Theory & Procedure, Saponification (The process of Making Soap) | Science Class 10 PDF Download

Our Objective
To study the saponification reaction for preparation of soap.

The Theory

Soaps and detergents are essential to personal and public health. They safely remove germs, soils and other contaminants and help us to stay healthy and make our surroundings more pleasant.   Soaps are made from fats and oils or their fatty acids.

What are fatty acids?

Fatty acids are merely carboxylic acids consisting of a long hydrocarbon chain at one end and a carboxyl group (-COOH) at the other end. They are generally represented as RCOOH. They are an important component of plants, animals and other microorganisms. They are found in various parts of the body, such as cell membranes, the nervous system and as lung surfactant.
There are two groups of fatty acids: saturated fatty acids and unsaturated fatty acids.

Saturated fatty acids:Fatty acids contain carbon-carbon single bonds called saturated fatty acids.
Examples: stearic acid (C₁₇H₃₅COOH) & palmitic acid (C₁₅H₃₁COOH)

Unsaturated fatty acids:Unsaturated fatty acids contain one or more double bonds between carbon atoms.
Example: Oleic acid (C₁₇H₃₃COOH)

If the fatty acid has a single carbon-carbon double bond in the molecule, it is known as a mono-unsaturated fatty acid. Oleic acid is a mono-unsaturated fatty acid.

If a fatty acid has two or more carbon-carbon double bonds in the molecule, it is known as poly-unsaturated fatty acid.

Linoleic acid {CH₃(CH₂)4CH=CHCH₂CH=CH(CH₂)7COOH} is a poly-unsaturated fatty acid. It contains two double bonds.

Long chain fatty acids always exist as triglycerides and are found in fats and oils. Triglycerides are esters of fatty acids and are formed by combining fatty acids with glycerol.

Glycerol has three alcohol functional groups (-OH group) and fatty acids have the carboxyl group (-COOH group).  Since glycerol has three –OH groups, three fatty acids must react with one glycerol molecule to make three ester functional groups and form triesters of glycerol or triglyceride. During this process three molecules of water are eliminated. The three fatty acids may or may not be identical.

The long chain fatty acids can be of either plant origin (linseed oil, castor oil, soya bean, coconut oil) or animal origin (tallow from cattle and sheep). In general, fats/oils from plant origin are high in unsaturated and low in saturated fatty acids. Fats/oils from animal source are high in saturated and low in unsaturated fatty acids.

SOAP

Soaps are sodium or potassium salts of long chain fatty acids. When triglycerides in fat/oil react with aqueous NaOH or KOH, they are converted into soap and glycerol. This is called alkaline hydrolysis of esters. Since this reaction leads to the formation of soap, it is called the Saponification process.

The soap molecule has two parts: a polar group (-COO-Na+) and a non-polar group (R-hydrocarbon part). The polar group is called the head and the non-polar group is called the tail. Thus, the soap molecule has a polar head and a non-polar hydrocarbon tail. The polar head is hydrophilic in nature (water loving) and the non-polar tail is hydrophobic (water repelling) in nature.

The saponification  reaction is exothermic in nature, because heat is liberated during the process. The soap formed remains in suspension form in the mixture. Soap is precipitated as a solid from the suspension by adding common salt to the suspension. This process is called Salting out of Soap.

Types of Soap

Depending upon the nature of alkali used in the production of soap, they are classified into two types.

The sodium salt of long chain fatty acid is known as hard soap. It is difficult to dissolve in water. It is used as laundry soap.

The potassium salt of long chain fatty acid is known as soft soap, as it produces more lather. It is used as toilet soap and shaving soap.

• Hard soap    
• Soft soap

In aqueous solution, soap ionises to form alkali ions.

Since soaps have free alkali ions, they are alkaline in nature. Hence, the soap solutions are slippery to the touch.

Learning Outcomes

• Students understand the terms: soap, saponification, salting out, hard soap and soft soap.
• Students identify the materials which are required for the preparation of soap.
• Students understand the use of common salt in saponification process.
• Students understand the alkalies required for the preparation of hard and soft soaps.
• Students understand the procedure of saponification process.
• Students acquire the skill to perform the preparation of soap in a real lab.

Materials required: 

• Vegetable oil (castor oil, olive oil, coconut oil or palm oil)
• 20% sodium hydroxide solution
• Common salt
• Measuring cylinders
• Glass beaker (250 ml)
• Blue and red litmus papers
• Glass rod
• Bunsen burner
• Wire gauze
• Tripod stand
• Filter funnel
• Filter paper
• Spatula
• Knife

Real Lab Procedure:

• Take 25 ml of coconut oil in a measuring cylinder and pour it into a 250 ml glass beaker.
• Measure 30 ml of 20% NaOH solution in another measuring cylinder and add it into the beaker containing vegetable oil.
• Vigorously stir the mixture using a glass rod.
• Touch the beaker from outside.  It is observed that the beaker is warm.
• Place the beaker on a wire gauze placed over a tripod stand.
• Heat the beaker using a Bunsen burner till the mixture become a whitish paste.
• Remove the beaker from the flame and allow it to cool.
• Dip a red litmus paper in the suspension formed.
• When dipped in the suspension, the red litmus paper changes its colour to blue.
• Dip a blue litmus paper in the suspension.
• The colour of blue litmus paper remains the same.
• To the above suspension, add 15g of common salt and stir it well with a glass rod.
• After adding common salt, soap in the suspension gets precipitated out as solid.
• Take a filter funnel and place a filter paper in it and fix it in a stand.
• Place a beaker below the funnel.
• Pour the contents of the beaker into the funnel over a glass rod and filter the contents of the beaker.
• After filtration, soap is left behind in the filter paper.
• Transfer the soap into another filter paper using a spatula and dry it by pressing with another filter paper.
• Then, cut it into desired shape with a knife.

Simulator Procedure:

• To start the simulator, click on the measuring cylinder containing 25 ml coconut oil to pour it into the beaker.
• Click on the measuring cylinder containing 30 ml 20% NaOH solution to pour it into the beaker containing coconut oil.
• Click on the glass rod to stir the mixture.
• Click on the information icon to see the inference.
• Click on the 'NEXT STEP' button to go to the next process.
• Turn on the burner by clicking on the 'knob' of the burner.
• Click on the beaker to place it over the Bunsen burner and heat it.
• Click on the information icon to see the inference.
• Click on the beaker to place it back from the burner for allowing it to cool.
• Turn off the burner by clicking on the 'knob' of the burner.
• Click on the 'NEXT STEP' button to go to the next process.
• Click on the red litmus paper to dip it into the soap suspension.
• Click on the information icon to see the inference.
• Click on the blue litmus paper to dip it into the soap suspension.
• Click on the information icon to see the inference.
• Click on the watch glass containing common salt to add it into the soap suspension.
• Click on the glass rod to stir the mixture.
• Click on the information icon to see the inference.
• Click on the 'NEXT STEP' button to go to the next process.
• Click on the beaker to filter the contents.
• Click on the spatula to take soap from the funnel.
• Click on the filter paper to dry the soap.
• Click on the information icon to see the inference.
• You can redo the experiment anytime by clicking on the 'Reset' button.

Observations:

• When 20% NaOH solution was added to the beaker containing vegetable oil, it was observed that the beaker was warm when touched from the outside.
• A whitish suspension was formed by heating the mixture of vegetable oil and 20% NaOH solution.
• Red litmus paper changed colour to blue when dipped in suspension.  Blue litmus paper was not affected by the suspension.
• After adding common salt, the soap in suspension form precipitated out as solid.

Conclusions:

• The reaction between vegetable oil and sodium hydroxide solution is exothermic in nature because heat is liberated during the reaction.
• The white suspension formed is made up of soap and glycerol. The process of formation of soap is called saponification.
• Test using red and blue litmus papers shows that soap suspension is basic in nature and not acidic in nature.
• The process of precipitation of soap from the suspension is called salting out.

Precautions:

• Do not touch the NaOH solution with bare hands as it may burn the skin.
• Do not breathe the fumes of NaOH or let the fumes get in your eyes. Keep the windows of the laboratory open.
• The mixture of oil and alkali should be stirred thoroughly.
• It is necessary to stir the soap solution after adding common salt to it, in order to precipitate out the soap in solid form.