What makes an acid an acid?

Answer 1

Definitions of acids and bases
The word "acid" comes from the Latin acidus meaning "sour," but in chemistry the term acid has a more specific meaning. There are three common ways to define an acid, namely, the Arrhenius,
the Brønsted-Lowry
and the Lewis definitions, in order of increasing generality.

Arrhenius: According to this definition, an acid is a substance that increases the concentration of hydronium ion (H3O+) when dissolved in water, while bases are substances that increase the concentration of hydroxide ions (OH-). This definition limits acids and bases to substances that can dissolve in water.

Around 1800, many French chemists, including Antoine Lavoisier, incorrectly believed that all acids contained oxygen. Indeed the modern German word for Oxygen is Sauerstoff (lit. sour substance). English chemists, including Sir Humphry Davy at the same time believed all acids contained hydrogen. The Swedish chemist Svante Arrhenius used this belief to develop this definition of acid.

Brønsted-Lowry: According to this definition, an acid is a proton (hydrogen nucleus) donor and a base is a proton (hydrogen nucleus) acceptor. The acid is said to be dissociated after the proton is donated. An acid and the corresponding base are referred to as conjugate acid-base pairs. Brønsted and Lowry formulated this definition, which includes water-insoluble substances not in the Arrhenius definition.

Lewis: According to this definition, an acid is an electron-pair acceptor and a base is an electron-pair donor. (These are frequently referred to as "Lewis acids" and "Lewis bases," and are electrophiles and nucleophiles, respectively, in organic chemistry; Lewis bases are also ligands in coordination chemistry.)

Lewis acids include substances with no transferable protons (ie H+ hydrogen ions), such as iron(III) chloride, and hence the Lewis definition of an acid has wider application than the Brønsted-Lowry definition. The Lewis definition can also be explained with molecular orbital theory.

In general, an acid can receive an electron pair in its lowest unoccupied orbital (LUMO) from the highest occupied orbital (HOMO) of a base. That is, the HOMO from the base and the LUMO from the acid combine to a bonding molecular orbital. This definition was developed by Gilbert N. Lewis.

Although not the most general theory, the Brønsted-Lowry definition is the most widely used definition. The strength of an acid may be understood by this definition by the stability of hydronium and the solvated conjugate base upon dissociation. Increasing stability of the conjugate base will increase the acidity of a compound.

This concept of acidity is used frequently for organic acids such as carboxylic acid. The molecular orbital description, where the unfilled proton orbital overlaps with a lone pair, is connected to the Lewis definition.

Solutions of weak acids and salts of their conjugate bases form buffer solutions.

Acid/base systems are different from redox reactions in that there is no change in oxidation state.


Properties
Generally, acids have the following properties:

Taste: Acids generally are sour when dissolved in water.

Touch: Acids produce a stinging feeling, particularly strong acids.

Reactivity: Acids react aggressively with or corrode most metals.

Electrical conductivity: Acids, while not normally ionic, are electrolytes.

Strong acids and most concentrated acids are dangerous, causing severe burns for even minor contact. Generally, acid burns are treated by rinsing the affected area abundantly with running water (15 minutes) and followed up with immediate medical attention. I

n the case of highly concentrated acids, the acid should first be wiped off as much as possible, otherwise the reaction of the acid dissolving in the water could cause severe thermal burns. In addition to dangers from the acidity, even dilute solutions of weak acids may also be dangerous, due to toxic or other effects of the ions involved.

Answer 2

Okay, there are three ways of looking at "what is an acid", and they're all very broad. They overlap alot, too.

First, there's the Arrhenius acid, which says that an acid tends to increase the amount of H+ when it's dissolved in water.
HCl --> H+ + Cl-
Hydrochloric acid is the acid.

Then there's the Bronsted-Lowry concept, which defines an acid as a proton (H+) donor.
HC2H3O2 + H2O --> H3O+ + C2H3O2-
The acetic acid is the acid.

Lastly, there's the Lewis definition of an acid--an acid is an electron acceptor. That's hard to show without dot diagrams, though.

I hope I helped!

Answer 3

There are three broad definitions of acid.

1. Arrhenius: this is the "pH" definition. Such acids, in aqueous solution, dissociate to form H3O+ (hydronium) ions.

2. Bronsted-Lowry: An acid is a proton (hydrogen nucleus) donor and a base is a proton (hydrogen nucleus) acceptor. Arrhenius acids are also B-L acids. However, B-L also includes acids that are not water soluble.

3. Lewis: An acid is an electron-pair acceptor and a base is an electron-pair donor.

http://en.wikipedia.org/wiki/Acid#Definitions_of_acids_and_bases

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Answer 4

An acid is a compound that donates a proton (hydrogen nucleus) during a chemical reaction. Also, it is a compound that produces a ph <7 when placed in water.

Answer 5

I think an acid is an acid because it's got hydrogen in it. A base has OH (hydroxide) in it.

Answer 6

It's ability to accept an electron pair in a reaction.

Answer 7

The pH level

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Answer 8

It has something to do with the pH scale.