Acids and Bases
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HCl H2O H+(q) + Cl-(aq)
Arrhenius defined bases as substances that dissolve in water to release
hydroxide ions (OH-) into solution. For example, a typical base
according to the Arrhenius definition is sodium hydroxide (NaOH):
NaOH
H2O Na+(aq) + OH-(aq) The Arrhenius definition of acids and bases
explains a number of things. Arrhenius's theory explains why all acids
have similar properties to each other (and, conversely, why all bases
are similar): because all acids release H+ into solution (and all bases
release OH-). The Arrhenius definition also explains Boyle's observation
that acids and bases counteract each other. This idea, that a base can
make an acid weaker, and vice versa, is called neutralization.
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The Brønsted definition of
bases is, however, quite different from the Arrhenius definition. The
Brønsted base is defined as any substance that can accept a hydrogen
ion. In essence, a base is the opposite of an acid. NaOH and KOH, as
we saw above, would still be considered bases because they can accept an
H+ from an acid to form water. However, the Brønsted-Lowry definition
also explains why substances that do not contain OH- can act like bases.
Baking soda (NaHCO3), for example, acts like a base by accepting a
hydrogen ion from an acid as illustrated below:
Acid Base Salt HCl
+ NaHCO3 H2CO3 + NaCl In this example, the carbonic acid formed
(H2CO3) undergoes rapid decomposition to water and gaseous carbon
dioxide, and so the solution bubbles as CO2 gas is released.
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