"Green Flame" experiment

How to make a green fire

This col­or­ful ex­per­i­ment shows how not only chem­i­cal prop­er­ties, but also phys­i­cal ones, change in a chem­i­cal re­ac­tion.

Reagents and equip­ment:

  • boric acid (5 g);
  • ethyl al­co­hol (50 ml);
  • con­cen­trat­ed sul­fu­ric acid;
  • porce­lain cups (2);
  • glass rod;
  • match­es.

Step-by-step in­struc­tions

Pour ethyl al­co­hol into two porce­lain cups and ig­nite it. We ob­serve that the al­co­hol burns with a yel­low flame. Now put out the fire. In one of the cups, add 5 g of boric acid and a few milliliters of con­cen­trat­ed sul­fu­ric acid. Mix to­geth­er well and wait for a while. Ig­nite the al­co­hol in the cups again. We see that the cup to which we added boric and sul­fu­ric acid, the flame has turned green.

Pro­cess­es de­scrip­tion

Boric acid can form com­plex es­ters with al­co­hol. Com­plex es­ters are a class of com­pounds which are the prod­ucts of the in­ter­ac­tion of acids with al­co­hols with the gen­er­al for­mu­la of R(C=O)OR in the pres­ence of a cat­a­lyst, for ex­am­ple sul­fu­ric acid. In the in­ter­ac­tion of acids and al­co­hols, the prop­er­ties may change fun­da­men­tal­ly. For ex­am­ple, in the in­ter­ac­tion of isoamyl al­co­hol with acetic acid in the pres­ence of sul­fu­ric acid, the com­plex es­ter isoamyl ac­etate forms, which has the odor of pears, while the es­ter of formic acid and ben­zyl al­co­hol has the odor of jas­mine. Boric acid is one of the few min­er­al acids which is ca­pa­ble of form­ing com­plex es­ters. Es­ters of boric acid burn with a green flame, and this prop­er­ty is used in an­a­lyt­i­cal chem­istry for de­tect­ing boric acid and its salts. Pure es­ters of boric acid burn at a tem­per­a­ture of around 30 °С, so you can safe­ly put your hand in the flame.

Safe­ty pre­cau­tions

The ex­per­i­ment uses con­cen­trat­ed al­co­hol and sul­fu­ric acid. Wear gloves and work in a well-ven­ti­lat­ed room. Ob­serve fire safe­ty rules.

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