For preparing soap in the laboratory we require an oil and a base. Which of the following combinations of an oil and a base would be best suited for the preparation of soap?

Also asked in:2016

Carbon and its Compounds

Soaps and Detergents

Options

(a)Castor oil and calcium hydroxide

(b)Turpentine oil and sodium hydroxide

(c)Castor oil and sodium hydroxide

(d)Mustard oil and calcium hydroxide

Official Answer

Castor oil and sodium hydroxide

Castor oilSodium hydroxideSaponificationTriglyceridesAlkaline hydrolysis

Marking Scheme

11 mark for selecting the correct combination of castor oil and sodium hydroxide (option c).

Hint

Soap is a sodium or potassium salt of long-chain fatty acids. You need a vegetable oil containing fatty acids and a strong alkali containing sodium or potassium ions.

Quick Oral Answer

We use castor oil because it contains triglycerides (esters of fatty acids) and sodium hydroxide because it is a strong alkali that hydrolyzes these esters to produce soluble sodium soap.

Analysis & Explanation

To prepare soap in the laboratory, we require a source of triglycerides (vegetable oil or animal fat) and a strong soluble base (alkali).

Castor oil is a vegetable oil rich in triglycerides of fatty acids (mainly ricinoleic acid), making it highly suitable for saponification.
Sodium hydroxide ( $NaOH$ ) is a strong alkali that hydrolyzes the ester bonds in the oil to form glycerol and sodium salts of fatty acids (soap).

Why other options are incorrect:

Turpentine oil (option b) is an essential oil consisting mainly of terpenes (hydrocarbons) and does not contain fatty acid esters; hence, it cannot undergo saponification.
Calcium hydroxide ( $Ca(OH)_2$ ) (options a and d) is a weak base and forms insoluble calcium salts of fatty acids (scum) rather than soluble, usable soap.
Therefore, option (c) is the best combination.

Common Mistakes

1Choosing turpentine oil, confusing it with a saponifiable vegetable oil.
2Choosing calcium hydroxide, which actually causes the precipitation of insoluble calcium salts (scum) instead of soluble soap.

Interesting Facts

The discovery of soap-making is ancient, dating back to the Babylonians around 2800 BC, who made soap from water, alkali, and cassia oil.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2016?

This question carries 1 mark in the CBSE Class 10 Science 2016 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Carbon and its Compounds" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Soaps and Detergents" from CBSE Class 10 Science.

Has this question appeared in other CBSE Science papers?

Yes, a similar question appeared in the 2016 CBSE Class 10 Science paper.

What type of question is this in the CBSE Class 10 Science 2016 paper?

This is a MCQ question from Section B in the CBSE Class 10 Science 2016 paper.