Ozone: the guardian of our planet

Oxygen is essential to life as we know it – but can it be poisonous?

We've put to­geth­er some ozone lay­ers for you. What to do next – see in this ex­per­i­ment!

Safe­ty pre­cau­tions

At­ten­tion! All ex­per­i­ments are per­formed by pro­fes­sion­als. Do not at­tempt.


  • flask with ozone;
  • rub­ber stop­per;
  • ozoniz­er;
  • so­lu­tions of iron(II) sul­fate and potas­si­um thio­cyanate;
  • porce­lain bowl;
  • potas­si­um io­dide so­lu­tion;
  • fil­ter pa­per.

Step-by-step in­struc­tions

A flask is filled with ozone us­ing an ozoniz­er and closed with a stop­per. When a strip of fil­ter pa­per is moist­ened with a so­lu­tion of potas­si­um io­dide and put in the flask of ozone, the pa­per turns brown. When the walls of an­oth­er flask are coat­ed with a so­lu­tion of iron(II) sul­fate and potas­si­um thio­cyanate, and the flask is filled with ozone us­ing an ozoniz­er, the flask turns red.

Process de­scrip­tion

Ozone is a col­or­less gas with ox­i­diz­ing prop­er­ties. It can be pro­duced in a lab­o­ra­to­ry via an elec­tric dis­charge, and it forms in the at­mos­phere af­ter light­ning strikes: its low con­cen­tra­tion in the air cre­ates the fa­mil­iar fresh smell af­ter a thun­der­storm.

3 O₂ → 2 O₃

Ozone eas­i­ly con­verts io­dide ions into molec­u­lar io­dine.

2KI + O₃ + H₂O → I₂ + 2KOH + O₂

And it con­verts iron(II) to iron(III), which forms a red com­pound Fe(SCN)₃ with potas­si­um thio­cyanate:

3Fe­SO₄ + 6KSCN + O₃ + H₂O → 2Fe(SCN)₃ + 3K₂­SO4 + Fe(OH)₂ + O₂

Ozone is poi­sonous, but it can be used to pu­ri­fy wa­ter, and it helps make life on Earth pos­si­ble: a lay­er of this gas in the strato­sphere pro­tects us from harm­ful so­lar ra­di­a­tion.

Safe ex­per­i­ments await you in the MEL Chem­istry sub­scrip­tion!