What's the difference between a strong and weak acid?

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The difference between strong and weak acids is that strong acids are likely to fully ionize whereas weak acids are not. Although a common misconception, when used in regards to the strength of an acid, the terms “strong” and “weak” do not refer to the terms “concentrated” and “diluted."  

Acids are compounds that donate hydrogen ions. The amount of ionization of an acid has to do with the stability of an acidic compound and its likelihood to dissociate. The amount of dissociation of protons within an acid has to do with the polarity of the covalent bond that connects hydrogen to another atom (the H-A bond). Polarity has to do with the electronegativity differences between the atoms that share a covalent bond. The more electronegative the conjugate base (A-) within an acid, the more polar the H-A bond, the more likely the hydrogen is to dissociate, and the stronger the acid.                                

Because strong acids are more likely to ionize and dissociate, their dissociation reactions are considered to be irreversible. 

Stronger acids have larger acid dissociation constants (Ka) than weak acids.

All acids have a pH between one and seven. However, when of equal concentrations, strong acids have lower pH’s than weak acids.

Examples of strong acids are hydrochloric acid, sulfuric acid, and hydrobromic acid.

Examples of weak acids are acetic acid and formic acid.

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