# Chemical Reactions and Chemical Equations

Chemical reactions are processes in which the atoms of some substances are rearranged to obtain new, different substances.

A chemical reaction is a process in which atoms from one or more substances rearrange, resulting in a different substance or substances. Chemical reactions involve breaking chemical bonds and forming new ones. A substance that reacts in a chemical reaction is a reactant. A substance that is formed during a chemical reaction is a product. During a chemical reaction, the products are formed from the reactants. To study chemical reactions, scientists categorize them into various classes.

A chemical reaction involves a rearrangement of valence electrons. A valence electron is an electron in the outermost shell of an atom. The nucleus, which contains protons and neutrons, is not affected. Electrons that are not in the valence shell (outermost shell) are also not affected. Furthermore, if there is no rearrangement of electrons, a chemical reaction does not occur. For example, a change of phase, such as melting or freezing, does not affect the arrangement of electrons. A phase change is a physical change, not a chemical reaction.

Consider the chemical reaction that occurs when a magnesium ribbon is heated in a flame. The magnesium burns brightly, forming a white powder, which is magnesium oxide. Chemical reactions are represented by equations in which an arrow points from the left (the reactant side) to the right (the product side). This representation of a chemical reaction in the form of symbols is a chemical equation.
${\rm{Mg}}(s)+{\rm{O_2}}(g)\rightarrow{\rm{2MgO}}(s)+\rm{heat}$

In this reaction magnesium and oxygen are the reactants, and magnesium oxide is the product. The equation shows that the reaction of magnesium and oxygen releases energy in the form of heat. A reaction in which energy is released in the form of heat or light is an exothermic reaction. In this chemical reaction, magnesium metal reacts with oxygen to produce the compound magnesium oxide.

A reaction in which energy is absorbed in the form of heat or light is an endothermic reaction.

When writing chemical reactions, elements are often represented by their symbols. For example, when hydrochloric acid is dripped onto a block of zinc, zinc chloride, a white salt, forms, and hydrogen gas is produced. This reaction can be represented by both words and by a chemical equation:
$\begin{gathered}\text{Zinc}\;&+&\;\text{Hydrochloric}\;\text{acid}\;&\rightarrow\;&\text{Zinc}\;\text{chloride}\;&+&\;\text{Hydrogen}\;\text{gas}\;\\{\rm{Zn}}(s)&+&\;{\rm{HCl}}({aq})&\rightarrow\;&{\rm{ZnCl}_2}({aq})&+&{\rm H_2}(g)\; (\text{unbalanced})\end{gathered}$

Note that the chemical equation is not balanced, because the number of chlorine (Cl) and hydrogen (H) atoms on the left and right sides of the equation are not equivalent. A balanced equation describes a chemical reaction in which the number of atoms of each element involved in the reaction and the total charge are balanced. The letters in parentheses following each reactant and product indicate their phase. The phases include (s) for solid, (l) for liquid, (g) for gas, and (aq) representing a solution in water. The role of the phase does not affect the outcome of balancing chemical reactions.

The example of the reaction between zinc and hydrochloric acid is a molecular equation. In a molecular equation, an ionic compound, such as zinc chloride (ZnCl2), is shown as a molecular formula and not as ions.