“Chameleon flowers” experiment

How to make paper flowers that change their color

In spring the flow­ers blos­som, in­clud­ing chem­i­cal ones! In this ex­per­i­ment we’ll see how they can change col­or de­pend­ing on pH.

A sim­i­lar ex­per­i­ment is in­clud­ed in the MEL Chem­istry sub­scrip­tion.

Safe­ty pre­cau­tions

  • wear pro­tec­tive gloves, a mask and glass­es;
  • work in a well-ven­ti­lat­ed room;
  • ob­serve safe­ty rules when work­ing with fire and heat­ing de­vices.

Warn­ing! Only un­der adults su­per­vi­sion.

Reagents and equip­ment:

  • 1 ml of sodi­um py­ro­sul­fite so­lu­tion (2.5 mol/l);
  • 2 ml of cit­ric acid so­lu­tion (1.5 mol/l);
  • aque­ous so­lu­tion of thy­mol blue (0.01 mol/l);
  • 3 pieces of white pa­per;
  • wood­en sticks;
  • con­i­cal flask;
  • burn­er.

Step-by-step in­struc­tions

Make origa­mi flow­ers and place them on sticks. Then paint the flow­ers with thy­mol blue. Pour so­lu­tions of sodi­um py­ro­sul­fite and cit­ric acid into the flask. Heat the mix­ture. Place the flow­ers in the flask and watch the col­or change from blue to red. With­out heat­ing the col­ors turn or­ange.

Pro­cess­es de­scrip­tion

In the re­ac­tion of sodi­um py­ro­sul­fate and cit­ric acid, sul­furous gas is re­leased, and the high­er the tem­per­a­ture, the more in­ten­sive­ly it is re­leased:

Na₂S₂O₅ + H8C₆O₇ → Na₂H₆C₆O₇ + H₂O + 2SO₂

The re­leased sul­furous gas dis­solves in the wa­ter that con­tains the thy­mol blue in­di­ca­tor. Sul­furous acid is formed, which changes the po­ten­tial of hy­dro­gen (pH) of the so­lu­tion. As a re­sult the in­di­ca­tor changes col­or from blue to red.

SO₂ + Н₂О = Н₂­SO₃

With­out heat­ing the gas is re­leased more slow­ly and evap­o­rates from the flow­ers more quick­ly than it reach­es them. This makes the col­or less sat­u­rat­ed, and the flow­ers turn or­ange.