- For alternative meanings see acid (disambiguation).
An acid (represented by the generic formula AH) is typically a water-soluble, sour-tasting chemical compound. In common usage an acid is a species that, when dissolved in water, gives a solution with a pH of less than 7. In general scientific usage an acid is a molecule or ion that is able to give up a proton (H+ ion) to a base, or accept an unshared pair of electrons from a base. An acid reacts with a base in a neutralization reaction to form a salt.
Chemical characteristics In water the following reaction occurs between an acid (AH) and water, which acts as a base:  The acidity constant is the equilibrium constant for the reaction of AH with water: ![Ka = {[A^-] cdot[ mbox{H}_3 mbox{O}^+] over [AH]}](http://en.wikipedia.org/math/06a17b5e79be3f9e37353990db6f3c0f.png) Strong acids have large Ka values (i.e. the reaction equilibrium lies far to the right, lots of H3O+ present; the acid is almost completely dissociated). For example, the Ka value for hydrochloric acid (HCl) is 107.
Weak acids have small Ka values (i.e. at equilibrium significant amounts of AH and A- exist together in solution; modest levels of H3O+ are present; the acid is only partially dissociated). For example, the Ka value for acetic acid is 1.8 x 10-5.
Strong acids include the hydrohalic acids - HCl, HBr, and HI. (However, hydrofluoric acid, HF, is realatively weak.) Oxyacids, which tend to contain central atoms in high oxidation states surrounded by oxygen, are also quite strong and include HNO3, H2SO4, HClO4. Most organic acids are weak acids.
A few clarifications:
- The terms "hydrogen ion" and "proton" are used interchangeble- both refer to H+.
- In chemical equations H+ is often written, although in water it will actually be H3O+.
- The strength of an acid is measured by its Ka value. pH measures how many hydrogen ions are present, which depends on both the type of acid (or base) and how much is there.
- Acid strength is also defined by pKa=-log(Ka).
Characteristics Acids are generally: - Taste: sour when dissolved in water
- Touch: strong acids have a stinging feeling
- Reactivity: acids react aggresively with many metals
- Electrical conductivity: acids are electrolytes
Different definitions of acid/base The word acid comes from the Latin acidus meaning sour. In chemistry the term acid has a more specific meaning.
The Swedish chemist Svante Arrhenius defined an acid to be a substance that gives up hydrogen ions (H+) when dissolved in water, while bases are substances that give up hydroxide ions (OH-). This definition limits acids and bases to substances that can dissolve in water. Later on, Bronsted and Lowry defined an acid to be a proton donor and a base to be a proton acceptor. In this definition, even substances that are insoluble in water can be acids and bases. The most general definition of acids and bases is the Lewis definition, given by the American chemist Gilbert N. Lewis. Lewis theory defines a "Lewis acid" as an electron-pair acceptor and a "Lewis base" as an electron-pair donor. It can include acids that do not contain any hydrogen atoms, such as iron(III) chloride. Acid/base systems are different from redox reactions in that there is no change in oxidation state. 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.
The Bronsted-Lowry definition, where an acid is treated as a proton donor, is sufficient for many situations. In this case, the proton (H+) is the actual acid and the acidity of the proton-donating-compound, such as an organic acid, is determined by its stability when it donates protons to the solution it is embedded in. So if the organic acid likes letting protons go, it has high acidity because it donates protons with empty molecular orbitals to the solution. This is how organic acids such as carboxylic acids work, here the Brønsted definition is nice for calculations while the Lewis definition is good for understanding.
Acid number This is used to quantify the amount of acid present, for example in a sample of biodiesel. It is the quantity of base, expressed in milligrams of potassium hydroxide, that is required to neutralize the acidic constituents in 1 g of sample.
AN = (Veq-beq)×N×56.1/Woil.
Veq is the amount of titrant (ml) consumed by the crude oil sample and 1ml spiking solution at the equivalent point, and beqbeq is the amount of titrant (ml) consumed by 1ml spiking solution at the equivalent point.
The molarity concentration of titrant (N) is calculated as such: N = 1000×WKHP/(204.23×Veq).
In which, WKHP is the amount (g) of KHP in 50ml of KHP standard solution, and Veq is the amount of titrant (ml) consumed by 50ml KHP standard solution at the equivalent point.
Acid number (mgKOH/g oil) for biodiesel is preferred to be lower than 3.
Neutralization Neutralization is a type of reaction between an acid and a base. The products include a salt and water. So, it is also called a water forming reaction 
Example: 
This type of reaction forms the basis of titration methods for analysing acids, where a pH indicator shows the point of neutralization.
Common acids
Strong inorganic acids
Weak inorganic acids
Weak organic acids
Acids in food
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