Weird Science

Water and Fire

Polish ver­sion is here

Che­mi­cal Oxy­mo­ron

What is an oxy­mo­ron? Let’s con­sult the dic­tio­nary:

Oxy­mo­ron (from Greek oksýmōron, com­bi­ning oksýs “sharp” and mōros “fo­o­lish”) – a figure of spe­ech that juxta­po­ses con­cepts with oppo­site mea­nings within a word or in a phrase that is a self-con­tra­dic­tion. As a rhe­to­ri­cal device, an oxy­mo­ron illu­stra­tes a point to com­mu­ni­cate and reveal a para­dox.

Lewis Ch, T., Short Ch., A Latin Dic­tio­nary, Cla­ren­don Press, Oxford, 1879

Can we link this some­how with che­mi­stry? Some expe­ri­ments indeed pro­duce results that appear con­tra­dic­tory to our eve­ry­day expe­rience. Take, for exam­ple, the sta­te­ment “wa­ter as a fire­star­ter”. In com­mon under­stan­ding, we use water pri­ma­rily as a coo­ling or fire-extin­gu­i­shing agent. Hence, this sta­te­ment bears the hall­marks of an oxy­mo­ron. Howe­ver, a che­mist can ignite a fire using just a drop of water.

What Do We Need?

In addi­tion to a small amo­unt of water, the expe­ri­ment requ­i­res the fol­lo­wing:

Ilustracja:

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Ilustracja:

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The zinc must be in the form of the finest pos­si­ble pow­der, as its degree of fine­ness stron­gly affects the out­come. Ammo­nium nitrate can be pur­cha­sed at gar­de­ning shops, since it is an effi­cient syn­the­tic fer­ti­li­zer (mar­ke­ted as ammo­nium salt­pe­ter). It sho­uld be puri­fied by cry­stal­li­za­tion and dried as tho­ro­u­ghly as pos­si­ble.

War­ning: Wear pro­tec­tive glo­ves and avoid con­ta­mi­na­ting your skin with ammo­nium nitrate. During the mixing of zinc pow­der and ammo­nium nitrate, both sub­stan­ces must be com­ple­tely dry! Do not grind them toge­ther, as this may cause a dan­ge­rous explo­sion. Gen­tly mix the com­po­nents on a piece of card­bo­ard using a small woo­den stick. Do not store the mixture! Per­form the expe­ri­ment under a fume hood or out­do­ors. The author assu­mes no respon­si­bi­lity for any dama­ges that may occur. You do this at your own risk!

The Expe­ri­ment

Care­fully mix equal amo­unts — a few grams (rou­ghly 0.07–0.18 oz) — of both ingre­dients on a piece of card­bo­ard, then pour the mixture onto a fire­proof sur­face (cera­mic, asbe­stos, or metal). Next, add a few drops of water and step away. You can see the effect in the fol­lo­wing video:

We obse­rve that, after a short while, the water eva­po­ra­tes and the mixture igni­tes dra­ma­ti­cally. So it really is pos­si­ble to start a fire with water!

Expla­na­tion

When dry, the com­po­nents do not react. Howe­ver, in the pre­sence of water, a highly exo­ther­mic redox reac­tion occurs between zinc and ammo­nium nitrate, as shown in the fol­lo­wing equ­a­tion:

Ilustracja

At such a high tem­pe­ra­ture, the resul­ting water instan­tly eva­po­ra­tes, and the nitro­gen also esca­pes. The zinc oxide pro­du­ced rema­ins as a light, white ash.

Fur­ther rea­dings:

Marek Ples

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