"Egyptian night" experiment

How to make "Iodine clock" reaction

Egyp­tian night is a clas­sic and very col­or­ful ex­per­i­ment. Two trans­par­ent so­lu­tions are mixed, and af­ter a while the re­sult­ing so­lu­tion sud­den­ly turns dark. Just as quick­ly as night falls in Egypt.

Reagents and equip­ment:

  • starch;
  • potas­si­um io­dide;
  • sodi­um thio­sul­fate;
  • hy­dro­gen per­ox­ide;
  • acid (sul­fu­ric, acetic);
  • wa­ter;
  • two beakers;
  • mea­sur­ing glass;
  • glass rod for stir­ring.

Step-by-step in­struc­tions

Mix ap­prox­i­mate­ly 0.2 g of starch and a small amount of cold wa­ter. Add a lit­tle hot wa­ter to dis­solve the starch. Di­lute the so­lu­tion to ap­prox­i­mate­ly 800 ml. Add 50 g of potas­si­um io­dide and 10 g of sodi­um thio­sul­fate. So­lu­tion A is ready. So­lu­tion B is a mix­ture of 30% hy­dro­gen per­ox­ide, wa­ter with a small amount of acid (sul­fu­ric, acetic etc.). This so­lu­tion may be of any con­cen­tra­tion – the re­ac­tion time de­pends on the con­cen­tra­tion of hy­dro­gen per­ox­ide. Quick­ly mix so­lu­tions A and B. Af­ter some time, the so­lu­tion ob­tained will sud­den­ly turn dark blue.

Pro­cess­es de­scrip­tion

In this ex­per­i­ment, three main re­ac­tions take place. The first is the slow re­ac­tion be­tween hy­dro­gen per­ox­ide and io­dine ions in the pres­ence of H⁺ ions. As a re­sult, molec­u­lar io­dine forms. The sec­ond re­ac­tion is the swift re­ac­tion be­tween io­dine and sodi­um thio­sul­fate. This re­ac­tion leads to the for­ma­tion of the tri­io­dide ion, which is ex­pend­ed more quick­ly than it forms. When all of the thio­sul­fate re­acts, this re­ac­tion stops and the tri­io­dide ion forms a dark-blue mix­ture with the starch. The high con­cen­tra­tion of H⁺ re­duces the time it takes the col­or to change. In­creas­ing the con­cen­tra­tion of thio­sul­fate­has a re­verse ef­fect.

Safe­ty pre­cau­tions

Wear pro­tec­tive glass­es and gloves. Avoid the hy­dro­gen per­ox­ide and acids touch­ing the skin and get­ting in the eyes or mouth. Fol­low gen­er­al safe­ty rec­om­men­da­tions. Chem­i­cal ex­per­i­ments must be car­ried out in full com­pli­ance with the leg­is­la­tion of your coun­try.

Warn­ing! Sub­stances of this ex­per­i­ment are tox­ic and high­ly dan­ger­ous for your health. Do not try this at home. Only un­der pro­fes­sion­al su­per­vi­sion.