Magic Lemons

Case of the disappearing pigment.

 

 

 

Category - Chemical & Reactions

Key Idea - Acids & Bases

 

Purpose - A very effective way to show the neutralization effect of acids and bases.

The equipment for ready for a magic lemon demo.

 

 

No colour!!!!!! 

 

C3H4OH(COOH)3 + NaOH => H2O + NaC3H4(COO)3
Nature -  Demonstration
Materials -  A lemon, a syringe with needle, .01 M Sodium Hydroxide, Phenolphthalein, small beaker, distilled water, knife, cutting board
Method - 
  1. In the small beaker mix some distilled water, sodium hydroxide and phenolphthalein to get a nice pink solution.
  2. Draw some pink solution into the syringe.
  3. Inject the solution into the lemon, trying to get it in the middle.
  4. Wait for a few moments, then cut the lemon in half, and note that the pink colour in not there.
Safety - Be careful of needle stick injuries.
Explanation - The citric acid in the lemon neutralizes the NaOH, resulting in the phenolphthalein turning from pink to colourless. The reaction between citric acid and sodium hydroxide is given above.
Notes - It may be easier to inject the phenolphthalein mixture if, before the demo, you squeeze the lemon a little bit to open up the fibers inside the lemon.

I usually present this as a magic trick by having three lemons and injecting one of them. I then juggle them for a while, telling the students to keep their eye on the one I injected. I then cut each one open (with much showmanship) to discover that none of them have pink in the centre.