Studying saponification reaction for preparation of soap

Aim

To study saponification reaction for preparation of soap.

Apparatus and Chemical

  • 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.

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

(i) (a) Change in colour of red litmus paper when dipped into the mixture : …………. (None/Blue)
(b) Change in colour of blue litmus paper when dipped into the mixture : …………. (None/Red)
(ii) The temperature of reaction mixture on adding sodium hydroxide solution to oil : ………. (increases/decreases)

Result and Discussion

On the basis of your observations with litmus paper, find out the nature of soap mixture (acidic/basic). Also comment whether the saponification reaction is exothermic or endothermic.

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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