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Aluminium and its Properties

Aluminium is the most abundant metal, and the third most abundant element after oxygen and silicon in the earth crust. It is found in large quantity in rocks and clays as silicate(IV).

However, the main source from which it is extracted is the mineral - bauxite, Al₂O₃.2H₂O.

Electrolytic method is used in extracting aluminium from the bauxite. Other important naturally occurring compounds of aluminium are:

mica; felspar (K₂Al₂Si₆O₁₆) ; Kaolin (or China clay, Al₂Si₂O₇.2H₂O); Cryolite (Na₃AlF₆); and Corundum (Al₂O₃).

The process of purifying bauxite involves the following steps:

(1). It is heated with caustic soda solution (NaOH) under pressure - soluble sodium aluminate (i.e. NaAl(OH)₄) is formed.

Al₂O_3(s) + 2NaOH_(aq) + 3H₂O_(l) → 2NaAl(OH)_4(aq)

Note: the impurities in the ore are mainly iron(III) oxide and different silicates(IV). They do not get involved or dissolved in the above reaction. Hence, they are subsequently filtered off as sludge.

(2). The filtrate (NaAl(OH)₄ is seeded with aluminium hydroxide crystals to cause aluminium hydroxide in the filtrate to precipitate.

NaAl(OH)_4(aq) → Al(OH)_3(s) + NaOH_(aq)

(3). The precipitate (Al(OH)_3(s) ) is filtered off, washed, dried and heated strongly. Pure aluminium oxide (Al₂O₃), which is also called alumina is formed.

2Al(OH)_3(s) → Al₂O_3(s) + 3H₂O_(l)

Aluminium is extracted by the electrolysis of pure alumina (i.e. Al₂O₃). See explanations of electrolysis here.

Physical Properties of Aluminium

Below are physical properties of aluminium:

1. Aluminium appears as a silvery-white solid.

2. It has a relative density of 2.7.

3. It is very malleable and ductile, and can therefore be rolled into a foil.

4. It has a melting point of about 660^oC.

5. It has a moderate tensile strength, but which is high in its alloys.

6. It is a very good conductor of heat and electricity.

The chemical properties of aluminium include the following:

1. Reaction with air

 Aluminium, when exposed to air reacts with oxygen to form aluminium oxide, Al₂O₃. At ordinary temperature and pressure, this oxide appears as a thin coat on the surface of the aluminium, thereby preventing further reaction with oxygen, water or steam. This is the reason aluminium is free of corrosion.

However, if the temperature is increased to 800^oC and above, the whole piece of aluminium would react with oxygen and nitrogen of the air to form aluminium oxide and nitride respectively.

4Al_(s) + 3O_2(g) → 2Al₂O_3(s)

2Al_(s) + N_2(g) → 2AlN_(s)

2. With non-metals

When aluminium is heated, it combines with a number of non-metals, such as the halogens, sulphur, nitrogen, phosphorus and carbon, with the liberation of heat.

Example, 2Al_(s) + 3Cl_2(g) → 2AlCl_3(s)

3. With acids - aluminium reacts to different degrees with certain acids at different concentrations. Example, with dilute HCl, aluminium reacts slowly, displacing hydrogen.

2Al_(s) + 6HCl_(aq) → 2AlCl_3(aq) + 3H_2(g)

With conc. HCl, the reaction is rapid; with dilute H₂SO₄ no reaction; and with hot conc. H₂SO₄ aluminium reacts to release sulphur(IV) oxide, SO₂.

2Al_(s) + 6H₂SO_4(aq) → Al₂(SO₄)_3(aq) + 6H₂O_(l) + 3SO_2(g)

With HNO₃ aluminium does not react with HNO₃ in both dilute and concentrated conditions. This is probably due to the formation of a protective thin layer of aluminium oxide on its surface. Hence, aluminium containers are commonly used in transporting HNO₃.

4. With Alkalis

When reacted with an alkalis such as sodium hydroxide or potassium hydroxide, aluminium dissolves to form a soluble aluminate (complex salt), with the release of hydrogen.

2Al_(s) + 2NaOH_(aq) + 6H₂O_(l) → 2NaAl(OH)_4(aq) + 3H_2(g)

sodium aluminate, 2NaAl(OH)₄ is a complex salt.

Note: washing soda, being alkaline, should not be used to clean aluminium articles. This is because aluminium is amphoteric, that is, it reacts with both acids (except HNO₃) and alkalis to form salts.

The following tests can confirm the presence of aluminium ions:

(1). Action with sodium hydroxide

With the addition of a few drops of sodium hydroxide solution, an aluminium salt solution will produce a white precipitate, which will dissolve in excess of the alkali.

Al³⁺_(aq) + 3NaOH_(aq) → Al(OH)_3(s) + 3Na⁺_(aq)

Al(OH)_3(s) + NaOH_(aq) → NaAl(OH)_4(aq)

Note: zinc and lead(II) salts also give the above reaction.

(2). With aqueous ammonia

An aluminium salt solution will give a white, gelatinous precipitate when a few drops of aqueous ammonia is added onto it. The precipitate is insoluble in excess of the ammonia solution.

Al³⁺_(aq) + 3OH^-_(aq) → Al(OH)_3(s)

Note: Zinc salts also give the above reaction. However, their precipitates are soluble in excess of aqueous ammonia.

Lead(II) salts give the above reaction and the precipitate is also insoluble in excess ammonia, but they can be distinguished from aluminium salts by the action of concentrated HCl on them.

Aluminium salts do not form precipitate with conc. HCl, while lead(II) salts give white precipitate.

The following are some uses for aluminium and its compounds:

(1). In alloys - several alloys are composed partly of aluminium. This is due to the fact that aluminium has both high tensile strength and lightness.

Hence, it is mostly used in aircraft construction.

(2). In cooking utensils. Aluminium is commonly used as cooking utensils because it is cheap and has low density. It also has good appearance and good conductivity of heat. It is also resistant to attack by cooking solutions.

Note: aluminium resists attack because it forms a thin film of aluminium oxide on its surface, thereby preventing the exposure of the inner part to the attacking substance.

However, aluminium vessels should not be exposed to alkaline solutions (as has been mentioned earlier) because the oxide (i.e. Al₂O₃) will dissolve to form salt.

(3). In overhead electric cables

 Aluminium is used in making overhead electrical cables due to its low density (i.e. lightness) and good electrical conductivity. Thick cables of low resistance can be employed without undue weight.

(4). In Themit process

The Themit process is used in welding together broken metallic parts of machines and equipment. Example, to weld together the broken ends of an iron rod.

(5). As foils

Aluminium foils are commonly used as packaging materials.

(6). As Paints

Powdered aluminium is used with oil to make paints used for mirrors, cars, petrol tanks and milk tanks. This is due to the high reflectivity of aluminium.