Surface Chemistry

Surface chemistry deals with studying chemical changes at the interface of two phases. Colloids are a mixture of two substances, one consisting of dispersed insoluble particles suspended in another. The substance which is suspended is called the dispersed phase, and the one in which it is suspended is called the dispersion medium.

For example, a colloidal solution of starch (dispersed phase) in water (dispersion medium).

Table of Content

• Aim

• Theory

• Procedure

• Result

Aim

To prepare a colloidal solution of starch in water.

Apparatus Required

1. 50ml beaker

2. 250ml beaker

3. Mortar and pestle

4. Glass rod 

5. Funnel

6. Filter paper

Reagents Required

1. Soluble starch

2. Distilled water

Theory

Based on the type of interaction between the dispersed phase and its dispersion medium, colloids are of two types - 

1. Lyophilic sols-Where dispersion phase has a strong attraction towards the dispersion medium. They are highly solvated and stable. For example, starch sol.

2. Lyophobic sols-Where the dispersion phase has very little or no attraction towards the dispersion medium. They are less stable. For example, ferric hydroxide sol.

Starch, when heated with water, forms a stable lyophilic sol, due to hydrogen bonding between the OH group of starch and water molecules. This causes the starch particles to be highly solvated in water. The boiling condition of the water is necessary, as starch is insoluble in cold water.

Procedure

1. Weigh 0.5g of soluble starch, and transfer it to a mortar. 

2. Add a few ml of distilled water (enough to make a thin paste) to the starch, and grind using a pestle, making it a thin paste. Transfer this paste to a 50ml beaker.

3. Take 100ml of distilled water in a 250ml beaker and bring it to a boil.

4. Once boiling is initiated, pour the starch paste using a glass rod, and stir continuously. 

5. Boil the starch-water mixture for about 10 minutes, and then allow it to cool.

6. Once the solution is cooled to room temperature, filter it through a filter paper fixed in a funnel over a 250ml beaker. The filtrate is the lyophilic starch sol.

Preparation of Starch Sol

Result

Following the above-mentioned procedure, a colloidal solution of starch in water was successfully prepared.

Precaution

1. All the apparatus should be cleaned before use.

2. Starch paste should not be lumpy.

3. Starch paste should be added slowly using a glass rod, to avoid precipitation.

4. The boiling solution must be stirred continuously.

Lab Manual Questions

1. What is starch?

Ans. Starch is a carbohydrate with the formula (C6H10O5)n

2. What is lyophilic sol?

Ans. Lyophilic sols are solutions in which the solute has a great affinity for the solvent.

3. What is lyophobic sol?

Ans. Lyophobic sols are solutions in which the solute has very little or no affinity for the solvent.

4. Give one example of a lyophobic sol.

Ans. Ferric hydroxide sol

Viva Questions

1. State the importance of surface chemistry.

Ans. It helps us study chemistry at the surface level, i.e. at the interface of two phases.

2. What is the formula of starch?

Ans.  (C6H10O5)n

3. What does lyophilic mean literally?

Ans. Solvent-loving

4. What does lyophobic mean literally?

Ans. Solvent fearing

5. Why is starch sol stable?

Ans. Due to strong hydrogen bonding between the OH group of starch and water

6. Gelatin powder is mixed in water. What is the dispersion medium here?

Ans. Water

7. Why does the starch paste need to add slowly?

Ans. To avoid precipitation.

8. Give one example of a liquid-liquid colloid.

Ans. Milk

9. How are carbohydrates important in the human body’s metabolism?

Ans. Carbohydrates break down into glucose, which is then used to produce
ATP.

10. What happens when starch is added to cold water?

Ans. Starch is almost insoluble in cold water.

Practical Based questions

1. Starch is a

1. Protein

2. Carbohydrate

3. Nucleic acid

4. Lipid

Ans: B)

2. Surface chemistry is the study of chemistry at _____ level

1. nascent

2. interface

3. endpoint

4. None of the above

Ans: B)

3. An emulsion consists of

1. liquid-solid

2. liquid-gas

3. liquid-liquid

4. solid-solid

Ans: C)

4. Which statement is not true about lyophobic sol?

1. They are less stable

2. They have high solvation

3. Ferric hydroxide sol is a lyophobic sol

4. None of the above

Ans: B)

5. Milk is an example of

1. emulsion

2. aerosol

3. foam

4. None of the above

Ans: A)

6. The dispersed phase in starch sol is

1. starch

2. water vapour

3. water

4. None of the above

Ans: A)

7. The formula of starch is

1. (C6N10O5)n

2. (C6H10O5)n

3. (C6H10O15)n

4. (C6N10O15)n

Ans: B)

8. Weak interaction between the dispersed phase and dispersion medium is shown in

1. Lyophobic sol

2. Lyophilic sol

3. Hydrophilic sol

4. None of the above

Ans: A)

9. Rubber is a

1. solid

2. liquid

3. colloid

4. none of the above

Ans: C)

10. To prepare starch sol, water should be

1. cold

2. Lukewarm

3. boiling

4. None of the above

Ans: C)

Conclusion

1. Starch is a long-chain polymer made up of sugar molecules connected through glycosidic linkage.

2. Starch, when heated with a dispersion medium such as water, forms a colloidal solution.

3. Colloids are mixtures where microscopic insoluble particles of one substance are dispersed in another.

4. The Starch solution is stable in the dispersion medium and remains unaffected by the electrolytic impurity.