Acid

An acid is a <a href="/facts/Molecule/N9ljSfFq">molecule</a> or <a href="/facts/Ion/3bePpDna">ion</a> capable of either donating a <a href="/facts/Proton/clCjepXP">proton</a> (i.e. hydrogen ion, H+), known as a <a href="/facts/Br%C3%B8nsted%E2%80%93Lowry_acid%E2%80%93base_theory/bqxQnJoX">Brønsted–Lowry acid</a>, or forming a <a href="/facts/Covalent_bond/EzMLf3Pz">covalent bond</a> with an <a href="/facts/Electron_pair/MENP5Gm0">electron pair</a>, known as a <a href="/facts/Lewis_acid/qlLqRvg2">Lewis acid</a>.
The first category of acids are the proton donors, or <a href="/facts/Br%C3%B8nsted%E2%80%93Lowry_acid%E2%80%93base_theory/bqxQnJoX">Brønsted–Lowry acids</a>. In the special case of <a href="/facts/Aqueous_solution/eRNr3MH2">aqueous solutions</a>, proton donors form the <a href="/facts/Hydronium_ion/HzE3BGYJ">hydronium ion</a> H3O+ and are known as <a href="/facts/Acid%E2%80%93base_reaction/z8pbL0X8">Arrhenius acids</a>. <a href="/facts/Johannes_Nicolaus_Br%C3%B8nsted/pw6sL4oc">Brønsted</a> and <a href="/facts/Martin_Lowry/XxL9Ftxo">Lowry</a> generalized the Arrhenius theory to include non-aqueous <a href="/facts/Solvent/LoMPMz8S">solvents</a>. A Brønsted–Lowry or Arrhenius acid usually contains a hydrogen atom bonded to a chemical structure that is still energetically favorable after loss of H+.
Aqueous Arrhenius acids have characteristic properties that provide a practical description of an acid. Acids form aqueous solutions with a sour taste, can turn blue <a href="/facts/Litmus/fHyzn1DW">litmus</a> red, and react with <a href="/facts/Base_(chemistry)/M9wA7qeM">bases</a> and certain metals (like <a href="/facts/Calcium/hf252CC0">calcium</a>) to form <a href="/facts/Salt_(chemistry)/lsSKwGpn">salts</a>. The word acid is derived from the <a href="/facts/Latin/pM3Kge85">Latin</a> acidus, meaning 'sour'. An aqueous solution of an acid has a <a href="/facts/PH/RnSv8D7y">pH</a> less than 7 and is colloquially also referred to as "acid" (as in "dissolved in acid"), while the strict definition refers only to the <a href="/facts/Solute/MGchhMQV">solute</a>. A lower pH means a higher acidity, and thus a higher concentration of <a href="/facts/Hydron_(chemistry)/v5Qkumvm">positive hydrogen ions</a> in the solution. Chemicals or substances having the property of an acid are said to be acidic.
Common aqueous acids include <a href="/facts/Hydrochloric_acid/HqItJlUT">hydrochloric acid</a> (a solution of <a href="/facts/Hydrogen_chloride/7Wzkhomd">hydrogen chloride</a> that is found in <a href="/facts/Gastric_acid/btVCyhYH">gastric acid</a> in the stomach and activates <a href="/facts/Digestive_enzymes/aRYCbJHe">digestive enzymes</a>), <a href="/facts/Acetic_acid/uyBKA2Pd">acetic acid</a> (vinegar is a dilute aqueous solution of this liquid), <a href="/facts/Sulfuric_acid/Wom6fymJ">sulfuric acid</a> (used in <a href="/facts/Car_battery/bCNgs1nc">car batteries</a>), and <a href="/facts/Citric_acid/EgKGQBdl">citric acid</a> (found in citrus fruits). As these examples show, acids (in the colloquial sense) can be solutions or pure substances, and can be derived from acids (in the strict sense) that are solids, liquids, or gases. <a href="/facts/Acid_strength/vJ4rj6Lu">Strong acids</a> and some concentrated weak acids are <a href="/facts/Corrosive_substance/7LXjT1XE">corrosive</a>, but there are exceptions such as <a href="/facts/Carborane/rBGVnA2R">carboranes</a> and <a href="/facts/Boric_acid/0so68XIK">boric acid</a>.
The second category of acids are <a href="/facts/Lewis_acids_and_bases/qlLqRvg2">Lewis acids</a>, which form a covalent bond with an electron pair. An example is <a href="/facts/Boron_trifluoride/2qa0NIH8">boron trifluoride</a> (BF3), whose boron atom has a vacant <a href="/facts/Atomic_orbital/tKhgl6mL">orbital</a> that can form a covalent bond by sharing a lone pair of electrons on an atom in a base, for example the nitrogen atom in <a href="/facts/Ammonia/MtLtJFtz">ammonia</a> (NH3). <a href="/facts/Gilbert_N._Lewis/wWnV6QXL">Lewis</a> considered this as a generalization of the Brønsted definition, so that an acid is a chemical species that accepts electron pairs either directly or by releasing protons (H+) into the solution, which then accept electron pairs. Hydrogen chloride, acetic acid, and most other Brønsted–Lowry acids cannot form a covalent bond with an electron pair, however, and are therefore not Lewis acids. Conversely, many Lewis acids are not Arrhenius or Brønsted–Lowry acids. In modern terminology, an acid is implicitly a Brønsted acid and not a Lewis acid, since chemists almost always refer to a Lewis acid explicitly as such.

Acid open-in-new

Acid