Chemical reactions can be described as the rearrangement of atoms to form new substances. The atoms cannot be created or destroyed (conserving mass). We use our observation of changes in temperature, color, smell, and phase to help us determine if a chemical reaction has occurred. All around us, chemical reactions take place: the burning of wood to heat our homes or fuel to propel our cars, the unfortunate rusting of our car bodies, leaves changing color, the formation of acid rain in our atmosphere due to the release of toxic emissions into the air from our factories (the combination of sulfur dioxide and water in air produces sulfuric acid), etc.
Scientists observed similarities chemical reactions, which led to the development of a system for documenting and communicating their observations. First, we use skeleton equations to distinctly identify the reactants (what you have initially) from the products (what you end up with after the reaction is complete). In addition, the skeleton equation also helps to identify the states of the substances involved in the reaction (solid, liquid, gas, or aqueous). Then, we write the chemical formulas for the substances involved in the reaction, substance can be in elemental or compound form. There are two types of chemical compounds: covalent and ionic (Chapters 8 and 9). In many ways, chemistry is very similar to cooking. We learned earlier that elements combine in unique whole number ratios to form compounds. Similarly, compounds combine together in unique whole number ratios to produce other compounds. In order to track where the atoms go, we use balanced chemical equations that describe the ratio of atoms of a given element used in a chemical reaction. In order to balance a chemical equation, we change the coefficients in front of the chemical formulas until the number of atoms of each element is the same on both sides of the equation.
Due to the overwhelming number of chemical reactions that were documented at the time, scientists needed a way of categorizing the reactions they observed. In order to do so, scientists came up with a system that describes all chemical reactions as being part of five categories: direct combination (synthesis reactions), decomposition (analysis reactions), single replacement, double replacement, or combustion. Each category describes a specific way the atoms rearrange to form new substances. In a synthesis reaction, two substances combine to form a single product. The substances that react can be two elements, an element and a compound, or two compounds. In contrast, a decomposition reaction occurs when a single compound breaks down into two or more elements or new compounds. A single replacement reaction occurs when the atoms of one element replace the atoms of another element in a compound. An activity series is used to predict whether a single replacement reaction can occur. Similarly, a double replacement reaction involves the exchange of cations between two compounds. Our last category, combustion, involves the reaction of a substance (usually hydro-carbons i.e. compounds containing hydrogen and carbon) with oxygen gas (O2) to produce oxides (usually carbon dioxide and water), heat and light.
Many solids will not react with each other unless one or both are dissolved in water. Knowing the phase of the substance will help you determine if the reaction will occur. Ionic compounds are solids under normal conditions. When these solids are dissolved in water they form ions. In addition, some, not all, molecular compounds will form ions when dissolved in water. The dissolution of compounds into water helps chemical reactions take place. In order to document these reactions, ionic equations are used to show how all the particles in a solution exist. More specifically, a net ionic equation is an ionic equation that includes ONLY the ions that participate in the reaction.