

CBSE Chemistry Experiment - 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
Preparation of AlCl3 solution.
Weigh 2g of pure AlCl3 in a beaker.
Take 100ml of D/W in a 250ml beaker and boil it.
Take the 2g of pure AlCl3 and add it to the boiling D/W and dissolve it.
A 2% Aluminium chloride solution is prepared.
Preparation of Al(OH)3
Clean 250ml conical flask by the process of steaming out.
Take 100ml of Distilled water in the above conical flask and boil it.
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.
Keep constantly stirring while adding the aluminium chloride solution.
Continue heating the flask until a white-coloured sol is obtained. Continuously replenish the water that is lost due to boiling action.
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
Use proper neat, and clean apparatus for the experiment.
Carefully handle the apparatus and carefully use the burner.
Remove the HCl produced by the process of dialysis.
Pour the aluminium chloride solution slowly and drop-wise.
Constantly stir the components of the flask.
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)
Commercially, aluminium hydroxide is produced by_____process.
Decomposition
Combustion
Bayer
Filtration
Ans: Bayer
Which of the following are alternative names of Al(OH)3
Aluminic acid
Aluminium hydroxide
Aluminium (III) hydroxide
All of the above
Ans: All of the above
Aluminium hydroxide is a _____
Acid
Base
Both A and B
Neutral
Ans: Both A and B
Aluminium ore is_____and contains_____and ____
Bauxite, Aluminium oxide, Aluminium oxide hydroxide
Bauxite, Alumina, Aluminium oxide
Alumina, Aluminium oxide, Aluminium oxide hydroxide
Alumina, Bauxite, Aluminium oxide hydroxide
Ans: Bauxite, Aluminium oxide, Aluminium oxide hydroxide
Commercially, aluminium metal is obtained from_____
Alumina only
Aluminium hydroxide
Aluminium oxide hydroxide only
Bauxite ore
Ans: Bauxite ore
Aluminium hydroxide is prepared by the process of______
Lipolysis
Proteolysis
Hydrolysis
Carboxylation
Ans: Hydrolysis
The pH of aluminium hydroxide is between____
5 to 10
5.7 to 6.7
7 to 8
0 to 3.0
Ans: 5.7 to 6.7
Al2O3 is____
Bauxite
Aluminium oxide
Aluminium hydroxide
Dialuminum trioxide
Ans: Aluminium oxide
Aluminium hydroxide is a____
Lewis acid
Lewis base
Normal acid
Normal base
Ans: Lewis acid
Gibbsite acts as _____and neutralises the acid.
Brønsted-Lowry base
Lewis base
Brønsted-Lowry acid
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.
FAQs on To Prepare Aluminium Hydroxide [Al(Oh)3] Sol
1. What is the expected procedure for preparing aluminium hydroxide sol for a 3-mark question in the CBSE Class 12 Chemistry practical exam 2025-26?
The procedure for preparing aluminium hydroxide sol involves the controlled hydrolysis of an aluminium salt. For full marks, the following steps should be mentioned:
- Prepare a 2% solution of aluminium chloride (AlCl₃) in distilled water.
- Take about 100 mL of distilled water in a clean beaker and heat it to boiling.
- Using a dropper, add the aluminium chloride solution drop by drop into the boiling water while stirring continuously.
- Continue heating the mixture until a white, milky, or translucent colloidal sol of aluminium hydroxide is formed.
- Stop heating and allow the beaker to cool down at room temperature. The resulting solution is the Al(OH)₃ sol.
2. Write the chemical equation involved in the preparation of aluminium hydroxide sol and identify the type of reaction.
The chemical reaction for the formation of aluminium hydroxide sol from aluminium chloride is a key question for the board exam. The balanced equation is:
AlCl₃ (aq) + 3H₂O (l) (boiling) → Al(OH)₃ (sol) + 3HCl (aq)
This reaction is an example of hydrolysis, where a salt is broken down by reacting with water to form a colloidal solution of its hydroxide.
3. Why is the freshly prepared Al(OH)₃ sol classified as a lyophobic colloid?
Aluminium hydroxide sol is classified as a lyophobic colloid (which means 'solvent-hating') for two main reasons:
- There is very little affinity or attraction between the dispersed phase particles, which are Al(OH)₃, and the dispersion medium, which is water.
- It cannot be prepared by simply mixing aluminium hydroxide with water. It requires a special chemical method, like hydrolysis, for its formation.
4. How would you explain the positive charge on the colloidal particles of aluminium hydroxide sol?
The positive charge on Al(OH)₃ sol particles is due to the phenomenon of preferential adsorption of ions. During the hydrolysis of AlCl₃, the Al(OH)₃ particles adsorb Al³⁺ ions, which are common to their own lattice, from the dispersion medium. This layer of adsorbed positive ions gives the entire colloidal particle a net positive charge, i.e., [Al(OH)₃]Al³⁺.
5. What is the consequence of adding the aluminium chloride solution too quickly or using cold water instead of boiling water during this experiment?
This is a common viva question to test conceptual understanding.
- Adding AlCl₃ too quickly: This would lead to the formation of a coarse white precipitate of Al(OH)₃ instead of a stable colloidal sol. Slow, controlled addition is essential for the particles to form within the colloidal size range.
- Using cold water: The rate of hydrolysis would be extremely slow in cold water, preventing the effective formation of a stable sol. Boiling water provides the necessary energy for the hydrolysis to proceed at an optimal rate for sol formation.
6. Why is the purification of the prepared aluminium hydroxide sol by dialysis often necessary?
Purification by dialysis is crucial because the hydrolysis of AlCl₃ produces hydrochloric acid (HCl) as a soluble impurity. The presence of excess electrolytes like HCl can destabilise the sol and cause it to coagulate or precipitate over time. Dialysis uses a semi-permeable membrane to remove these dissolved ionic impurities, thereby increasing the stability of the final colloidal solution.
7. State two important precautions a student must take to score full marks in the practical exam for preparing Al(OH)₃ sol.
To ensure a successful experiment and secure full marks, students must adhere to these precautions:
- All glassware, including the beaker and stirrer, must be meticulously cleaned. Any impurity can act as a nucleus, causing premature coagulation of the sol.
- The aluminium chloride solution must be added dropwise with constant stirring. This prevents localised high concentrations and ensures the formation of a uniform, stable sol instead of a precipitate.
8. Aluminium hydroxide is amphoteric in nature. How can you justify this statement for a 2-mark question?
The amphoteric nature of aluminium hydroxide, Al(OH)₃, means it can react with both acids and bases. To justify this, you can show its reaction with a strong base like sodium hydroxide (NaOH):
Al(OH)₃ (s) + NaOH (aq) → Na[Al(OH)₄] (aq)
In this reaction, Al(OH)₃ acts as a Lewis acid by accepting a hydroxide ion, forming the soluble complex sodium tetrahydroxoaluminate(III). This demonstrates its acidic character, complementing its well-known basic character when reacting with acids.

















