A study of chemistry involves the study of chemical changes or, as they are more commonly called, chemical reactions. Examples of chemical reactions are: the combination of elements to form compounds, the decomposition of compounds (such as sodium hydrogen carbonate or mercury(II) oxide), and reactions between compounds, such as the reaction of vinegar (a solution of acetic acid) with baking soda (sodium hydrogen carbonate). Reactions are usually described using chemical equations. Equations may be expressed in words: Mercury(II) oxide decomposes to mercury and oxygen. Using formulas, we state this reaction as:

2 HgO 2 Hg + O₂

A chemical equation has several parts: The reactants are those substances with which we start (here mercury(II) oxide, HgO, is the reactant). The arrow () means "reacts to form" or "yields." The products are those substances formed by the reaction (here mercury and oxygen are the products). The numbers preceding the formulas are called coefficients. Sometimes the physical state of the reaction component is shown; we use a lowercase, italic letter in parentheses following the substance to show its state. For example, if the equation for the decomposition of mercury(II) oxide were written as:

2 HgO(s) 2 Hg(l) + O₂(g)

we would know that the mercury(II) oxide was a solid, the mercury was a liquid, and the oxygen was a gas when the reaction was carried out. The same equation is repeated below with all the parts labeled:

Table 3.7 lists the parts of an equation and the notations commonly used.

TABLE 3.7 Parts of an equation

Reactants	The starting substances, which combine in the reaction. (Formulas must be correct.)
Products	The substances that are formed by the reaction. (Formulas must be correct.)
Arrows
	Found between reactants and products, means "reacts to form."
	Used between reactants and products to show that the equation is not yet balanced.
	Placed after the formula of a product that is a gas.
	Placed after the formula of a product that is an insoluble solid, also called a precipitate.
Physical state	Indicates the physical state of the substance whose formula it follows.
	(g) Indicates that the substance is a gas
	(l) Indicates that the substance is a liquid
	(s) Indicates that the substance is a solid
	(aq) Means that the substance is in aqueous (water) solution
Coefficients	The numbers placed in front of the formulas to balance the equation.
Conditions	Words or symbols placed over or under the horizontal arrow to indicate conditions used to cause the reaction.
	Heat is added hv Light is added elec Electrical energy is added

A. Writing Chemical Equations

A correctly written equation obeys certain rules.

1. The formulas of all reactants and products must be correct.
Correct formulas must be used. An incorrect formula would represent a different substance and therefore completely change the meaning of the equation. For example, the equation

2 H₂O₂ 2 H₂O + O₂

describes the decomposition of hydrogen peroxide. This reaction is quite different from the decomposition of water, which is described by the equation

2 H₂O 2 H₂ + O₂

When an uncombined element occurs in an equation, the guidelines in Section 3.3D (parts 1, 2 and 3) should be used to determine its formula.

2. An equation must be balanced by mass.
An equation is balanced by mass when the number of atoms of each element in the reactants equals the number of atoms of that element in the products. For example, the equation shown for the decomposition of water has four atoms of hydrogen in the two molecules of water on the reactant side and four atoms of hydrogen in the two molecules of hydrogen gas on the product side; therefore, hydrogen is balanced. It has two atoms of oxygen in the two reacting molecules of water and two atoms of oxygen in the single molecule of oxygen produced; therefore, oxygen is also balanced.

2 H₂O 2 H₂ + O₂

four (2 X 2) H atoms on the left = four (2 X 2) H atoms of the right

two (2 X 1) O atoms on the left = two (1 X 2) O atoms on the right

When the atoms are balanced, the mass is balanced and the equation obeys the Law of Conservation of Mass.

You can write and balance equations in three steps:

1. Write the correct formulas of all the reactants. Use a plus sign (+) between the reactants and follow the final reactant with an arrow. After the arrow, write the correct formulas of the products, separating them with plus signs.

2. Count the number of atoms of each element on each side of the equation. Remember that all elements present must appear on both sides of the equation.

3. Change the coefficients as necessary so that the number of atoms of each element on the left side of the equation is the same as that on the right side. Only the coefficients may be changed to balance an equation; the subscripts in a formula must never be changed.