“Green flame” experiment

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

Safe­ty pre­cau­tions

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

Reagents and equip­ment:

• boric acid (1 g);
• 5 ml of ethyl al­co­hol;
• con­cen­trat­ed sul­fu­ric acid;
• test tube;
• spir­it burn­er;
• lighter.

Step-by-step in­struc­tions

Add boric acid to the test tube, then pour in the ethyl al­co­hol, and a few drops of con­cen­trat­ed sul­fu­ric acid. Heat the test tube in the flame of the spir­it burn­er for a few min­utes.

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 (non-or­gan­ic) 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.