How Can We Prepare Soap in the Lab?

Carbon and Its Compounds

What are Soaps?

Soaps are sodium or potassium salts of higher fatty acids. Vegetable oils, such as olive oil and castor oil, contain esters of fatty acids like stearic acid, palmitic acid, ricinoleic acid, etc.

Let us take a look at the preparation of soap in the lab with an example: 

Take a mixture of 100 g of olive oil and 125 mL of 20% sodium hydroxide solution in an evaporating dish. Place this dish in a beaker containing water. Start heating and stir the mixture continuously at around 50 °C. After a few minutes, the oil and the sodium hydroxide layers merge. 

Here, sodium hydroxide splits the tristearin present in the olive oil into glycerol and three moles of sodium salt of stearic acid. This sodium salt of stearic acid, that is, sodium stearate, is nothing but soap. Perfumes, such as lavender oil or any other, can be added to give fragrance.

Other chemicals, such as sodium carbonate or alcohol, can also be added at this time to improve the texture of the soap. After a while, the solution starts becoming thick. The product is now a mixture of soap and glycerol formed in the reaction.

The reaction mixture is allowed to cool, and then a small amount of common salt is added to this mixture to precipitate out the soap. This is known as the salting-out process. This mixture is then poured carefully into the mould. The mould is kept undisturbed for 24-36 hours. The cake of the soap is then removed from the mould.

Let us understand the process through a simulation: ‘How can we prepare soap in a lab?’. 

Oil or fats known as triglycerides (such as castor oil) react with sodium hydroxide solution and get converted into sodium salts of fatty acids called soap and glycerol. This reaction is called saponification. The evolution of heat accompanies this reaction, and hence, it is an exothermic reaction. The soap has a pH of around 8-10, making it basic in nature.

Sodium chloride is added to the reaction mixture to make the soap precipitate out of the solution easily. When sodium chloride is added to the solution, the solubility of soap present in it decreases. All the soap precipitates to form a solid. This process is called salting out of soap. The liquid glycerol is formed as a by-product.

This experiment aims to study the saponification reaction for the preparation of soap.

The apparatus and chemicals required for this experiment are: 

 Apparatus

• Glass rod
• Litmus Paper strips (both blue and red)                 
• Beakers                                                        
• Measuring cylinder (50 mL)

Chemicals

• Castor oil (20 mL) 
• Sodium hydroxide pellets 
• Common salt ( About 10 g)
• Distilled water  

The procedure followed for this experiment is as follows: 

1. Take about 20 mL of castor oil (triglyceride) in a beaker (250 mL) and mix it with 30 mL of  20% solution of sodium hydroxide with the glass rod.

2. Successively dip the red and blue litmus paper strips into this reaction mixture to check the nature of the mixture.

3. Touch the beaker from the outside to observe if heat is generated.

4. Add 5 g to 10 g of common salt to this mixture and use a glass rod to stir the mixture continuously till the soap begins to set.

5. Leave it for a day till the mixture cools and solidifies.

6. Remove the soap cake and cut it into desired shapes and sizes.

Hence, it can be concluded that

• Castor oil reacts with NaOH to form soap (sodium salt of fatty acids) and glycerol as the by-product. 
• The reaction that occurs is

3NaOH + (C₁₇H₃₅COO)₃C₃H₅ → (CH₂OH)₂CHOH + 3C₁₇H₃₅COONa

• This process is called saponification, which is an exothermic reaction. 
• The mixture is basic in nature, as it turned red litmus blue.
• Sodium chloride was used to precipitate the soap.

FAQs on Preparing Soap in the Lab