Weird Science

Purple Smoke

Polish ver­sion is here

Halo­gens

Halo­gens belong to Group 17 of the perio­dic table. The name "halo­gen" comes from Greek and means "salt for­mer" or "salt maker", reflec­ting this group's abi­lity to cre­ate a wide variety of che­mi­cal com­po­unds. The halo­gens inc­lude flu­o­rine F, chlo­rine Cl, bro­mine Br, iodine I, asta­tine At, and ten­nes­sine Ts (for­merly known as unun­sep­tium).

These ele­ments exhi­bit high elec­tro­ne­ga­ti­vity and strong che­mi­cal reac­ti­vity. In their free state, they exist as dia­to­mic mole­cu­les. Halo­gens typi­cally form sin­gle nega­tive ions because they need just one more elec­tron to attain a sta­ble noble gas elec­tron con­fi­gu­ra­tion. Regar­ding phy­si­cal sta­tes at room tem­pe­ra­ture: flu­o­rine and chlo­rine are gases, bro­mine is a liquid, and iodine, asta­tine, and ten­nes­sine are solids.

Ilustracja

Source: http://upload.wiki­me­dia.org/wiki­pe­dia/com­mons/f/f5/Halo­gene.jpg, acces­sed: 08/25/2011

Halo­gens react par­ti­cu­larly well with metals, for­ming salts known as hali­des. Below, I pre­sent the reac­tion between iodine and alu­mi­num. This reac­tion is par­ti­cu­larly inte­re­sting due to its stri­king visual effect.

What Do We Need?

To con­duct this expe­ri­ment, we need:

Alu­mi­num is a silvery metal. Iodine appe­ars as dark pur­ple-black cry­stals. It sho­uld not be unne­ces­sa­rily hea­ted, as it sub­li­ma­tes very easily.

Ilustracja:

Kliknij, aby powiększyć

Ilustracja:

Kliknij aby powiększyć

Both sub­stan­ces must be as finely pow­de­red as pos­si­ble. The iodine cry­stals can be gro­und using a mor­tar and pestle.

War­ning: Pure iodine is highly toxic! The lethal dose for humans is appro­xi­ma­tely 4 grams (0.14 oz). Avoid any con­tact with eyes or skin, as iodine is an irri­tant! Iodine vapors are also irri­ta­ting and, in higher con­cen­tra­tions, can be suf­fo­ca­ting. This expe­ri­ment must be con­duc­ted out­do­ors and away from flam­ma­ble mate­rials. Safety gog­gles are essen­tial, as bur­ning rea­gents may scat­ter. The author assu­mes no respon­si­bi­lity for any pos­si­ble dama­ges. You pro­ceed at your own risk!

The Demon­stra­tion!

Weigh out 1 gram (0.035 oz) of alu­mi­num pow­der and 3 grams (0.11 oz) of finely gro­und iodine sepa­ra­tely. It is cru­cial that both alu­mi­num and iodine remain dry! These sub­stan­ces can be sto­red sepa­ra­tely for a long time. Howe­ver, just before the demon­stra­tion, they must be mixed and for­med into a small mound on a fire-resi­stant sur­face (cera­mic, asbe­stos-free fire­proof board, or metal).

After mixing the dry sub­stan­ces, no visi­ble reac­tion occurs. Howe­ver, upon adding a few drops of water, a vigo­rous reac­tion occurs, rele­a­sing a pur­ple-brown smoke! This can be seen in the image below:

Ilustracja:

Kliknij, aby powiększyć

A simi­lar expe­ri­ment can be per­for­med using zinc instead of alu­mi­num, as shown in the video below.

The reac­tion effect is also beau­ti­fully cap­tu­red in the fol­lo­wing pho­to­graph:

Ilustracja:

Kliknij, aby powiększyć

Expla­na­tion

In its dry form, the reac­tants do not inte­ract. Howe­ver, in the pre­sence of even a small amo­unt of moi­sture, an exo­ther­mic reac­tion occurs, syn­the­si­zing alu­mi­num iodide accor­ding to the fol­lo­wing equ­a­tion:

2 Al + 3I2 → 2 AlI3

The heat gene­ra­ted cau­ses any rema­i­ning iodine to rapi­dly sub­li­mate, for­ming a distinc­tive pur­ple vapor.

The reac­tion mecha­nism between iodine and zinc is ana­lo­gous to the one descri­bed above.

Enjoy your expe­ri­ments! :)

Fur­ther rea­dings

Marek Ples

Aa