Compounds of Aluminium- Definition, Important compounds, Physical Properties, Chemical properties, Uses, Practice Problems, FAQs.

You must have seen your mother using thin shiny foil for wrapping chapatis or food to stay hot for a longer time. What is this shiny foil made up of? You can ask this question to your teacher. 
You have seen the utensils in your kitchen, some of them are made up of aluminium. Aluminium is commonly used to make household utensils, trays, frames, and other items due to its low cost and lightweight. It is a good conductor of heat and electricity that’s why it is used in electrical wiring and kitchen utensils. 
It's impossible to mention all of the aluminium's applications in our daily lives. Buildings, boats, planes, and cars, as well as household appliances, packaging, computers, cellphones, and food and beverage containers, all benefit from the better design, sustainability, corrosion resistance, and lightweight strength of aluminium.
Let us study this metal and some important compounds related to aluminium in detail.

Table of content:
What is Aluminium?
Important compounds of Aluminium
Aluminium Oxide
Aluminium Chloride
Alums
Ultramarine
Practice Problems
Frequently asked questions

What is Aluminium?
Aluminium is the 13th element in the periodic table, and it is a silvery-white metal. 
One startling statistic about aluminium is that it is the most common metal on the planet, accounting for more than 8% of the planet's core mass. After oxygen and silicon, it's the third most common chemical element on the earth.
Aluminium is the third most plentiful element on the planet, accounting for 8.3% of the earth's crust. It's found in clay, slate, and a variety of silicate rocks. It can only be found in the combined state.

Important compounds of Aluminium:
There is a wide range of Aluminium compounds available, but few are very important for our daily purpose. Let's study a few of them. 
Aluminium Oxide
Aluminium Chloride
Alums
Ultramarine

Aluminium Oxide:
It is found in nature as colourless corundum that has been coloured by metallic oxides such as ruby (red), sapphire (blue), amethyst (violet), emery (green), and others. These coloured oxides are valuable gems.
Bauxite is a dihydrated oxide (Al2O3.2H2O).
Preparation: Aluminum hydroxide, aluminium sulphate, or ammonium alum are used to make it.
2Al(OH)3(s)Al2O3(s)+3H2O(aq)
Al2(SO4)3(s)Al2O3(s)+3SO3(g)
(NH4)2SO4.Al2(SO4)3.24H2O(s)Al2O3(s)+2NH3(g)+4SO3(g)+25H2O(aq)
It's made from a mixture of aluminium fluoride and boric oxide that's been heated to a crystalline state.
2AlF3(s)+B2O3(s)Al2O3(s)+2BF3(g)

Properties:
It is a white solid that is water-insoluble. It is a non-reactive and stable chemical. 
Around 1750°C, it begins to volatilize and melts at 2050°C. At 2250°C, it boils.
It exemplifies amphoteric character. Synthetic semi-precious stones are created by fusing it with chromium, iron, and cobalt oxides.

Uses:
It's most typically utilised to make bauxite bricks, which are used to line furnaces.
Fused alumina is utilised as a refractory material. Alundun, a hard powder that is used as an abrasive, is generated when alundum is burned in an electric arc at 3000°C.
When mixed with lime, it is used as a bauxite cement that is not affected by seawater.
It's used in chromatography, aluminium extraction, and the production of precious stones.

Aluminium Chloride:
Two types of Aluminium chloride are found:
I. Anhydrous aluminium chloride
II. Hydrated aluminium chloride

Anhydrous aluminium chloride: 
It is made by passing dry HCl or chlorine gas over hot aluminium turnings in the absence of oxygen. When solid anhydrous aluminium chloride is heated, the vapours of aluminium chloride condense.
2Al(s)+6HCl(g)2AlCl3+3H2(g)
2Al(s)+3Cl2(g)2AlCl3(s)
It can also be made by heating at 1000 ℃ an alumina and carbon in a chlorine-rich environment.
Al2O3(s)+3C+3Cl2(g)2AlCl3(g)+3CO(g)
Vapours of AlCl3 when cooled, solid anhydrous aluminium chloride is obtained.
AlCl3(g)AlCl3(s) (anhydrous aluminium chloride)

Hydrated aluminium chloride:
When aluminium metal or aluminium hydroxide is dissolved in dilute hydrochloric acid, AlCl3.6H2O is produced.
2Al(s)+6HCl(g)2AlCl3+3H2(g)
Al(OH)3(s)+3HCl(g)AlCl3+3H2O(g)
To obtain hydrated aluminium chloride crystals, HCl gas is cycled throughout the solution.

Properties:
Aluminium chloride, anhydrous, is a white solid. It is effervescent and emits fumes into the atmosphere. When heated to 180°C, it sublimes, and the vapour density matches the formulaAlCl6. When anhydrous, it is covalent because it does not conduct current in the fused state. It can be dissolved in organic solvents like alcohol, benzene, ether, etc. When dissolved in non-polar solvents, the dimeric formula is preserved, but when dissolved in polar solvents, it is broken down into [Al(H2O)6].Cl3 due to the high heat of hydration, in water. The molecule (dimer) is an auto complex with the following representation:
 

Due to the evolution of HCl, anhydrous aluminium chloride smokes in the damp air.
Al2Cl6(s)+6H2O(aq)2Al(OH)3+6HCl
It transforms into hydrated aluminium chloride, which is ionic in nature when dissolved in water.
Al2Cl6(s)+6H2O(aq)2AlCl3.6H2O(aq)
AlCl3 (s)⇌Al3+(g)+3Cl- (g)
With ammonia gas, anhydrous aluminium chloride creates an addition product.
Al2Cl6(s)+12NH3(g)2[AlCl3.6NH3](s)
Due to hydrolysis, the solution of aluminium chloride in water is acidic.
AlCl3 (s)+3H2O(aq)Al(OH)3(s)+3HCl (g)(strong acid)

Uses:
Friedel Craft's reaction uses it as a catalyst.
It's also employed in the production of gasoline by cracking high-boiling petroleum fractions.
It is widely used in the production of dyes, pharmaceuticals, and perfumes.

Alums: 
Previously, the name alum was reserved for a single double sulphate with 24 molecules of water of crystallisation, potassium aluminium sulphate, K2SO4.Al2(SO4)3.24H2O , but it is now applied to all double sulphates with the composition, M2SO4.M'(SO4)3.24H2O.
Monovalent basic radicals such as Na+, K+, Rb+, Cs+, Ag+, Tl+, and NH4+ are represented by the letter M, whereas trivalent are represented by M', these are Al3+,Cr3+, Fe3+ etc.
Alums are typically made by mixing heated solutions of equimolar concentrations of their constituent sulphates and then crystallising the resultant solution.
Alum is a crystalline substance. Each metal ion in alums is surrounded by six water molecules, and alum crystals are made up of [M(H2O)6]+, [M'(H2O)6]3+ and SO42- ions.
In hot water, alums are fairly soluble, but in cold water, they are less soluble. The solutions are acidic and bitter in flavour. The properties of the constituent salt ions are displayed in the solutions. The alums are isomorphous and form mixed crystals together.
The melting point of alum varies. When alum is heated, the water of crystallisation evaporates. The alum expands up when burned to a high temperature quickly, leaving a porous mass known as burnt alum behind.
The alums are efficient coagulants because they precipitate colloids. Germicide characteristics are found in alums. As a result, alums are utilised in water purification, leather tanning, colour mordants, and antiseptics.

Potash Alum:
Aluminium sulphate is made from bauxite or aluminium sulphate is made by boiling bauxite with sulphuric acid. A calculated amount of K2SO4 is added to this solution. The solution is cooled and concentrated. Potash alum crystals are formed after a period of time.
Al2O3(s)+3H2SO4(aq)Al2(SO4)3(s)+3H2O
Al2(SO4)3(s)+K2SO4(s)+24H2OK2SO4.Al2(SO4)3.24H2O(potash alum)

From alum stone or alunite: Alum stone is heated after being treated with weak sulphuric acid. To the solution, a calculated amount of K2SO4 is added. Potash crystals form when the solution cools.
K2SO4.Al2(SO4)3.4Al(OH)3

Properties:
It is a white crystalline substance.
It is soluble in water and has an acidic aqueous solution due to Al2(SO4)3.hydrolysis.
It swells up when heated due to the elimination of water molecules.

Uses:
Used in dyeing and calico printing as a mordant.
In sizing of low-quality paper.
In water purification.
Used as an antiseptic and in the treatment of cuts.
Used in the tanning of leather.

Ultramarine:
Ultramarine is an artificial Lapis-Lazuli. Lapis-Lazuli is a magnificent blue mineral that is extremely rare. 

It's a sodium-aluminium complex silicate with around 12% sulphur, most likely in the form of sodium sulphide form.
Artificially, it can be made by heating a mixture of kaolin, soda ash, sulphur, and charcoal to a bright red temperature. In the beginning, a white mass is formed, which then transforms into a green mass in the air.
The blue variation with the composition Na5Al3Si3S3O12 is created by powdering it and heating it with additional sulphur. When heated in a dry chlorine current, the blue variant transforms into a violet version.

Uses: 
Used in making blue paint.
Used in the laundry for blueing purposes.
Used in making wallpaper and blue-tinted paper.
Used in calico printing.

Practice Problems:

Q1._____________ is the famous alloy of Aluminium
A. Magnatite
B. Duralumin
C. Celestrium
D. Steel
Answer: B
Solution: Duralumin is one of the famous alloys of aluminium. Its composition is 95% Al + 4% Cu + 0.5% Mg + 0.5% Mn.

Q2. Sodium alum has molecular mass equal to___________u
A. 428.38
B. 458.28
C. 348.68
D. 243.54
Answer: B
Solution: Sodium alum has molecular mass equal to 458.28 u, ,Molecular formula od sodium alum is NaAl(SO4)2.12H2O. Soda alum, sodium alum, or SAS is another name for it.

Q3._____________ process is used purification of bauxite ore
A. Froth flotation
B. Blast furnace
C. Baeyer's Process
D.Thermite
Answer: B
Solution: Baeyer's Process is mostly used on bauxite ore with ferric oxide as the major contaminant. Because the ore is usually red in colour, it is known as red bauxite.

Q4.Aluminium being more electropositive in nature displaces ____________ from their salt solutions.
A. Zn metal
B. Cu metal
C. Pb metal
D. All of the above
Answer: D
Solution: Because Al is more electropositive, it displaces copper, zinc, and lead from their salt solutions.
Here are the reactions of these metals with Al.
3ZnSO4(s)+2Al(s)Al2(SO4)3(s)+3Zn(s)
3CuSO4(s)+2Al(s)Al2(SO4)3(s)+3Cu(s)
3PbSO4(s)+2Al(s)Al2(SO4)3(s)+3Pb(s)

Frequently asked questions- FAQ

Q1. Is aluminium prone to catching fire?
Solution: The molecular structure of aluminium is such that its molecules are densely packed, necessitating a high temperature to burn it. Aluminium powder, on the other hand, is easily flammable. Thermite welding, which uses Aluminium powder and Ferric oxide, is an example of powder Aluminium combustion.

Q2. Can we burn aluminium foil? Is this dangerous or not?
Solution: Aluminium foil has the ability to melt and burn, but only at extremely high temperatures. This is why you can use it in the oven to cook dishes and it will often stay intact. It's better not to try this at home because the gases produced by burning aluminium are quite harmful.

Q3. Cooking on aluminium Utensils is safe. Why?
Solution: The amount of aluminium you are exposed to on a daily basis through your food and cooking is deemed safe. This is due to the fact that healthy persons can effectively eliminate the little amounts of aluminium absorbed by the body.

Q4. Is aluminium foil environmentally friendly?
Solution: Aluminum foil isn't necessarily harmful to the environment. It decomposes quickly and can be recycled or reused. However, aluminium production and the amount of foil that ends up in landfills make it unfriendly to the environment.