Acid

(in beaker) reacting with ammonia fumes to produce ammonium chloride (white smoke).]]

An **acid** is a type of chemical compound. The definition of an acid has changed as people discovered more about chemistry. Acids were originally grouped together by their properties: they taste sour, change the color of litmus paper to red, and make salts when reacted with certain other chemicals.

There are two main definitions of acid used by chemists today. A Brønsted–Lowry acid is a chemical that can donate a hydrogen ion () (generally speaking, this will be a proton) to another substance. A Lewis acid is a chemical that can receive a pair of electrons from another substance. In both cases, the other substance the acid reacts with is called a base.

pH is used to measure the strength of acids that are dissolved in water.

A chemical can donate a proton if the hydrogen atom is attached to an electronegative atom like oxygen, nitrogen, or chlorine. Some acids are strong and others are weak. The weak acids hold on to some of their protons, while the strong acids let go of all of them. All acids will release hydrogen ions into solutions. The amount of ions that get released per molecule will determine if the acid is weak or strong. Weak acids are acids that partially release the hydrogen atoms that are attached. These acids, then, may lower pH by dissociation of hydrogen ions, but not completely. Weak acids generally have a pH value of 4-6 while strong acids have a pH value of 1 to 3.

A base is an acid's "chemical opposite." A base is a substance that will accept the acid's hydrogen atom. Bases are molecules that can  split apart in water and release hydroxide ions.

## How acids work
Acids and bases typically exist together in equilibrium. This means that within a sample of an acid, some molecules will give up their protons and others will accept them. Even water is a mixture of an acidic ion, H3O+ (called a hydronium ion) and a basic ion, OH- (called a hydroxide ion). When acids are diluted (mixed in water) they release H+ ions. These H ions cannot exist independently, so they combine with water to form Hydronium ion.  A hydronium ion will give up its proton to a hydroxide ion, forming two molecules of H2O, which is neutral. This reaction happens continuously in a sample of water, but overall the sample is neutral because there are equal amounts of hydronium and hydroxide in the sample. For most reactions, however, the acids and bases are not present in equal amounts, and this imbalance is what allows a chemical reaction to occur.

Every acid has a conjugate base formed by removing the acid's proton. Hydrochloric acid (HCl), for example, is an acid and its conjugate base is a chlorine anion, or Cl-. An acid and its conjugate base are opposite in strength. Since HCl is a strong acid, Cl- is a weak base.

## Properties 
Acids can have different *strengths*, some are more reactive than others. More reactive acids are often more dangerous.

Acids can have a lot of different properties depending on their molecular structure. Most acids have the following properties:

* taste sour when they are eaten
* Lactic acid, C3H6O3 was found by Carl Wilhelm Scheele in 1780. It can be found in sour milk products, such as Yogurt.
* Sulfuric acid, H2SO4 was probably discovered by Geber. Today it can be found in batteries.

## Related pages
* Alkali
* Folic acid

## References