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For alternative meanings see acid (disambiguation).

Acids and Bases:
Acid-base reaction theories
Self-ionization of water
Redox reactions
Strong acids
Weak acids
Weak bases
Strong bases

An acid (represented by the generic formula AH) is typically a water-soluble, sour-tasting chemical compound. An acid always has a pH of less than 7. It is a molecule or ion that contains hydrogen or that is able to give up a proton 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.

Table of contents
1 Chemical Characteristics
2 Characteristics
3 Acids in Food
4 Different Definitions of Acid/Base
5 Acid number
6 Neutralization

Chemical Characteristics

In water, there is the following reaction:

There is a distinction between weak acids and strong acids. For a strong acid, no AH remains in solution:

That is why the
acidity constant is only defined for weak acids:

Some of the stronger acids include the hydrohalic acids - HCl, HBr, and HI - and the oxyacids, which tend to contain central atoms in high oxidation states surrounded by oxygen - including HNO3, H2SO4, HClO4.

Acidic (chemistry), the opposite to basic, reacting with basics to form salts. Acidic (geology), of rock: containing more than 65% of silica.


Acids are generally:

Acids in Food

Different Definitions of Acid/Base

The word acid comes from the
Latin acidus meaning sour. Chemically though the term acid has a more specific meaning.

The Swedish chemist Svante Arrhenius defined an acid to be a substance that gave up hydrogen ions (H+) when dissolved in water, while bases are substances that give up hydroxide ions (OH-). Notice that 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. A Lewis acid is an electron acceptor, while a Lewis base is an electron donor. Acid/base systems are different from redox reactions in that there is no change in oxidation state.

Acid number

This is used to quantify oxidation. 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)N56.1/Woil ).

Veq is the amount of titrant (ml) consumed by 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 = 1000WKHP/(204.23Veq).

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 is a type of reaction between an acid and a base. The products include salt and water. So, it is also called a water forming reaction