“Egyptian Night” experiment

To be­come the lord of dark­ness, all you need the va­pors of clear so­lu­tions. In this ex­per­i­ment you’ll find out how to cause dark­ness in a beaker.

Safe­ty pre­cau­tions

• Wear pro­tec­tive glass­es and gloves.
• Avoid the hy­dro­gen per­ox­ide and skin from com­ing into con­tact with the skin and get­ting into the eyes and mouth.

Warn­ing! Only un­der adults su­per­vi­sion.

Reagents and equip­ment:

• starch (10 g);
• 500 ml of a so­lu­tion of sodi­um thio­sul­fate (10g) with potas­si­um io­dide (50g);
• con­cen­trat­ed hy­dro­gen per­ox­ide;
• 20% sul­fu­ric acid so­lu­tion;
• wa­ter;
• beakers.

Step-by-step in­struc­tions

Sprin­kle starch into a beaker, add 200 ml of cold wa­ter and stir thor­ough­ly. Then add 300 ml of hot wa­ter and stir again. Pour the so­lu­tion of sodi­um thio­sul­fate with potas­si­um io­dide into the re­sult­ing so­lu­tion. Then pour the mix­ture into beakers, adding di­lut­ed sul­fu­ric acid and con­cen­trat­ed hy­dro­gen per­ox­ide. Ob­serve the so­lu­tions turn dark in the beakers.

Pro­cess­es de­scrip­tion

Sev­er­al par­al­lel re­ac­tions take place in the so­lu­tion.

The first in­volves the ox­i­da­tion of the io­dide ions of hy­dro­gen per­ox­ide to molec­u­lar io­dide:

H₂O₂ + 2I⁻ + 2H⁺ → I₂ + 2H₂O

The io­dine that forms en­ters into a re­ac­tion with sodi­um thio­sul­fate un­til the sodi­um thio­sul­fate runs out.

2S₂O₃²⁻ + I₂ → S₄O₆2⁻ + 2I⁻

When the sodi­um thio­sul­fate has thor­ough­ly re­act­ed, io­dine con­tin­ues to form, en­ter­ing into a re­ac­tion with starch and form­ing a blue io­dine-starch com­plex.