SOAPS AND DETERGENTS
Soap and Cleaning Action
- Take 10 mL of water each in two test tubes A and B.
- Add a drop of cooking oil to both test tubes.
- To test tube B, add a few drops of soap solution.
- Shake test tubes vigorously to get unclear mixtures.
- Leave the test tubes undisturbed for some time. The 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 or potassium salts of long-chain carboxylic acids.
The ionic-end (hydrophilic) of soap interacts with water, while the 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.
Effect of Soap in Cleaning
At the surface of water, soap aligns such that its ionic end is in water and the hydrocarbon tail protrudes out of water.
Inside water, these molecules form clusters in which the hydrophobic tails are oriented towards the interior and the ionic ends towards the exterior. This cluster is called a micelle.
The oily dirt is collected in the center of the 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.
Hard Water and Soap
Water containing sulphates, chlorides, or hydrogen carbonates of calcium or magnesium is called hard water. E.g., water from a tube well or hand-pump.
It is difficult to produce foam with 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 rainwater) and 10 mL hard water in separate test tubes (hard water can be prepared by dissolving salts of Ca or Mg in water).
- Add a few drops of soap solution to both and shake well for the same period.
- The test tube with distilled water gets more foam.
- The test tube with hard water gets a white curdy precipitate.
This scum or precipitate is caused by the reaction of soap with the calcium and magnesium salts.
Detergents
Detergents are sodium salts of sulphonic acids or ammonium salts with chlorides or bromides ions, etc.
Both have long hydrocarbon chains. The charged ends of these compounds do not form insoluble precipitates with Ca and 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 the test tube with soap, a curdy precipitate is formed.
Detergents are used to make shampoos and products to clean clothes.