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Acids and Alkalis

What is the Difference between Strong and Weak Acids?

Acids and alkalis can be described as strong or weak.
This does not mean the same as concentrated or dilute.

The strength of an acid (or alkali) depends on how ionised it is in water.

What is a Strong Acid?

A strong acid is completely (100%) ionised.
An example of a strong acid is hydrochloric acid.
A strong acid has a pH of 1.

hydrogen chloride (in water)  arrow  hydrogen ion  +  chloride ion
HCl(aq)                               arrow     H+(aq)       +          Cl-(aq) 

All of the hydrogen chloride molecules become
hydrogen ions and chloride ions when they
are dissolved in water (see examples of other strong acids).

What is a Weak Acid?

A weak acid is only partly (less than 100%) ionised.
An example of a weak acid is
ethanoic acid.
A weak acid has a pH of 3 to 5.

ethanoic acid (in water)  reversible arrow    hydrogen ion  +   ethanoic ion
reversible arrow       H+(aq)       +      CH3CO2-(aq)

Some of the ethanoic acid molecules become ions in water
but most of them stay as molecules.
The reaction is reversible (shown by the reversible arrow arrow).

See also Concentration and
Reaction Rates of Strong and Weak Acids.

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