CBSE Class 10 Science Practical Skills – Soap Preparation
BASIC BUILDING CONCEPTS
Esters react with base to give alcohol and salt of carboxylic acid. This reaction is known as saponification reaction.
Ester + Base ——> Alcohol + Salt of Carboxylic acid
This reaction is used for making soap.
To study saponification reaction for preparation of soap.
Vegetable oil like castor oil (25 ml), sodium hydroxide pellets, common salt (about 10 g), distilled water, red and blue litmus strips, two beakers (250 ml), two test tubes, glass rod, measuring cylinder (50 ml), a knife, weight box and weighing bottle.
Soaps are sodium or potassium salts of long chain fatty acids. The process in which soap is produced is called saponification. Oils and fats are triesters or triglycerides of fatty acids. When a fat or oil reacts with a base like sodium hydroxide or potassium hydroxide, salts of fatty acids (or soaps) are formed along with glycerol. Glycerol is a. viscous liquid that is widely used in pharmaceutical formulations. Saponification reaction is an exothermic reaction, i.e., heat is liberated during saponification. Addition of sodium chloride causes the precipitation of soap.
- Take about 25 ml vegetable oil in a 250 ml beaker.
- Prepare 50 ml 20% solution of NaOH by weighing 10 g of NaOH and dissolving it in 50 ml of distilled water.
- Add 30 ml of 20% solution of NaOH to the vegetable oil.
- Successively dip the red and blue litmus paper strips into the reaction mixture and note down any change observed in the colour of litmus paper.
- Touch the beaker containing the reaction mixture from outside and note whether it is hot or cold.
- Now, add approximately 10 g of common salt to the reaction mixture and stir using a glass rod till the soap begins to set.
- Leave the mixture undisturbed for one day. This will allow the soap to set and solidify.
- Remove the soap cake, use a knife to cut it into various shapes and sizes.
- colour of red litmus turns blue, whereas the colour of blue litmus does not change.
- Beaker containing reaction mixture gets warm. .
- The medium of soap solution is basic in nature.
- Saponification reaction is an exothermic reaction as heat is released during the course of the reaction.
- The by-product of the saponification reaction is glycerol.
- Soap is a salt of a fatty acid and is precipitated out of the reaction medium by addition of common salt.
- Carefully handle the solution of sodium hydroxide.
- Stir the soap solution carefully.
- Wash your hands properly after the experiment.
What is soap?
Soap is a sodium or potassium salt of long chain fatty acid.
Name the process by which soap is prepared.
The reaction in which oil or fat reacts with base to yield soaps and glycerol is termed as saponification.
Give an equation representing saponification.
ExplainWhat is the use of glycerol?
Glycerol is used for pharmaceutical purposes.
ExplainIs saponification endothermic or exothermic?
ExplainInstead of NaOH can any other base be used for saponification?
Yes, KOH can also be used for saponification.
ExplainWhat is a fatty acid?
A fatty acid is a carboxylic acid which consists of a long unbrahched, saturated or unsaturated aliphatic tail.
Explain Why does the colour of red litmus change in the reaction mixture during preparation of soap?
Red litmus changes to blue indicating a basic medium in which the reaction is occurring.
NCERT LAB MANUAL QUESTIONS
Why does a red litmus paper change its colour when dipped in soap solution? Explain your observation.
Red litmus paper changes its colour when dipped in soap solution because soap solution is basic in nature. Soap is a salt of a strong base and a weak acid and hence, has a pH > 7.
Why is it advised to add common salt while preparing the soap?
Common salt is added while preparing soap as common salt is more soluble in water as compared to soap. As a result solubility of soap in solution decreases and it is precipitated out.
Was heat evolved or absorbed when sodium hydroxide was added to oil?
Heat evolved when sodium hydroxide was added to oil.
What is the chemical reaction involved in the manufacture of soap?
The chemical reaction involved in the manufacture of soap is as follows:
Can you suggest a method to separate glycerine from the reaction mixture?
Glycerine is separated from the reaction mixture by the addition of salt. Solubility of soap in the reaction mixture decreases, soap curdles and floats to the top. It can then be skimmed off from the reaction mixture which would then contain glycerine.
PRACTICAL BASED QUESTIONS
Multiple Choice Questions/VSA (1 Mark)
Soaps are .
(а) sodium salts of fatty acids.
(b) calcium salts of fatty acids.
(c) magnesium salts of mineral acids.
(d) all of these.
The reaction involved in making soap is
The by-product in the saponification reaction is
Apart from sodium hydroxide, the other base which can be used for preparing soap is
(a) calcium hydroxide
(b) potassium hydroxide
(c) magnesium hydroxide
(d) all of these
Soap is formed from
(a) two hydrophobic compounds.
(b) a physical change when fats are heated.
(c) fats by reacting them with alkali.
(d) two hydrophilic compounds.
Sodium hydroxide (NaOH) pellets should be handled very carefully because
(a) they are highly corrosive.
(b) they are very reactive.
(c) they react with moisture/water very vigorously and exothermally.
(d) all of these.
Identify the correct equation involved in making soap.
(a) Fat + Ca(OH)2 ——Δ—> soap + glycerol
(b) Fat + Mg(OH)2 —Δ——> soap + glycerol
(c) Fat + LiOH ——Δ—> soap + glycerol
(d) Fat + NaOH ——Δ—>soap + glycerol
Which of the following is not required for preparing soap in laboratory?
(a) vegetable oil
(b) sodium chloride
(c) sodium hydroxide
(d) none of these
Saponification is a
(a) endothermic reaction
(b) exothermic reaction
(c) physical change
(d) all of these
Saponification is the hydrolysis of an ester under basic conditions to form
(a) alkane and salt of carboxylic acid.
(b) ketone and salt of carboxylic acid.
(c) aldehyde and salt of carboxylic acid.
(d) alcohol and salt of carboxylic acid.
What does the colour change of litmus paper by the reaction mixture in saponification reaction indicate?
(a) The reaction mixture is basic.
(b) The reaction mixture is acidic.
(c) The reaction mixture is first acidic then becomes basic.
(d) The reaction mixture is neutral.
When soap is formed during saponification, a liquid separates out. What is the name of this liquid?
(a) Fatty acid
(b) Ethanoic acid
Why is common salt added to the reaction mixture?
(а) To make the reaction mixture neutral.
(б) To convert soap into its ions.
(c) To make the reaction mixture a good electrolyte.
(d) To facilitate the precipitation of soap.
While studying saponification reaction for the preparation of soap we make certain observations. Select from the following the correct observation about the reaction mixture and the type of reaction:
(а)the reaction mixture is acidic and reaction is exothermic.
(b)the reaction mixture is basic and reaction is exothermic.
(c)the reaction mixture is basic and reaction is endothermic.
(d)the reaction mixture is acidic and reaction is endothermic.
In the following equation representing saponi-fication X and Y are
(a) alcohol and alkali
(b) ester and alkali
(c) alkali and acid
(d) ester and alcohol
Short Answer Questions
1. What is saponification reaction?
2. Why is saponification reaction called exothermic reaction?
3. State the procedure of preparing soap in laboratory.
4. Why is common salt added in soap making?
5. What is the by-product of saponification reaction? When a strip of red litmus paper is dipped into reaction mixture of saponification reaction what change would you observe? What does it show?
Multiple Choice Questions/VSA V
Short Answer Questions
2. During saponification reaction, heat is liberated. 4. Hence, it is called exothermic reaction.
- Take about 25 ml of castor oil in a beaker.
- Add about 30 ml of 20% NaOH solution to oil.
- Heat the mixture with constant stirring.
- Add 10 gm of common salt and stir.
- Cool the mixture to separate out the soap.
4.Common salt is added while preparing soap since common salt is more soluble in water as compared to soap. As a result solubility of soap in solution decreases and it is precipitated out.
5.Glycerol is the by-product of saponification reaction. The red litmus paper turns blue. This shows that reaction mixture of saponification reaction is basic in nature.