An acid is a substance that can donate a proton (a hydrogen ion, H⁺) or increase the concentration of hydrogen ions in an aqueous solution. This is the core idea behind several key definitions of acids in chemistry:
- Arrhenius definition: An acid is a substance that increases the concentration of H⁺ ions when dissolved in water
- Brønsted–Lowry definition: An acid is a proton donor, meaning it can release a hydrogen ion to another molecule or ion
- Lewis definition: An acid is an electron pair acceptor, broadening the concept beyond just proton donation
Acids typically have a sour taste, turn blue litmus paper red, and react with bases to form salts. They can also react with certain metals, such as calcium or iron, to release hydrogen gas
. Common examples of acids include hydrochloric acid (found in stomach acid), sulfuric acid (used in car batteries), acetic acid (vinegar), and citric acid (found in citrus fruits)
. Acids can be strong or weak depending on their ability to dissociate in water. In biological contexts, acids are crucial: hydrochloric acid aids digestion, amino acids build proteins, fatty acids support tissue repair, and nucleic acids like DNA and RNA carry genetic information
. In summary, an acid is fundamentally a molecule or ion capable of donating a hydrogen ion (proton) or increasing hydrogen ion concentration in solution, leading to characteristic chemical behaviors such as sourness, reactivity with bases, and pH less than 7 in aqueous solutions
