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3. pH
of a solution
To
understand if a given solution is acidic or not, H+ ion
concentration is generally measured. This is called the pH
of a solution. It has been found experimentally that in neutral water, the
concentration of H+
and (OH)-
is 10-7 grams ions per liter. For acidic solution, the
concentration of H+ ions has to be > 10-7 grams
ions per liter, for an alkaline solution or for a base, the (OH)-
ion concentration has to be < 10-7 grams ions per
liter. To decide the acidic or alkaline nature of a solution, it is
necessary to express the hydrogen ion concentration on a more convenient
scale. Such a scale was suggested in 1909 (by Sorensen) and is known as the pH scale. In the symbol pH,
p stands for potenz, meaning
strength (or power) and pH
indicates the strength of the hydrogen ion concentration expressed in
gram-ions per liter.
The pH of a solution is
defined as the negative of the exponent or power of 10 expressing the H+
ion concentration.
Thus
[H+ ]
= 10- pH
(concentration is shown as a boxed bracket)
log 10[H+
] = -
pH
pH
of a solution is also defined as the negative logarithm of H+ ion
concentration.
In
a neutral solution pH is 7. For an
acidic solution pH is between 1
and 7. For an alkaline solution, pH
is between 7 and 14.
The pH
scale
4.
Neutralization and salts
If
the number of H+ and (OH)- ions are equal in a
solution, then the solution is called as a neutral
solution. Water, H2O is a neutral solution as the number of H+
and (OH)- ions in it are equal. When acids and bases react with
each other, they nullify the effects of acidity and alkalinity in a
solution and the process is known as neutralization.
In neutralization, salt and water are the byproducts. Salt in chemistry is a
term that denotes all compounds whose positive radical is derived from a
base and the negative radical is derived from an acid. In addition to the
salt, the H+
ion from acid and (OH)-
radical (or ion) from a base form neutral non-ionised water.
NaOH
Na+
+ (OH)-
(ionization)
HCl
H+ +
Cl-
(ionization)
NaOH
+ HCl
Na+
+ (OH)- + H+
+ Cl -
NaCl
+ H2O
(neutralisation)
Table
below gives some examples of salts.
|
Salt
|
Chemical
Formula
|
Derived
From
Base
Acid
|
|
Sodium
Chloride
|
NaCl
Na+
Cl-
|
NaOH
HCl
Na+
Cl- |
| Copper
Sulphate |
CuSO4
Cu++
SO4--
|
Cu(OH)2
H2 SO4
Cu++
SO4-- |
| Calcium
Nitrate |
Ca(NO3)
2
Ca++(NO)
3-
|
Ca(OH)2
H(NO)3
Ca++
NO 3- |
Summary
We
have seen in this chapter what are acids, bases and salts. We have also seen
how we can find the strengths of acids and alkalis by determining the pH
of a solution. When acids and alkalis react together, they neutralize each
other and produce salt and water.
Acids and Alkalis in
Our Everyday Lives
In our daily
lives we come across many compounds which are acids and alkalis.
Citric acid is responsible for the sour taste in lemons and oranges.
Acetic acid is commonly known as vinegar that we use in cooking. All
forms of soaps contain sodium hydroxide which is an alkali.
When
you chop an onion, you break open some of the onion's cells. This
releases enzymes which act on one of the flavoring components of
onions, allicin. Allicin is a compound containing sulphur. This
volatile irritant "fumes" up from the onion and irritates
your eyes. The stinging sensation is because of the sulphuric acid
produced when allicin dissolves in the water of your eyes.
Our stomachs
have hydrochloric acid which is needed for breaking down ingested
food. Our stomach lining is thick and is immune to the effects of
the hydrochloric acid, but occasionally this acid spills into the
esophagus. The lining of the esophagus is not as thick as the
stomach lining. So when the stomach acid touches esophagus, we get a
burning sensation. When you have stomach burns, you take a medicine
called antacid. Antacid is a chemical that has aluminum hydroxide.
This alkali neutralizes the excess acid in your stomach.
Baking powder is
actually sodium bicarbonate and is alkaline. On heating, it
degenerates and produces carbon dioxide. This is the reason why
sodium bicarbonate is used in bakery products; on heating the carbon
dioxide is released. This makes the cake ãriseä. It is also the
same soda that is in most of the soft drinks. |
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