Lab Manual: Soap Preparation | Lab Manuals for Class 10 PDF Download

Objective

Materials Required
20% sodium hydroxide solution, castor oil, common salt, filter paper, distilled water, litmus paper (red and blue) strips, glass rod, borosil beakers, Bunsen burner, wire gauze and tripod stand.

Theory

• A soap is the sodium or potassium salt of fatty acids containing 15 to 18 carbon atoms, e.g. palmitic acid (C₁₅H₃₁COOH), stearic acid (C₁₇H₃₅COOH).
• Glycerides are the esters of higher fatty acids formed after combining of higher fatty acid with atrihydric alcohol, e.g. glycerol, which has three hydroxyl groups. This process is called esterification
  
• Oil or fats (like castor oil, olive oil, palm oil) when treated with sodium hydroxide solution, these oils/fats gets converted into sodium salt of fatty acids (soap) and glycerol. This reaction is known as saponification.This reaction is accompanied by the evolution of heat, hence it is exothermic in nature.
• Common salt is added to the mixture to make the soap come out of solution. Common salt precipitate out all the soap from the aqueous solution. Actually, when it is added to the solution, then the solubility of soap present in it decreases, due to which all the soap separates out from the solution in the form of a solid. This process is called salting out of soap.
  Excess of Na⁺ will help in the formation of soap.

Procedure

• Take 20 ml of castor oil (triglyceride) in a beaker (250 ml).
• Prepare 20% NaOH solution (by adding 10 g of NaOH in 50 ml of water) in another clean beaker and add 30 ml of this solution in 20 ml castor oil. Stir it well with a clean glass rod so that a thick paste is formed (Fig.1).
  
• Dip the red and blue litmus paper strip successively into this reaction mixture. Note the change in colour of litmus paper strips and record your observation.
• Touch the beaker from outside and determine whether is it hot or cold. Note the change and record your observations.
• Place the beaker on a Bunsen burner. Heat the solution with constant stirring till a thick whitish paste is formed (in suspension form) [Fig. 2(a)]. Remove the beaker from the burner and allow to cool.
  Note: To check whether saponification is complete or not, add few drops of the solution in a test tube containing distilled water, if the oil drops are not visible then the saponification is complete, but if oil droplets floats on the top of the surface of water that means saponification is incomplete (heating should be continued.)
• Add 5 to 10 g of common salt to the above mixture and stir the mixture continuously with the help of qlass rod till the soap begins to set. Add colour, perfume and fillers to make the soap harder [Fig. 2(b)].
• Leave the mixture for a day till it cools and becomes solid [Fig. 2(c)].
• Remove the soap cake and cut it into desired shapes and sizes.

Observation

• The colour of red litmus paper when dipped into mixture turns blue while the colour of blue litmus paper remains unchanged.
• The temperature of reaction mixture increases on adding sodium hydroxide to the castor oil.
• After adding common salt, the soap from suspension, precipitated out as a solid.

Result
The medium of soap solution is basic in nature (as it turns red litmus blue) and the saponification reaction is exothermic as heat is released during the course of the reaction. Glycerol is a by-product of this reaction.

Precautions

• Stir the soap solution carefully so that it does not spill out.
• Do not touch and taste the pellets of sodium hydroxide.
• Do not taste the castor oil.
• Handle the beaker carefully as it becomes too hot.
• Use a clean beaker and glass rod.
• Mixture of oil and NaOH solution should not be overheated.
• Continuous and thorough stirring of the reaction mixture is necessary.