According to the Arrhenius definition, an acid is a substance that increases the concentration of hydrogen ions, often written as \(H^+\) or \(H_3O^+\), when dissolved in water, as in the dissociation of hydrochloric acid. An Arrhenius base, by contrast, increases the concentration of hydroxide ions, \(OH^-\), in water, as sodium hydroxide does when it dissociates in solution. The pH scale quantifies the acidity or basicity of a solution on a range from 0 to 14, with values below 7 indicating acidic solutions, a value of 7 indicating neutrality, and values above 7 indicating basic, or alkaline, solutions. Mathematically, pH is defined as the negative logarithm of the hydrogen ion concentration, written as \(pH = -\log[H^+]\).
When an acid and a base react, they undergo a neutralization reaction to produce a salt and water, such as in the classic reaction of hydrochloric acid with sodium hydroxide. Buffer solutions resist changes in pH when small amounts of acid or base are added, and they typically consist of a weak acid together with its conjugate base. This buffering capacity is essential in biological systems and in many industrial processes where stable pH conditions are required.
Every chemical reaction also involves energy changes, which are described by the concept of enthalpy, or the total heat content of a system at constant pressure. The change in enthalpy, denoted \(\Delta H\), indicates whether a reaction releases or absorbs heat. Exothermic reactions release heat to the surroundings and have a negative \(\Delta H\), with combustion being a familiar example. Endothermic reactions absorb heat from the surroundings and have a positive \(\Delta H\), as in photosynthesis. For a reaction to proceed, the reactants must overcome an energy barrier called the activation energy, often visualized as the peak in an energy diagram. Several factors influence how fast a reaction proceeds, including the concentration of reactants, temperature, surface area of solids, the presence of catalysts, and the inherent chemical nature of the reacting substances.