Defining Acids and Bases
The fundamental chemical difference between an acid and a base is defined by how each substance behaves when dissolved in water. Acids are compounds that increase the concentration of hydrogen ions, represented as $\text{H}^+$, in an aqueous solution. Bases, conversely, are substances that increase the concentration of hydroxide ions, denoted as $\text{OH}^-$, when they dissolve in water.
Acids and bases are classified as either strong or weak, depending on the extent to which they ionize in water. A strong acid, such as hydrochloric acid in your stomach, completely dissociates to release all its $\text{H}^+$ ions into the solution. A weak acid, like the acetic acid in vinegar, only partially ionizes, meaning most of the acid molecules remain intact and release far fewer $\text{H}^+$ ions.
The same principle applies to bases, where a strong base fully dissociates to yield a high concentration of $\text{OH}^-$ ions. The concentration of these ions in a solution dictates the substance’s chemical strength and its ability to react with other compounds. This ionic balance is what chemists use to quantify and compare the properties of all acidic and basic solutions.
The pH Scale as a Measurement Tool
The $\text{pH}$ scale is a measurement system used to quantify the degree of acidity or basicity of an aqueous solution. This scale ranges from 0 to 14 and is an inverse measure of the hydrogen ion concentration. A solution with a $\text{pH}$ below 7 is considered acidic, while a solution with a $\text{pH}$ above 7 is classified as basic, or alkaline.
The neutral point is precisely at $\text{pH}$ 7, which represents a state where the concentrations of $\text{H}^+$ ions and $\text{OH}^-$ ions are perfectly balanced, as is the case with pure water. The scale is logarithmic, which means that a change of just one unit represents a tenfold change in the concentration of $\text{H}^+$ ions. For instance, a solution with a $\text{pH}$ of 3 is ten times more acidic than a solution with a $\text{pH}$ of 4, and one hundred times more acidic than a solution with a $\text{pH}$ of 5.
Distinct Properties of Acids and Bases
Acids and bases can be differentiated by several observable physical and reactive characteristics. Acids are commonly associated with a sour taste. They also react vigorously with certain active metals, such as zinc, producing hydrogen gas as a byproduct of the reaction.
Bases, in contrast, are characterized by a bitter taste and a slippery, soapy feel when touched. A base’s primary chemical function is its ability to neutralize an acid, which occurs when the $\text{H}^+$ ions and $\text{OH}^-$ ions combine to form water.
A simple, visible test to distinguish between the two involves the use of chemical indicators, such as litmus paper. An acid will cause blue litmus paper to turn red, while a base will cause red litmus paper to turn blue. This color change is a reliable, macroscopic manifestation of the microscopic difference in ion concentration.
Acids and Bases in Daily Life
Acids and bases play significant roles in human biology and common household activities. The human digestive system relies on a strong acid, hydrochloric acid, which is secreted in the stomach to break down food. Common acidic household items include vinegar, which contains acetic acid, and citrus fruits that are rich in citric acid.
Bases are just as common, serving purposes from cleaning to medicine. Baking soda, or sodium bicarbonate, is a mild base used as a leavening agent in baking and as a gentle cleaner. Antacids, which contain bases like magnesium hydroxide, are ingested to neutralize the excess stomach acid that causes indigestion.
This concept of neutralization is frequently applied to restore chemical balance. For example, the slightly alkaline nature of toothpaste helps to neutralize the acids produced by bacteria in the mouth that contribute to tooth decay.