Viva Voce: Soap Preparation | Lab Manuals for Class 10 PDF Download

Q.1. Why is it advised to add common salt while preparing the soap?

Common salt is added while preparing the soap, to precipitate out all the soap from the aqueous solution. When we add common salt to the solution, then the solubility of soap present in it decreases, due to which all the soap separated out from the solution in the form of a solid.

Q.2. Was heat evolved or absorbed when sodium hydroxide was added to oil?

When sodium hydroxide was added to oil, then heat evolved during the reaction. It is an exothermic reaction.

Q.3. Can you devise a method to separate glycerine from the reaction mixture?

Common salt is added to precipitate out all the soap from the aqueous solution. The solution left after the removal of soap is called lye, it contains glycerol (glycerine) which can be obtained by fractional distillation under low pressure.

Q.4. During commercial preparation of soap, fillers are added to it. Explain.

For commercial preparation of soap, fillers are added as it hardens the soap and make the cutting of soap easy.

Q.5. The reaction of castor oil with sodium hydroxide solution produces a soap molecule. Name the chemical composition of this soap molecule.

Castor oil contains glyceryl oleate. Thus, the soap molecule prepared from the reaction of castor oil with sodium hydroxide solution is sodium oleate.

Q.6. What are esters?

Esters are organic compounds having general formula
e.g. CH₃-COOC₂H₅ (Ethyl ethanoate)

Q.7. Apart from sodium hydroxide, name the other base which can be used for preparing soap.

Potassium hydroxide

Q.8. What are glycerides?

Esters of higher fatty acids formed with glycerol are called glycerides.

Q.9. How the temperature of reaction mixture vary when we add sodium hydroxide solution to oil?

The temperature will increase on adding sodium hydroxide solution to oil.

Q.10. Why does soap solution appears cloudy?

Soaps micelles scatter light because of their large size.

Q.11. What will be the product formed, when groundnut oil is Heated with caustic soda solution?

A soap is formed.

Q.12. Name the raw materials used in soap industry.

Raw materials used in soap industry are:

• Vegetable oil
• Caustic soda
• Common salt

Q.13. What is the use of glycerol?

Glycerol is used for pharmaceutical industry.

Q.14. What is a fatty acid?

A fatty acid is a carboxylic acid which consists of a long unbranched, saturated or unsaturated aliphatic chain.

Q.15. Give two examples of soap.

Sodium palmitate (C₁₅H₃₁COONa) and sodium stearate (C₁₇H₃₅COONa).

Q.16. Name the by-product obtained in a saponification reaction.

Glycerol (commonly known as glycerine) is obtained as a by-product in saponification reaction.

Q.17. Reena added 20% NaOH solution to 20 ml of castor oil taken in a beaker. Suddenly, the beaker becomes hot. What conclusion can you drawn from this reaction?

The addition of NaOH solution to castor oil is an exothermic reaction as the heat is evolved during the reaction. Hence, the beaker becomes hot.

Q.18. What is the chemical reaction involved in the manufacture of soap?

The chemical reaction involved is

Q.19. Can we use Na2CO3 instead of NaOH? Explain.

No, Na₂CO₃ cannot be used instead of NaOH in saponification as it is a weak base, not able to break strong bond of ester and also it does not contain -OH group to participate in the reaction as -OH group containing compounds like NaOH helps in hydrolysis [decomposition of ester (triglyceride)].

Q.20. Why does a red litmus paper changes 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. A soap is the salt of a strong base (sodium hydroxide) and a weak acid (carboxylic acid). So, a solution of soap in water is basic in nature, i.e. pH > 7.