The H atom balance was upset by this change, but it is easily reestablished by changing the coefficient for the H 2 product to 2. One dozen methane molecules and two dozen oxygen molecules react to yield one dozen carbon dioxide molecules and two dozen water molecules. Comparing the number of \(\ce{H}\) and \(\ce{O}\) atoms on either side of this equation confirms its imbalance: Element Special conditions necessary for a reaction are sometimes designated by writing a word or symbol above or below the equation’s arrow. For example, a reaction carried out by heating may be indicated by the uppercase Greek letter delta (\(\ce{Δ}\)) over the arrow. Given the abundance of water on earth, it stands to reason that a great many chemical reactions take place in aqueous media. When ions are involved in these reactions, the chemical equations may be written with various levels of detail appropriate to their intended use. To illustrate this, consider a reaction between ionic compounds taking place in an aqueous solution. When aqueous solutions of CaCl 2 and AgNO 3 are mixed, a reaction takes place producing aqueous Ca(NO 3) 2 and solid AgCl:

Unlike these three ionic compounds, \(\ce{AgCl}\) does not dissolve in water to a significant extent, as signified by its physical state notation, s. Though nitrogen is balanced, changes in coefficients are needed to balance the number of oxygen atoms. To balance the number of oxygen atoms, a reasonable first attempt would be to change the coefficients for the O 2 and N 2O 5 to integers that will yield 10 O atoms (the least common multiple for the O atom subscripts in these two formulas).

The equation for the reaction between methane and oxygen to yield carbon dioxide and water is confirmed to be balanced per this approach, as shown here: Explicitly representing all dissolved ions results in a complete ionic equation. In this particular case, the formulas for the dissolved ionic compounds are replaced by formulas for their dissociated ions:

This results in seven O atoms on the product side of the equation, an odd number—no integer coefficient can be used with the O 2 reactant to yield an odd number, so a fractional coefficient, is used instead to yield a provisional balanced equation: A balanced chemical equation often may be derived from a qualitative description of some chemical reaction by a fairly simple approach known as balancing by inspection. Consider as an example the decomposition of water to yield molecular hydrogen and oxygen. This process is represented qualitatively by an unbalanced chemical equation: The relative numbers of reactant and product species are represented by coefficients (numbers placed immediately to the left of each formula). A coefficient of 1 is typically omitted.

Following the usual inspection approach, one might first balance C and H atoms by changing the coefficients for the two product species, as shown: The N atom balance has been upset by this change; it is restored by changing the coefficient for the reactant N 2 to 2. These coefficients yield equal numbers of both H and O atoms on the reactant and product sides, and the balanced equation is, therefore: The substances undergoing reaction are called reactants, and their formulas are placed on the left side of the equation. Write a balanced equation for the decomposition of ammonium nitrate to form molecular nitrogen, molecular oxygen, and water. (Hint: Balance oxygen last, since it is present in more than one molecule on the right side of the equation.) Answer