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Reactants are substances consumed at the start of a chemical reaction and appear on the left side of the equation. Products are substances formed during the reaction and appear on the right side. An arrow separates reactants from products, showing the reaction direction. Chemical formulas represent the atoms, molecules, or compounds involved in the transformation.

Chemical equations must be balanced to satisfy the law of conservation of mass, which states that the total mass of all participating elements is conserved. Balancing ensures the cumulative number of atoms of each element matches on both the reactant and product sides, accurately representing how atoms rearrange during the reaction.

Balance equations by adjusting numerical coefficients placed before formulas to equalize atom counts for each element on both sides. Change only coefficients, never subscripts within formulas. Start with atoms in compounds, then balance free elements. Fractional coefficients can be used initially, then multiplied by a whole number to convert all coefficients to integers.

Parenthetical abbreviations indicate the physical states of reactants and products. Standard abbreviations include 's' for solids, 'l' for liquids, 'g' for gases, and 'aq' for substances dissolved in water. These notations provide complete information about the reaction conditions and the form of each substance involved.

Polyatomic ions are always balanced as complete units rather than individual atoms. When balancing equations with ionic compounds, cations are adjusted first, followed by anions. The polyatomic ion maintains its identity and charge throughout the balancing process, ensuring the equation accurately represents the chemical transformation.

Numerical coefficients represent the relative ratios of reactants and products in a chemical reaction. They indicate the smallest whole-number proportions of substances involved and can be interpreted as ratios for any unit, including individual molecules or moles. A coefficient of 1 is typically not shown in the equation.

Subscripts define the chemical identity of a compound and cannot be altered during balancing. Changing subscripts would create different substances entirely, misrepresenting the actual reaction. Only numerical coefficients should be adjusted to balance atom counts while preserving the identity and composition of each reactant and product.