SOAPS AND DETERGENTS
Any substance which has cleansing action in water is called a detergent. It is of two types
- Soaps and
- Synthetic detergents
Soaps are sodium or potassium salts of higher fatty acids such as oleic acid (C17H33COOH), stearic acid (C17H35COOH) and palmitic acid (C15H31COOH). These acids are present in the form of their esters, with glycerol an alcohol containing 3 hydroxyl groups. Soaps are effective cleansing agents only in soft water. Thus, these are not effective in hard water because hard water contains salts of magnesium and calcium. When soap is dissolved in hard water, it reacts with salts of calcium and magnesium to produce water insoluble precipitates called scum. Due to formation of scum the efficiency of soap reduces drastically. So, if we will use soap for washing clothes then a lot of soap will go waste in reacting with salts of magnesium and calcium present in hard water.
Preparation of Soap
Take about 20 ml of castor oil or cottonseed oil in a beaker and add an equal volume of conc. sodium hydroxide in it. Heat the mixture with a continuous stirring for a few minutes, till a thick paste of soap is formed. Now, add 5-10 gm of common salt to it. Stir the mixture and allow it to cool. Ultimately, a pale yellow solid cake will separate out as soap.
The process of making soap by the hydrolysis of fats and oils with sodium hydroxide is called saponification.
Working of Soap
A soap molecule is made up of two parts a long hydrocarbon part and a short ionic part containing – COO-Na+.
Structure Of Soap Molecule
The long hydrocarbon part is hydrophobic (water repelling) in nature, whereas the ionic part of soap is hydrophilic (water loving) in nature. So, when soap is dissolved in water, its hydrocarbon part attaches itself to grease and oil particles whereas its ionic part remains attached to water. Therefore, when dirty clothes are agitated in soap solution then dirt particles sticks to the hydrocarbon part of soap and at the same time the water loving ionic part pulls away this dirt from clothes.
When soap is dissolved in water, the soap molecules cluster together in the form of micelles. In a micelle the hydrocarbon part remains directed towards the centre while the ionic part remains directed outwards.