If hydrochloric acid (HCl) is added to water, it dissolves and ionizes, releasing the ions H+ and Cl-:
Because its H+ concentration has increased, such a solution is acidic. Just like the combustion reaction of propane and oxygen (see Figure 2.15), the dissolution of HCl to form its ions is a complete reaction. HCl is therefore called a strong acid.
Acids release H+ ions in solution. HCl is an acid, as is H2SO4 (sulfuric acid). One molecule of sulfuric acid may ionize to yield two H+ and one SO42-. Biological compounds that contain —COOH (the carboxyl group; see Figure 2.20) are also acids (such as acetic acid and pyruvic acid), because
Not all acids dissolve fully in water. For example, if acetic acid is added to water, at the end of the reaction, there are not just the two ions, but some of the original acid as well. Because the reaction is not complete, acetic acid is a weak acid.
Bases accept H+ in solution. Just as with acids, there are strong and weak bases. If NaOH (sodium hydroxide) is added to water, it dissolves and ionizes, releasing OH- and Na+ ions:
Because the concentration of OH- increases and OH- absorbs H+ to form water, such a solution is basic. Because this reaction is complete, NaOH is a strong base.
Weak bases include the bicarbonate ion (HCO3-), which can accept a H+ ion and become carbonic acid (H2CO3), and ammonia (NH3), which can accept a H+ and become an ammonium ion (NH4+). Amino groups (—NH2) in biological molecules can also accept protons, thus acting as bases:
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