SOAPS AND DETERGENTS std 8 to 10 GSEB CBSE

Preparation of soap

SOAPS AND DETERGENTS

Soaps – preparation of soaps – 

Detergents – 

advantages and disadvantages.

REVIEW

You have learnt the reasons for hardness of water.

Let us recall some of them.

·       Water which readily produces lather with soap is called soft water and that which does not readily produce lather is called hard water.

·       Hard water contains dissolved sulphates, chlorides and bicarbonates of calcium and magnesium.

·       The natural water is generally impure.

·       The polar nature of water makes it an excellent solvent.

  INTRODUCTION

When water is used for washing, it dissolves dirt; but some dirt will not dissolve in water. Soaps and detergents help water to remove dirt. The rod detergent means something which cleans. Soapnut powder has been inuse for about 3,000 years. In India even now some still use soapnut powder as a natural soap to remove oily substance. People have been using soap for a long time.

 SOAPS

Metallic salts prepared from natural fats are called soaps. Soap is a sodium salt or potassium salt of long chain fatty acide that has clensing action in water. Sodium stearate, sodium oliate and sodium plamitate 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 plamitic acid forming sodium oleate (C₁₇H₃₃COONa) and sodium palmitate C₁₇H₃₁ COONa) 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, ginegely oil, animal fat etc are all esters of fatty acids. Fats and fatty oils are called glyceryl esters or glycerides.

While writing the formulae of inorganic csalts. It is the convention to write cation first However cation js written at end in the case of organic salts. Consider the following examples

Sodium chloride -        NaCL

Sodium acetate   -        Ch₃COONa

PRPARATION OF SOAP

Experiment : Take  about 30 ml of a vegetable oil in a beaker. Add about 60 ml of 20% sodium hydroxide solution to it. Heat slowly until the mixture boils. After 5-10 mimutes 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.

When a mixture of a oil/fat and a strong solution of sodium hydroxide is boiled in an iron tank, a sodium salt of the fatty acid and glycerol ane formed. The sodium salt of the fatty acid thus formed is nothing but soap.

Preparation of soap Experiment.

Video  how to make soap by cold process in Laboratory

A Swedish chemist, Carl Wihelm Scheel, discovered accidentally the process of preparation of soap in 1783. He boiled olive oil with lead oxide and obtained a substance which was sweet to taste. This substance is now known as glycerine.

Oil/fat + Sodium hydroxide – soap + glycerol

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, flots 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 cleans process

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 (Ex:CH₃(CH₂)₁₆) and  a short ionic end containing COONA₊. The non-ionic hydrocarbon end attaches to the dirt or grease to form structures called ‘micelles’.

1.    water 2. soap molecule 3. Dirt

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 soapless soaps. Detergents do not from 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.

Let us now consider a method of making detergents. 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 that soap. Therefore. Detergents are more widely used that soap. Let as 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 evening acidic medium. Soap does not cleanse well in such a medium.

Detergents have their disadvantages.

·       Detergents are non bio-degradable. They pollute water and soil.

Points to remember :

·       Metallic salts prepared from natural fats are called soaps.

·       Sodium hydroxide, fatty acid and sodium chloride are the raw materials used in the preparation of soap.

·       Sodium oleate and sodium palmitate are example for soap.

·       Detergents are also called soapless soap.

·       Detergents cleanse well even in hard water.

·       Detergents cleanse well in acid medium.

·       Detergents pollute water and soil.

·       The process of preparing soap using sodium hydroxide or potassium hydroxide is called saponification.