To Prepare Aluminium Hydroxide [Al(Oh)3] Sol

Colloidal solutions are solutions containing solutes and solvents which are immiscible within each other. As a result, the solute particles remain suspended giving a cloudy appearance to the solution. Colloidal solutions are of various types; one of them is lyophobic, sols-wherein the dispersed phase is a solid and the dispersion medium is liquid. The dispersed phase here has less affinity towards the dispersion medium, hence the name lyophobic. The stability of lyophobic sols is due to the charge on the colloidal particles. Common examples of lyophobic sols are Ferric hydroxide, Aluminium hydroxide, Arsenic sulphide etc.

Table of Contents

• Aim

• Preparation of Aluminium Chloride AlCl3

• Preparation of Aluminium Hydroxide Al(OH)3 Sol

• Result

Aim

To prepare aluminium hydroxide Al(OH)3 sol

Apparatus required

• Conical flask

• Beaker

• Glass-rod

• Funnel

• Filter paper 

• Burner 

• Wire-gauze 

• Tripod Stand 

• Stand with clamp 

• Dropper

• 2% Aluminium chloride solution 

• Distilled water

Theory

Metal hydroxides and metal sulphides form lyophobic sols because they have a reduced affinity towards the solvent into which they are made. Due to the lyophobic nature of metal hydroxides, they are prepared indirectly through hydrolysis by using aluminium chloride in boiling distilled water. The reaction is as follows- 

AlCl3 + 3H2O→Al(OH)3 + 3HCl

The Al(OH)3 forms insoluble colloidal particles which aggregate together, forming larger colloidal particles. The aluminium hydroxide colloidal sol is white. The excess HCl produced during the above process is removed by dialysis.

Procedure

1. Preparation of AlCl3 solution.

1. Weigh 2g of pure AlCl3 in a beaker.

2. Take 100ml of D/W in a 250ml beaker and boil it.

3. Take the 2g of pure AlCl3 and add it to the boiling D/W and dissolve it.

4. A 2% Aluminium chloride solution is prepared.

2. Preparation of Al(OH)3

1. Clean 250ml conical flask by the process of steaming out.

2. Take 100ml of Distilled water in the above conical flask and boil it.

3. Take 10ml of freshly prepared 2% AlCl3 solution and add it to the boiling water in a drop-wise manner using a dropper/pipette/ burette.

4. Keep constantly stirring while adding the aluminium chloride solution.

5. Continue heating the flask until a white-coloured sol is obtained. Continuously replenish the water that is lost due to boiling action.

6. Allow the contents of the flask to cool till room temperature. Aluminium hydroxide sol is prepared.

Observations

As soon as AlCl3 solution is added to the boiling water, small colloidal particles of aluminium hydroxide are formed, slowly forming a colloidal sol. The Al(OH)3 solution thus prepared is white.

Result

A lyophobic colloidal sol of Aluminium hydroxide is prepared using the process of hydrolysis of aluminium chloride with boiling water.

Precautions

1. Use proper neat, and clean apparatus for the experiment.

2. Carefully handle the apparatus and carefully use the burner.

3. Remove the HCl produced by the process of dialysis.

4. Pour the aluminium chloride solution slowly and drop-wise.

5. Constantly stir the components of the flask.

6. Use proper protective gear in the laboratory.

Lab Manual Questions

1. Give the physical properties of aluminium hydroxide.

Ans: Following are the properties of aluminium hydroxide-

• Aluminium hydroxide is an amorphous white-coloured powder.

• It is insoluble in water but readily soluble in acids and alkalis.

• Its molecular weight is 78.00 g/mol, its density is 2.42 g/dm3, and its melting point is 300℃.

• It is amphoteric.

2. Why does dialysis of the colloidal sol have to be conducted?

Ans: Lyophobic sols such as aluminium hydroxide are stabilised due to the charges present on their surfaces. When acids such as HCl are present in the sol, these charges on the surface of the colloidal particles get disturbed and may lead to the precipitation of colloidal particles. Hence, to prevent this, HCl formed during the chemical reaction is removed through dialysis.

3. How is aluminium hydroxide made commercially? 

Ans: Aluminium hydroxide is manufactured commercially through the Bayer process. It involves dissolving bauxite in NaOH solution at temperatures up to 270℃. The waste products and the sodium aluminate solution is precipitated. This precipitate contains aluminium hydroxide.

4. What is the process of steaming out?

Ans: Steaming-out process cleans the conical flask in which the aluminium hydroxide sol is prepared. The conical flask to be cleaned is inverted over a round bottom flask which contains boiling water. The steam that is produced due to the boiling water is used for cleaning the conical flask. This helps remove any impurities that might be present in the conical flask in which the aluminium hydroxide sol is prepared.

Viva Questions 

1. How does the preparation of aluminium hydroxide gel take place?

Ans: Aluminium hydroxide gel is produced via the process of hydrolysis of aluminium chloride with boiling water.

2. Give the chemical formula of aluminium hydroxide

Ans: The chemical formula of aluminium hydroxide is Al(OH)3.

3. What is aluminium hydroxide?

Ans: Aluminium hydroxide is a metal hydroxide that, in nature, is present as the mineral gibbsite or hydragillite. It is an amorphous white-coloured powder and has various daily life applications.

4. What is bauxite?

Ans: Bauxite is a sedimentary rock with high amounts of aluminium and gallium.

5. Give the constituents of bauxite.

Ans: Bauxite comprises minerals such as gibbsite, boehmite, diaspore and two iron oxides, clay mineral kaolinite and small amounts of anatase and ilmenite.

6. What is aluminium hydroxide gel?

Ans: Aluminium hydroxide gel is the aqueous solution of aluminium hydroxide. 

7. How is aluminium hydroxide gel prepared?

Ans: Aluminium hydroxide gel is prepared by reacting aluminium oxide with water.

8. Define lyophobic sols.

Ans: Sols wherein the dispersed phase (solid)  has weak interactive forces with the dispersion medium (solvent) or is solvent-repelling are known as lyophobic sols.

9. Give the two types of aerosols present.

Ans: There are two types of aerosols-

• Solid aerosols-here, a solid dispersed phase is suspended in the gas dispersion medium. 

• Liquid aerosols-here, a liquid dispersed phase is suspended in a gas dispersion medium.

10. What are emulsions?

Ans: Emulsions are liquid-liquid colloidal solutions. An emulsifying agent stabilises them. E.g. milk, ice-creams etc.

Practical-Based Questions (MCQs)

1. Commercially, aluminium hydroxide is produced by_____process.

1. Decomposition

2. Combustion

3. Bayer

4. Filtration

Ans: Bayer

2. Which of the following are alternative names of Al(OH)3

1. Aluminic acid

2. Aluminium hydroxide

3. Aluminium (III) hydroxide

4. All of the above

Ans: All of the above

3.  Aluminium hydroxide is a _____

1. Acid

2. Base

3. Both A and B

4. Neutral

Ans: Both A and B

4. Aluminium ore is_____and contains_____and ____

1. Bauxite, Aluminium oxide, Aluminium oxide hydroxide

2. Bauxite, Alumina, Aluminium oxide

3. Alumina, Aluminium oxide, Aluminium oxide hydroxide

4. Alumina, Bauxite, Aluminium oxide hydroxide

Ans: Bauxite, Aluminium oxide, Aluminium oxide hydroxide

5. Commercially, aluminium metal is obtained from_____

1. Alumina only

2. Aluminium hydroxide 

3. Aluminium oxide hydroxide only

4. Bauxite ore

Ans: Bauxite ore

6. Aluminium hydroxide is prepared by the process of______

1. Lipolysis

2. Proteolysis

3. Hydrolysis

4. Carboxylation

Ans:  Hydrolysis

7. The pH of aluminium hydroxide is between____

1. 5 to 10

2. 5.7 to 6.7

3. 7 to 8

4. 0 to 3.0

Ans: 5.7 to 6.7

8. Al2O3 is____

1. Bauxite

2. Aluminium oxide

3. Aluminium hydroxide

4. Dialuminum trioxide

Ans: Aluminium oxide

9. Aluminium hydroxide is a____

1. Lewis acid

2. Lewis base

3. Normal acid 

4. Normal base

Ans: Lewis acid

10. Gibbsite acts as _____and neutralises the acid.

1. Brønsted-Lowry base

2. Lewis base

3. Brønsted-Lowry acid

4. Lewis acid and base

Ans: Brønsted-Lowry base

Conclusion

From the above experiment, we conclude that aluminium hydroxide forms lyophobic sol. Lyophobic sols have a solid solute particle with a low affinity towards the solvent particles. Aluminium hydroxide is a white-coloured colloidal solution which is formed by the process of hydrolysis. It is an amphoteric chemical component and has various industrial and medical applications.