Evolution of a Chemical Equation

If a piece of sodium is dropped into water, a vigorous reaction occurs (Fig. 8.7). A full qualitative description of the chemical change is solid sodium plus liquid water 

The (s) after the symbol of sodium indicates it is a solid. Similarly, the ( ) after H2O and the (g) after H2 show that they are a liquid and a gas, respectively. When a substance is dissolved in water, the mixture is an aqueous solution and is identified by (aq). Aqueous comes from the Latin aqua for water.) These state symbols are sometimes omitted when writing equations, but they are included in most of the equations in this book. They are discussed in more detail in Chapter 9. We suggest that you use or not use them at this time according to the directions of your instmctor. State symbols are summarized in Table 8.2. 

Nearly all chemical reactions involve some transfer of energy, usually in the form of heat. Generally, we omit energy terms from equations unless there is a specific reason for including them. 

The Law of Conservation of Mass (Section 2.9) says that the total mass of the products of a reaction is the same as the total mass of the reactants. Atomic theory explains this by saying that atoms involved in a chemical change are neither created nor destroyed, but simply rearranged. Equation 8.1 does not satisfy this condition. There are two hydrogen atoms in HjO in the left side of the equation, but three atoms of hydrogen on the right—two in Hj and one in NaOH. At this point, the equation is only a qualitative description of the reaction. The equation is not balanced. 

An equation is balanced by placing a coefficient in front of one or more of the formulas, indicating that it is used more than once. Once balanced, an equation is both a qualitative and a quantitative description of the reaction. 

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Evidence of a Chemical Change Evolution of a Chemical Equation... 

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