Characteristics of aluminum and its combustion

Making it burn

Alu­minum is a sil­very-white met­al that swift­ly ox­i­dizes in air and be­comes cov­ered with an ox­ide film. This re­ac­tion also oc­curs when the met­al re­acts with con­cen­trat­ed acids.

[Deposit Photos]

Gen­er­al char­ac­ter­is­tics of alu­minum and its phys­i­cal prop­er­ties

Alu­minum is the 13th el­e­ment of the main group (IIIa, or boron group) of the pe­ri­od­ic ta­ble. Alu­minum has strong metal­lic prop­er­ties, and its atom­ic weight is 26.98; the met­al does not have sta­ble iso­topes in na­ture and ex­ists in a sin­gle form. Alu­minum has 3 va­lence elec­trons, and a great ma­jor­i­ty of alu­minum com­pounds have an ox­i­da­tion state of +3. Like all ac­tive met­als, alu­minum is a strong re­duc­er, as it has a low elec­tron affin­i­ty and a large atom­ic ra­dius.

Chunk of aluminium [Wikimedia]

Alu­minum is a light, soft, cor­ro­sion-re­sis­tant, high­ly-durable met­al. Not ev­ery sub­stance can boast such char­ac­ter­is­tics. Alu­minum’s main phys­i­cal prop­er­ties are:

  • melt­ing point – 660 °C;
  • face-cen­tered cu­bic crys­tal struc­ture;
  • boil­ing point – 2470 °C;
  • den­si­ty – 2.7 g/cm3;
  • type of bond – metal­lic;
  • as alu­minum is high­ly duc­tile and pli­able, it is used to make durable, light, thin foil. It is also rolled into wire.
Aluminum coils [Deposit Photos]

Alu­minum’s re­duc­tive abil­i­ty and chem­i­cal prop­er­ties

Alu­minum’s re­duc­tive prop­er­ties can be ob­served in the el­e­ment’s re­ac­tions with ox­ides of less ac­tive met­als. Here’s an ex­am­ple of one such re­ac­tion equa­tion:

Cr₂O₃ + 2Al = Al₂O₃ + 2Cr

In in­dus­try, alu­minum’s re­duc­tive qual­i­ties al­low for its use in ob­tain­ing oth­er met­als. When pure, alu­minum is a strong re­duc­er with high chem­i­cal ac­tiv­i­ty. To in­crease alu­minum’s ac­tiv­i­ty, its ox­ide film must be re­moved. The chem­i­cal prop­er­ties of the sim­ple sub­stance are de­ter­mined by its abil­i­ty to re­act with al­ka­lis, acids, sul­fur, and halo­gens. The met­al does not re­act with wa­ter in or­di­nary con­di­tions. At the same time, the only halo­gen that alu­minum re­acts with when un­heat­ed is io­dine. Oth­er re­ac­tions re­quire the ap­pli­ca­tion of heat. Here you can learn more about how alu­minum re­acts with hy­dro­gen and oth­er sub­stances.

Com­bus­tion of alu­minum — re­ac­tion de­scrip­tion

Pure alu­minum par­ti­cles do not com­bust in air or wa­ter va­por at tem­per­a­tures be­low 1727 °C. To ig­nite alu­minum in air, burn­ing mag­ne­sium par­ti­cles are placed on the sur­face of a heat­ing el­e­ment, and alu­minum par­ti­cles are placed on nee­dle points above them.

The alu­minum par­ti­cles ig­nite in the va­por phase, and the in­ten­si­ty of the glow that ap­pears around the par­ti­cles slow­ly in­creas­es. Com­bus­tion is char­ac­ter­ized by the pres­ence of a glow­ing zone, which does not change size un­til the met­al has burnt al­most com­plete­ly. In this zone, small drops of the ox­ide form and col­lide with one an­oth­er. The par­ti­cles re­main­ing af­ter com­bus­tion are shells with no met­al in­side. Here is the for­mu­la of the re­ac­tion of the com­bus­tion of alu­minum in oxy­gen:

4Al + 3O₂ = 2Al₂O₃

Ball-and-stick model of part of the crystal structure of Al₂O₃ [Wikimedia]

Com­bus­tion in wa­ter va­por: the ig­ni­tion of alu­minum in wa­ter va­por is het­ero­ge­neous. The hy­dro­gen re­leased in the re­ac­tion helps de­stroy the ox­ide film, and the liq­uid alu­minum ox­ide scat­ters in the form of drops. This ox­ide film forms and is de­stroyed re­peat­ed­ly, as a sig­nif­i­cant per­cent­age of the met­al burns on the sur­face of the par­ti­cles. Alu­minum com­busts five times more quick­ly in wa­ter va­por than in air.

Dis­cov­ery of alu­minum com­bus­tion

The com­bus­tion of alu­minum pow­der in a mix­ture with oxy­gen gas was first ap­plied in 1930 by chemists Beck­er and Strong in an oxy­gen-alu­minum blow­torch they in­vent­ed. The sci­en­tists used fine alu­minum pow­der as fuel. To sta­bi­lize com­bus­tion, a de­vice formed and con­stant­ly sup­plied a ho­moge­nous sus­pen­sion of alu­minum pow­der in oxy­gen. The mix­ture was lit with a Bun­sen burn­er. It burned with a blind­ing white flame, re­leas­ing a large quan­ti­ty of alu­minum ox­ide smoke. These par­ti­cles were so small that the smoke did not set­tle for 24 hours. Beck­er and Strong es­tab­lished that the com­bus­tion prod­ucts con­tained around 2% free alu­minum. By test­ing the ef­fect of the blow­torch’s flame on var­i­ous ma­te­ri­als, the sci­en­tists ap­prox­i­mate­ly de­ter­mined the tem­per­a­ture of the flame. Molyb­de­num melt­ed, but a tung­sten thread with a thick­ness of 1 mm did not. The sci­en­tists thus es­tab­lished that the com­bus­tion tem­per­a­ture of alu­minum in the mix­ture with oxy­gen was be­tween 2535 °C (the melt­ing point of molyb­de­num) and 3400 °C (the melt­ing point of tung­sten). To ob­serve the com­bus­tion re­ac­tion of alu­minum, and ad­mire the im­pres­sive sparks that ap­pear as a re­sult, you can con­duct the fol­low­ing ex­per­i­ment: sprin­kle alu­minum pow­der into the flame of a burn­ing spir­it burn­er, us­ing a porce­lain spoon or spat­u­la to add it in very small dos­es. You can also per­form this same ex­per­i­ment with zir­co­ni­um, ti­ta­ni­um, or mag­ne­sium pow­der. When the met­al pow­ders burn, the ox­ides Al₂O₃, ZrO₂, TiO₂, and MgO form. You shouldn’t use ex­treme­ly fine pow­ders of these met­als, as they may ex­plode in the flame.