of each type of atom appears on both sides of the equation. This balance is required by the law of conservation of mass, which states that mass is neither created nor destroyed during a chemical reaction, so the total mass of reactants must equal the total mass of products.
Chemists classify reactions into five main types. In a synthesis reaction, two or more substances combine to form a single product. A decomposition reaction reverses this, breaking a single compound into simpler substances. In a single replacement reaction, one element replaces another in a compound, while a double replacement reaction involves an exchange of ions between two compounds. Combustion reactions occur when a fuel reacts with oxygen, typically producing carbon dioxide and water, as in the burning of hydrocarbons.
Quantitative chemistry depends on the concept of the mole, the SI unit for amount of substance. One mole contains Avogadro's number of particles, which is \(6.022 \times 10^{23}\). The molar mass of a substance, expressed in grams per mole, is calculated by summing the atomic masses of all atoms in its chemical formula. Stoichiometry uses these relationships and balanced equations to calculate the amounts of reactants and products involved in a chemical change. In any reaction, the limiting reactant is the substance consumed first, and it determines the maximum amount of product that can be formed.