CBSE Class 12 Chemistry: Stepwise Guide to Colloidal Arsenious Sulphide Preparation (2025-26)

Colloidal solutions are heterogeneous solutions containing an insoluble dispersed phase. The solvent medium here is a dispersion medium in which solute particles are suspended. There are various types of colloidal solutions, examples-sols, emulsions, foam, and aerosols. Sols are a common type of colloid wherein the solute is solid, and the solvent is liquid. Sols are of two types, lyophobic, which is solvent-repelling and lyophilic, which is solvent-attracting. Lyophobic sols and lyophilic sols are differentiated based on the nature of the interaction between the solute and solvent particles. Common lyophobic sols are arsenious sulphide, aluminium hydroxide, ferric hydroxide etc

Table of Contents

• Aim

• Apparatus Required

• Theory

• Observation

• Result

Aim

To prepare a colloidal solution of arsenious sulphide As2S3

Apparatus required

• Conical flask

• Beaker

• Glass-rod

• Funnel

• Filter paper 

• Burner 

• Wire-gauze 

• Tripod Stand 

• Stand with clamp 

• Distilled water

• Glass tube

Theory

Arsenious sulphide forms a lyophobic colloidal solution. Arsenious sulphide sol is prepared through hydrolysis of arsenious oxide As2O3 in boiling distilled water. H2S gas is passed through the above arsenious oxide solution, giving the final arsenious sulphide. In this colloidal sol, each colloidal particle is surrounded by HS- ions, formed from the dissociation of H2S gas. A layer of H+ ions again surrounds this sulphide ion layer.

Procedure

1. Take a clean conical flask and clean it by the steaming-out process.

2. Add 0.2g of arsenious oxide solid into the above flask and then pour 100 ml of Distilled water.

3. Boil this solution for approximately 10 mins.

4. Using a fluted filter paper, filter out the above contents and collect the filtrate in another beaker.

5. Pass a slow current of H2S gas into an Arsenious oxide solution until a stable yellow-coloured solution is not obtained.

6. Remove the excess H2S gas by boiling the sol slowly.

7. Filter the bright yellow-coloured filtrate in a dry conical flask using a fluted-filter paper and cork it. 

8. Arsenious sulphide sol is prepared.

Preparation of As2S3 solution

Observations

The hydrolysis of the arsenious oxide process was followed by the passing of H2S in the arsenious oxide solution, which led to the formation of a yellow-coloured solution. With every step of passing H2S gas in the solution, the intensity of the yellow colour increased until there was no change in the intensity of the yellow-coloured form.

Result

A lyophobic colloidal sol of arsenious sulphide was prepared by the process of hydrolysis of arsenious oxide with distilled water followed by passage of H2S gas from the above solution.

Precautions

1. Use properly cleaned apparatus for the experiment.

2. Be careful while working with H2S gas.

3. Arsenious oxide is highly poisonous, hence use protective gear while working with it.

4. Do not eat or drink anything while performing this experiment.

5. Clean your hands properly after the experiment is completed.

Lab Manual Questions

1. What are the properties of arsenious sulphide?

Ans: Following are properties of arsenious sulphide

• It is an inorganic compound.

• Its chemical formula is As2S3.

• It is a dark yellow solid compound.

• It is insoluble in water.

• It is found in two mineral forms- orpiment and realgar.

2. How is Arsenious sulphide sol prepared?

Ans: Arsenious sulphide is a lyophobic sol; hence, it contains a solid dispersed phase-arsenious oxide and a liquid dispersion medium-water. To form arsenious sulphide, sol-hydrolysis of arsenious oxide is done with boiling water, followed by the passing of hydrogen sulphide gas in the above solution. After which, the entire solution is filtered, and yellow-coloured arsenious sulphide sol is obtained.

3. Why is clean and washed lab apparatus required for this experiment?

Ans: The apparatus used for the above experiment has to be cleaned properly because the preparation of such inorganic compounds is highly sensitive, and even the minute presence of impurities or contamination will hinder the formation of arsenious sulphide, and a pure As2O3 solution will not be prepared. The steaming-out process is used to clean the apparatus to remove all impurities.

4. What is responsible for the stability of arsenious sulphide sol?

Ans: Lyophobic sols such as arsenious sulphide, aluminium hydroxide, and ferric hydroxide are stabilised due to the charge present on the surfaces of the colloidal particles. They are easy to coagulate if these charges on the surfaces are disturbed or neutralised.  

Viva Questions

1. What is the reason for the yellow-coloured solution of As2O3?

Ans: The yellow colour of As2O3 solution when hydrogen sulphide gas is passed is due to the formation of arsenious sulphide solution.

2. The colloidal solution of silver is prepared by which process?

Ans: A colloidal solution of silver is prepared by Bredig’s method.

3. Which apparatus is used for the generation of H2S?

Ans: Kipp’s apparatus is used for the generation of H2S.

4. What is an alternative way of preparation of H2S gas?

Ans: Another way to obtain H2S gas via Kipp’s apparatus is by using zinc flakes with glass marbles and pouring dil. HCl into this mixture.

5. How to bring coagulation of arsenious sulphide?

Ans: Coagulation of 100ml of arsenious sulphide can be brought about by using a 5ml of 1M NaCl solution.

6. What is an associated colloid?

Ans: Associated colloids are colloids which, at lower concentrations, behave as strong electrolytes, but at higher concentrations behave as colloids due to aggregation of particles. 

7. Name the mineral forms of arsenious sulphide.

Ans: Orpiment and Realgar are the two mineral forms of arsenious sulphide.

8. What are lyophobic sols?

Ans: Sols wherein the solid dispersed phase has a low affinity with the liquid dispersion medium are known as lyophobic sols.

9. What is the range of particle size in colloids?

Ans: The colloidal particles have a diameter in the range between 1 and 1000 nm.

10. What is the charge present in arsenious sulphide?

Ans: Negative charge is present on arsenious sulphide.

Practical Based Questions (MCQs) 

1. To prepare a colloidal solution of starch, which of the following is important?

1. Iodine solution

2. Starch paste

3. Cold water

4. -20℃ temperature

Ans: Starch paste

2. Arsenic sulphide formula is_____

1. As4S6

2. As3S

3. As5S7

4. As2S3

Ans: As2S3

3. Arsenious sulphide sol is prepared by passing

1. Ferrous sulphide gas

2. Hydrogen peroxide gas

3. Hydrogen sulphide gas

4. Carbon dioxide gas

Ans: Hydrogen sulphide gas

4. A colloidal solution of platinum is prepared by____

1. Bredig’s method

2. Solvation method

3. Hydrolysis

4. Combustion 

Ans: Bredig’s method

5. What does hydrolysis mean?

1. Chemical breaking of a compound due to water

2. Lysis of compound due to hydrogen

3. Ligation of a compound due to water

4. The chemical breakdown of the compound due to water vapour

Ans: Chemical breaking of a compound due to water

6. Sols can be destroyed using which of the following?

1. Adding water

2. Adding inert metals

3. By increasing the dispersion medium

4. By adding electrolytes

Ans: By adding electrolytes

7. Orpiment is _____in colour

1. Pink

2. Orange

3. Yellow

4. Black

Ans: Yellow

8. A red-orange mineral of arsenious sulphide is

1. Realgar

2. Arsenious sulphide

3. Orpiment

4. Arsenious oxide

Ans: Realgar

9. State which of the following is false.

1. Colloidal solutions are true solutions

2. The emulsion is a type of colloidal solution

3. Gas particles trapped in a solid medium are known as foam.

4. Starch forms a lyophilic sol.

Ans: Colloidal solutions are true solutions

10. Find the odd man out

1. Starch 

2. Protein

3. Aluminium hydroxide

4. Gelatin

Ans: Aluminium hydroxide

Conclusion

From the above experiment, we can conclude that colloidal solutions are made of two parts: a dispersed phase and the dispersion medium. The dispersed phase remains suspended in the dispersed medium. Sols are of two types lyophobic, which are solvent hating and hence remain suspended in the solution due to the presence of charges on their surface. Arsenious sulphide is an example of a negatively charged sol, i.e. its colloidal particles have a negative charge on their surface. As2S3 is a yellow-coloured colloidal solution which has various industrial applications.