Studying saponification reaction for preparation of soap

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Aim

To study saponification reaction for preparation of soap.

Materials Required

• Two beakers (250 mL)
• Two test tube
• A glass rod
• A measuring cylinder (50 mL)
• Burner
• Stand
• Wire gauge
• Knife
• 20% sodium hydroxide solution : 30 mL
• Distilled water
• Vegetable oil (cotton seed oil, soyabean oil, palm oil) : 20 mL
• sodium chloride (common salt) : 10 g
• Red and blue litmus paper strips.

Theory

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. Addition of NaCl causes the precipitation of soap.

Procedure

• Take nearly 20 mL of vegetable oil (preferably palm oil) in a 250 mL beaker. Warm it gently to melt, if needed.
• Add 30 mL of 20% sodium hydroxide solution with constant stirring.
• Successively dip the red and blue litmus paper into this reaction mixture. Do you find any change in colour of any litmus paper strip. Note and record your observation.
• Touch the beaker from outside. Is it hot or cold? Note and record your observation.
• Heat the beaker containing the reaction mixture on a wire gauge with constant stirring till it thickness.
• Add 10 g of common salt to this. Stir the mixture well and allow it to cool.
• Leave the mixture for some time (nearly 24 hours) till the soap set to a semi-solid mass floating on the top a solution.
• Remove the soap cake and cut it into desired shape.

Observations

• The colour of red litmus turns blue, whereas the colour of blue litmus does not change.
• Beaker containing reaction mixture gets warm.

Result

• Soap solution is basic in nature.
• Saponification reaction is an exothermic reaction as heat is released during the course of the reaction.

Precautions

• Stir the reaction mixture carefully so that it does not spill out.
• Use a wire gauge to heat the reaction mixture.
• Let the soap set and float on the spent lye before removing it form the beaker.
• Sodium hydroxide is hydroscopic in nature. Therefore, store it in an air-tight container.
• Sodium hydroxide is highly corrosive in nature. Therefore, handle it carefully.

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