4. CARBON AND ITS COMPOUNDS
SOAPS AND DETERGENTS
Take 10 mL of water each in two test tubes A & B.
Add a drop of
cooking oil to both the test tubes.
To the test tube
B, add few drops of soap solution.
Shake test tubes
vigorously to get unclear mixtures.
Leave the test
tubes undisturbed for some time. Oil layer separates out in both test tubes. But
this happens first in test tube A.
This activity
demonstrates the effect of soap in cleaning.
Most dirt is oily
and does not dissolve in water.
Soap molecules
are sodium / potassium salts of long-chain carboxylic acids.
Ionic-end (hydrophilic) of
soap interacts with water while carbon chain (hydrophobic tail) interacts
with oil. The soap molecules, thus form structures called micelles. In
this, one end of the molecules is towards the oil droplet and the ionic-end
faces outside. It forms an emulsion in water. The soap micelle thus
helps in pulling out the dirt in water and clothes become clean.
At the surface of water, soap aligns such that its ionic end is in water and hydrocarbon tail protrude out of water.
Inside water, these molecules form clusters in which the hydrophobic tails are oriented towards interior and the ionic ends towards exterior. This cluster is called a micelle.
The oily dirt is collected in the centre of micelle. The micelles stay as a colloid and will not come together to precipitate because of ion-ion repulsion. So, the dirt in micelles is easily rinsed away.
The soap micelles are large enough to scatter light. Hence a soap solution appears cloudy.
The water containing sulphates/ chlorides/ hydrogen carbonates of calcium or magnesium is called hard water. E.g. Water from tube well or hand-pump.
It is difficult
to produce foam by soaps in hard water. So bathing and washing become difficult.
After washing, an insoluble substance (scum) remains in hard water. It
can be demonstrated by the following experiment:
- Take 10 mL distilled water (or rain water) and 10 mL hard water in separate test tubes (hard water can be prepared by dissolving salts of Ca or Mg in water).
- Add few drops of soap solution to both and shake well for same period.
- Test tube with distilled water gets more foam.
- Test tube with hard water gets white curdy precipitate.
This scum or
precipitate is caused by the reaction of soap with the calcium and magnesium
salts.
Detergents
Both have long
hydrocarbon chain. Charged ends of these compounds do not form insoluble
precipitates with Ca & Mg ions. Thus, they are also effective in hard
water.
It can be demonstrated
by the following experiment:
- Take two test tubes with 10 mL hard water in each.
- Add five drops of soap solution to one and five drops of detergent solution to the other.
- Shake both test tubes for the same period.
- The test tube with detergent gets more foam.
- In test tube with soap, curdy precipitate is formed.
Detergents are used to make shampoos and products to clean clothes.
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