10th Chemistry : Soaps and Detergents [Carbon and Its Compound]
SOAPS
Metallic salts prepared from
natural fats are called soaps. Soap is a sodium salt or potasium salt of long
chain fatty acids that has cleansing action in water. Sodium stearate, sodium
oliate and sodium palmitate are some
examples of soaps.
Stearic acid is a long chain
fatty acid. Its formula is C17H35COOH. This reacts with sodium
hydroxide to give a compound called sodium stearate, which has properties of soap.
Similarly, sodium hydroxide
reacts with oleic acid and palmitic acid forming sodium oleate (C17H33COONa)
and sodium palmitate (C17H31COONa) respectively.
Please
Note!
Fatty acids are organic
compounds. The molecules of these compounds contain a carboxyl group {-COOH}. These molecules react with a
trihydric alcohol called glycerol to form a class of compounds called esters.
Groundnut oil, coconut oil,
palm oil, caster oil, gingely oil, animal fat etc are all esters of fatty
acids. Fats and fatty oils are called glyceryl
esters or tglycerides.
PREPARATION
OF SOAP
Experiment : Take about 30
ml of a vegetable oil in a beaker. Add about 60ml of 20% sodium hydroxide
solution to it. Heat slowly until the mixture boils. After 5-10 minutes of
boiling, add about 5g of sodium chloride to separate soap from the solution.
Cool the solution. Observe the creamy layer of soap floating on the solution.
We need animal fat or
vegetable oil, sodium hydroxide and sodium chloride for preparing soap. Fat or
vegetable oil is a glyceride or a glyceryl ester. Soap is prepared by hydrolyzing
fat or oil with bases such as sodium hydroxide or potassium hydroxide. This
process of preparing soap is called saponification.
When a mixture of a oil/fat
and a strong solution of sodium hydroxide is boild in an iron tank, a sodium
salt of the fatty acid and glycerol are formed. The sodium salt of the fatty
acid thus formed is nothing but soap.
Please
Note!
A Swedish chemist, Carl
Wilhelm Scheel, discovered accidentally the process of preparation of soap in
1783. He boiled olive oil with lead oxide and obtained a sweetish substance
which was seat to taste. This substance is now known as glycerine.
Oil/fat
+ sodium hydroxide -----> soap
+ glycerol
Fig-01
Please
Note!
Glycerol
Glycerol is a sweet viscous
liquid soluble in water. It is a byproduct of saponification process. It is
commonly called glycerine. Its formula is CH2OH.CHOH.CH2OH.
It is used in the manufacture of explosives, plastics, and pharmaceuticals. It
is also used to prevent freezing.
Soap thus formed is
separated with the help of sodium chloride. Addition of sodium chloride reduces
the solubility of soap considerable. Soap, being lighter, floats like a cream
on the solution. It is separated from the solution; suitable chemicals of
suitable colour and odour are added to it and cast into moulds. When soap cools
and solidifies, it is cut into desired shapes and packed. Glycerol exists in
dissolved state in the solution. It is separated by distillation.
How
does soap clean
Soap dissolves in water.
This is due to the contrasting properties of the ends of the soap molecules.
A soap molecule has a long
hydrocarbon end [E.g. CH3(CH2)16] and a short
ionic end containing COONa+. The non-ionic hydrocarbon end has the
property to repel water. The other end, which is ionic, has the property to
attract water. This end attaches itself to water. The hydrocarbon end attaches
to the dirt or grease to form structures called ‘micelles’.
Figure-02
In micelles soap molecules
are arranged radially. The end attached to water pulls out the dirt from the
fabric. This can be washed off with water.
DETERGENTS
The term detergent is
derived from a Latin word which means surface active. Synthetic detergents are
commonly known as detergents. Detergents are also called soap less soaps.
Detergents do not form
insoluble ‘scum’ with magnesium and calcium ions present in hard water.
Therefore, detergents clean better than soap even in hard water.
Detergents are sodium salts
of long chain benzene sulphonic acid or sodium salts of long chain alkyl
hydrogen sulphate. A detergent molecule consists of a large hydrocarbon group
that is non-ionic and a sulphonate (SO3- Na+)
or a sulphate (SO4- Na+) group that is ionic. Sodium n-dodecyl benzene
sulphonate and Sodium dodecyl sulphate are two well-known detergents.
Note:
Long chain hydrocarbons obtained
from petroleum are treated with concentrated sulphuric acid. The organic acids
produced during this process are neutralized with sodium hydroxide. The sodium
salt obtained is a detergent.
ADVANTAGES OF DETERGENTS
OVER SOAPS
Detergents have more
advantages than soap. Therefore, detergents are more widely used than soap. Let
us now turn our focus towards the advantages of detergents.
! Detergents cleanse well
even in hard water. Soap is not so good for washing when water is hard.
! Detergents are made from
hydrocarbons obtained from petroleum. Soap is prepared from vegetable oil or
animals fat that are usually edible. Detergents are not bio-degradable. Soaps
are biodegradable.
! Detergents cleanse well
even in acidic medium. Soap does not cleanse well in such a medium.
Detergents
have their disadvanges.
! Detergents are not
bio-degradable. They polutes water and soil.
Total Fatty Matter (TFM)
Soaps are graded in terms of total fatty
matter or TFM. Bureau of Indian Standards (BIS) has catogorised bath or toilet
soaps as ‘normal’, ‘baby, transparent,
and antibacterial soaps. The last three are called specialty soaps targeted to specific users. A toilet soap is a
cosmetic by law and it must fulfil the requirements
of the relevant Indian standard.
T. F. M or total fatty matter is a measure for
identifying the amount of fatty matter present
in soaps. TFM of a sample of soap can be determined as follows. A known weight of the soap is dissolved in water and
the solution is treated with dilute sulphuric
acid. The soap decomposes to sodium suplphate and fatty acids. The fatty acids
so formed can be estimated. From this TFM can be calculated. On the basis TFM, toilet soaps can be classified into
three grades.
Grade
|
TFM
|
Moisture
|
Free
salt (NaCl)
|
||
I
|
above
80
|
Max
:
|
13.5
|
Max
:
|
0.7
|
II
|
65
– 80
|
13-
15
|
0.8
|
||
III
|
55-
65
|
15-20
|
1.5
|
||
Q. Swapnesh, living in Ooty, was
washing clothes in cold water. He found that the clothes were not getting clean. Geeta ,
his niece, suggested that he wash the
clothes in warm water. Washing of clothes with soaps or detergents is easier in Luke warm water than cold water.
(a)Why?
(b) What value do you derive from
this?
Ans:
(a) Action of soaps is due to
formation of micelles. But the formation of micelles takes place at a minimum
temperature called ‘Kraft temperature’.
This temperature is reached in lukewarm water.
(b) Be humble to accept a
scientific fact from a younger person
