This lab was performed to determine the enthalpy of a neutralization reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH) in both solid and aqueous form, thereby showing an enthalpy difference depending on the state of matter. A neutralization reaction is one that takes place between an acidic substance and a basic substance. The products of this type of reaction will yield an ionic salt and in many cases water. In order for this type of reaction to take place and form these types of products, an exothermic process will typically occur. In some cases however, such as between a weak acid, or base, (or both), the process can be endothermic. An exothermic process is one in which the products are more stable than the reactants because the energy required to make their bonds is less than the energy required to break the bonds of the reactants. The opposite is true for an endothermic reaction, in which the reactants are more stable than the products because it takes more energy to break the bonds of the reactants than those of the products. The Law of Conservation of Energy states that energy cannot be created or destroyed. Through the application of this law to the lab, it can be determined that energy must be preserved in an isolated chemical system (a chemical system which does not allow the escape of energy or chemicals). With this established, a relationship between a chemical system and its surroundings can be established; Heat Lost = Heat Gained, or more mathematically, ΔH = -ΔH. Using calorimetry, the heat energy (enthalpy) of a neutralization reaction can be calculated.