Magic Lemons

Case of the disappearing pigment

HOME  | SAFETY  | NEW | TEACHERS | PARENTS | CATEGORIES | LIST | AUTHOR

Purpose -

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

Nature - 

Demonstration

Materials - 

A lemon, a syringe with needle, .01 M Sodium Hydroxide, Phenolphthalein, small beaker, distilled water, knife, cutting board

Method - 

In the small beaker mix some distilled water, sodium hydroxide and phenolphthalein to get a nice pink solution.

Draw some pink solution into the syringe.

Inject the solution into the lemon, trying to get it in the middle.

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 here.

  C₃H₄OH(COOH)₃ + NaOH => H₂O + NaC₃H₄(COO)₃

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.