Chemistry Reference
In-Depth Information
With very few exceptions, naturally occurring acids and alkalies are
weak. All acids known in antiquity were of organic origin; some occur in
fruits, especially in unripe fruitjuices. Most ancient alkalies were derived
from the ash of plants such as barilla,
Salsola soda
and
Salsola kali
(Russian
thistle), and kelp.
Properties of Acids and Bases
All
acid solutions
taste sour and are more or less corrosive and chemically
quite reactive; they react with most metals, many of which are corroded
and dissolved by acids.
Alkaline solutions
, also chemically reactive, are
caustic
(they burn or corrode organic tissues), taste bitter, and feel slippery
to the touch. Both acids and bases change the color of
indicators
(sub-
stances that change color, hue, or shade depending on whether they are
in an acid or basic environment).
Neutral Solutions: Salts
When an acid solution is mixed with an alkaline solution in the appropri-
ate quantities, the two solutions are said to neutralize each other: the
hydrogen (H
+
) cations of the acid and the hydroxyl (OH
−
) anions of the base
combine to form water molecules (HOH or H
2
O), canceling the (acid or
alkaline) properties of the other. The term
neutralization
is used to refer
to this reaction because the acid and basic properties of the two solutions
are
neutralized
and the solution is
neutral
, neither acid nor basic:
Acid solution
+
alkaline solution
→
water
+
salt
neutralization
Neutral solutions also contain a dissolved
salt
, derived from the neutral-
ization of the acid and the base (a
salt
is an ionic compound formed in a
neutralization reaction and is composed of the cation of an alkali and the
anion of an acid). When a solution of carbonic acid (formed when atmos-
pheric carbon dioxide dissolves in water), for example, reacts with an alka-
line solution of lime, the two solutions neutralize each other and form a
salt, calcium carbonate:
(
)
2H CO
+
Ca OH
→
CaCO
+
HO
2
3
3
2
carbonic acid
acid
lime
alkali
calcium carbonate
salt
water
(
)
(
)
(
)
pH
The acidity and alkalinity of water solutions and, therefore, differences in
their acidity or alkalinity, can be quantified and assigned numerical values.
One way of doing this is to express the concentration of hydrogen ions in
solutions on a numerical scale. Such a scale is provided by the widely
accepted
pH scale
, in which the strength or weakness of acid or alkaline
