d. Properties of Bases. In the same manner that all acids had certain
properties in common, all bases have related properties. The ones that are important to
the medical personnel are as follows:
(1) Bases change red litmus paper to blue. This is just the opposite of the
change which acids cause in litmus paper.
(2)
Bases possess a bitter taste and feel soapy when in contact with the
skin.
(3) Bases react with acids to form salts and water (neutralization reaction).
This is the same type reaction as previously discussed under acids.
e. Classification of Acids and Bases. Even though all acids possess certain
properties in common, as do bases, not all possess them to the same degree. Some
acids, for example, will completely neutralize sodium hydroxide with equal
concentrations while others will only partially neutralize this base. As you might
suspect, the differences in the strengths of acids results from differing abilities to donate
hydrogen ions and the differences in bases from differing abilities to donate hydroxyl
ions or accept hydrogen ions.
(1) Some acids and bases dissociate more readily than others when placed
in solution. Those that dissociate at a rate greater than 50 percent are considered to be
strong acids or bases. Weak acids and bases dissociate at a rate that is less than 50
percent. Examples:
(a) When hydrochloric acid (HCl) is placed in solution, most of the
molecules will dissociate to form free H+ ions and Cl- ions. Hydrochloric acid is
therefore considered a strong acid.
(b) When carbonic acid (H2CO3) is placed in solution, less than 50%
will ionize into free H+ ions and HCO3 - ions. Most of the molecules will remain in
molecular form.
HOH
+
H2CO3 -----> H + HCO3 -----> H2CO3
(2) This means one mole (gram molecular weight) of HCl will produce more
properties to a greater degree than will carbonic acid. A simpler way to say this is that
HCL is a stronger acid than H2CO3.
(3) The same rationale holds for bases as well as acids. Therefore, we can
divide or classify acids or bases into groups based on their dissociation--strong acids or
bases (those that dissociate completely) and weak acids or bases (those that dissociate
to a small degree).
MD0803
2-15
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