What is the difference between a strong acid and a weak acid? 

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A strong acid is one that completely ionizes in solution to produce hydrogen ions and weak acid is one that shows less than 100% ionization in solution.

The strength of an acid isn't the same thing as its concentration. A strong acid can be either concentrated or dilute, but a concentrated strong acid will have more hydrogen ions than a weak acid of the same volume and concentration.

The strong acids are:

hydrochloric acid, `HCl`

hydroiodic acid, `HI`

hydrobromic acid, `HBr`

nitric acid, `HNO_3`

sulfuric acid, `H_2SO_4`

perchloric acid, `HClO_4`

Some common weak acids are:

acetic acid, `HC_2H_3O_2`

carbonic acid,` H_2CO_3`

phosphoric acid, `H_3PO_4`

nitrous acid, `HNO_2`

When these weak acids dissolve in water most of the acid molecules stay intact and only a small percentage ionize.

When an acid ionizes in water it produces the hydronium ion, `H_3O^+` , which is also represented as the hydrogen ion `H+` . Here's an example of ionization of HCl shown both ways: 

`HCl + H_2O -gt H_3O^+ + Cl^-`

`HCl -gt H^+ + Cl^-`


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