# Chemical Reactions

In a chemical reaction, atoms of one or more substances rearrange to form one or more different substances.

The chemical properties of a substance describe how it undergoes chemical reactions. A chemical reaction is a process in which atoms from one or more substances rearrange, resulting in a different substance or substances. Bonds between atoms of substances are broken, and new bonds are formed, resulting in a change in the chemical structure. The occurrence of a chemical reaction depends on factors such as the energies of the bonds and the electron configurations of reactants and products. The beginning substances are the reactants, and the resulting substances are the products. Hence, a chemical reaction is a process in which reactants produce products. Reactants and products can be elements, compounds, or both. Chemical reactions include processes such as burning fuels, extracting metals, preparing medicines, and making glass. In fact, without chemical reactions, life is not possible. Digestion, blood clotting, respiration, and many other life processes are chemical reactions.

There are set standards to follow in communicating chemical reactions. In writing a chemical reaction, reactants are written to the left of an arrow pointing to the right, and products are written to the right of the arrow. Elements and compounds are written using specific symbols and notations, as are their relative quantities, electrical charges, and states. The type of information given depends on the intended purpose and the type of substances involved.

### Chemical Reaction Examples

Reaction Description
${2\rm{Mg}}(s)+{\rm{O}_2}(g)\rightarrow2{\rm{MgO}}(s)$ Magnesium (Mg) is burned in the presence of air, which contains oxygen. Hence magnesium reacts with oxygen in air, forming magnesium oxide (MgO), a new substance. The reaction of a substance with oxygen is called a combustion reaction.
${\rm{CaCO}_3}(s)+{\rm{heat}}\rightarrow{\rm{CaO}}(s)+{\rm{CO}_2}(g)$ When calcium carbonate (CaCO3) is heated, it separates into calcium oxide (CaO) and carbon dioxide (CO2). A reaction that breaks down a substance is called a decomposition reaction.
${4\rm{Fe}}(s)+{3\rm{O}_2}(g)\rightarrow{2\rm{Fe}_2\rm{O}_3}(s)$ Iron (Fe) reacts with oxygen (O2) to form iron oxide (Fe2O3), also known as rust. A reaction that forms a single product is called a synthesis reaction. The iron reacts with oxygen, so it is a combustion reaction.
${\rm{CO}_2}(g)+\rm{H}_2\rm{O}(l)\rightarrow \rm{H}_2 \rm{CO}_3(aq)$ Carbon dioxide (CO2) is dissolved in water (H2O) to form carbonic acid (H2CO3). A single product is formed, which is a synthesis reaction.
${\rm{CO}_2}(g)+{\rm{H}_2\rm{O}}(l)\rightarrow {\rm{H}_2 \rm{CO}_3}(aq)$ Zinc (Zn) reacts with hydrochloric acid (HCl) to produce zinc chloride (ZnCl2). A reaction in which an element reacts with a compound and replaces an element in the compound is known as a substitution reaction or a single displacement reaction.
${\rm{H_2SO_4}}(aq)+{2\rm{KOH}}(aq)\rightarrow{\rm{K_2SO_4}}(aq)+2{\rm{H_2O}}(l)$ Sulfuric acid (H2SO4) reacts with potassium hydroxide (KOH) to produce potassium sulfate (K2SO4) and water. A reaction such as this in which an acid reacts with a base to form a salt and water is known as a neutralization reaction. Notice also that the sulfate ion (SO42–) and the hydroxide ion (OH) switch places. A reaction in which ions switch places in the compounds is known as a double displacement reaction.

Chemical reactions are classified into types based on reactants and products.