having a pH less than 7.0 (contrast with alkaline).

Merriam-Webster Online Dictionary
acid (adjective)
1.
a) sour, sharp, or biting to the taste - an acid flavor
b) sharp, biting, or sour in manner, disposition, or nature - an acid individual an acid personality
c) sharply clear, discerning, or pointed - an acid wit acid criticism
d) piercingly intense and often jarring - acid yellow
2.
a) of, relating to, or being an acid , also having the reactions or characteristics of an acid - acid soil an acid solution
b) of salts and esters derived by partial exchange of replaceable hydrogen - acid sodium carbonate NaHCO3
c) containing or involving the use of an acid (as in manufacture) - an acid bath
d) marked by or resulting from an abnormally high concentration of acid - acid indigestion
3.
relating to or made by a process (as in making steel) in which the furnace is lined with material and an acidic slag is used - acidic
4.
rich in silica - acid rocks
acid (noun)
1.
a sour substance , specifically any of various typically water-soluble and sour compounds that in solution are capable of reacting with a base to form a salt, redden litmus, and have a pH less than 7, that are hydrogen-containing molecules or ions able to give up a proton to a base, or that are substances able to accept an unshared pair of electrons from a base
2.
something incisive, biting, or sarcastic - a social satire dripping with acid
3.
- lsd
acid (Wikipedia)

Zinc, a typical metal, reacting with hydrochloric acid, a typical acid

An acid is a molecule or ion capable of donating a proton (hydrogen ion H+), or, alternatively, capable of forming a covalent bond with an electron pair (a Lewis acid).

The first category of acids is the proton donors or Brønsted–Lowry acids. In the special case of aqueous solutions, proton donors form the hydronium ion H3O+ and are known as Arrhenius acids. Brønsted and Lowry generalized the Arrhenius theory to include non-aqueous solvents. A Brønsted 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 which provide a practical description of an acid. Acids form aqueous solutions with a sour taste, can turn blue litmus red, and react with bases and certain metals (like calcium) to form salts. The word acid is derived from the Latin acidus/acēre meaning sour. An aqueous solution of an acid has a pH less than 7 and is colloquially also referred to as 'acid' (as in 'dissolved in acid'), while the strict definition refers only to the solute. A lower pH means a higher acidity, and thus a higher concentration of positive hydrogen ions in the solution. Chemicals or substances having the property of an acid are said to be acidic.

Common aqueous acids include hydrochloric acid (a solution of hydrogen chloride which is found in gastric acid in the stomach and activates digestive enzymes), acetic acid (vinegar is a dilute aqueous solution of this liquid), sulfuric acid (used in car batteries), and citric acid (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. Strong acids and some concentrated weak acids are corrosive, but there are exceptions such as carboranes and boric acid.

The second category of acids are Lewis acids, which form a covalent bond with an electron pair. An example is boron trifluoride (BF3), whose boron atom has a vacant orbital which can form a covalent bond by sharing a lone pair of electrons on an atom in a base, for example the nitrogen atom in ammonia (NH3). Lewis 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. However, hydrogen chloride, acetic acid, and most other Brønsted-Lowry acids cannot form a covalent bond with an electron pair 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 a Lewis acid.

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