A strong acid is an acidic compound which ionizes completely in an aqueous solution.
HA(aq) 
H+(aq) + A-(aq)
pH is the measure of acidity. It is defined for aqueous solutions as the -log10[H+], the negative of the log10 of the concentration of hydrogen (or more accurately, hydronium H3O+) ions in solution. Pure water at Standard temperature and pressure has an equilibrium concentration of one hydronium ion (and therefore one hydroxide ion) per 10,000,000 water molecules (M = 10-7 ) and thus has a pH of 7.
Acidic solutions have a pH lower than 7.
Basic solutions have a pH higher than 7.
Examples of strong acids (in order from strongest to weakest):
The strength of strong acids in aqueous solutions is equivalent at equal concentrations because the concentration of hydrogen ions is always equal to the concentration of the acid. Only through replacing the solvent with a more acidic one, is it possible to observe and measure the differences in proton dissociation among the strong acids.
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