By Patricia Grant Updated Mar 24, 2022
In chemistry, an acid is a substance that imparts distinct properties such as a sour taste, a pH below 7, and the ability to react with bases, metals, and litmus paper. Depending on its strength, an acid can be classified as strong or weak, influencing its reactivity, conductivity, and pH level.
The term “acid” derives from the Latin word acidus, meaning sour. This sensory cue is one of the simplest ways to distinguish acids from other classes of compounds, such as salts and bases. While a few benign acids—like lemon juice and vinegar—are common in households, many industrial acids pose health risks if ingested and should never be tasted.
The pH scale, ranging from 0 to 14, quantifies how acidic or basic a solution is. A pH of 7 denotes neutrality; values below 7 indicate acidity, while those above 7 signify alkalinity. Blue litmus paper turns red upon contact with acidic substances, providing a quick visual test. Bases, in contrast, leave the paper unchanged.
When an acid neutralizes a base, the resulting mixture is typically neutral, often forming water and a salt—especially evident with strong acids and strong bases. Acids also react with many metals to produce metal salts and hydrogen gas, a process that can corrode the metal. While noble metals such as platinum and gold resist this reaction, more common metals like aluminum and zinc readily corrode in acidic environments.
Acids are electrolytes: they conduct electricity when dissolved in water. The degree of conductivity distinguishes strong acids, which dissociate almost completely, from weak acids, which partially ionize. This property underpins their use in batteries and other electrochemical applications.
