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Properties of Colloids - Physical, Kinetic, Optical and Electric Properties of Colloids, Practice Problems and FAQ

Properties of Colloids - Physical, Kinetic, Optical and Electric Properties of Colloids, Practice Problems and FAQ

It's experiment time!

Take two glasses, one with sugar solution and the other with milk. Put them in the dark and shine a light through both of them. 

What do you notice?

The sugar solution appears clear, but when light is passed through a glass of milk, a beam of light is dispersed. This scattering of light is caused by a phenomenon known as the Tyndall effect. 

Only colloids exhibit this effect. Milk is a fat and protein mixture, whereas sugar solution is a homogeneous solution.


TABLE OF CONTENTS

  • Physical Properties of Colloids
  • Kinetic Properties of Colloids
  • Optical Properties of Colloids
  • Electric Properties of Colloids
  • Practice Problems
  • Frequently Asked Questions - FAQ

Physical Properties of Colloids

  • Heterogeneous nature

Colloidal solutions are inherently heterogeneous. It is made up of a dispersed phase and a dispersion medium.

  • Visibility

Colloidal particles are too small to be seen with the naked eye, but when viewed through an ultramicroscope, they appear as dark spots against a dark background due to light scattering caused by them.

  • Filterbility

Because the size of the solute particles is smaller than the pores of the filter paper, they pass through easily. Colloidal particles, on the other hand, cannot pass through the animal membrane, parchment paper, or ultrafilters.

  • Surface tension and Viscosity

Surface tension and viscosity of lyophobic sols are similar to those of the dispersion medium. Lyophilic sols, on the other hand, have higher viscosity and lower surface tension than the dispersion medium.

  • Colligative properties

The properties of a solution are determined by the number of solute particles present, regardless of the identity or nature of the solute particle, i.e., shape, size, charge, and so on.

Colloidal particles are more substantial aggregates. In comparison to a true solution, the number of particles per unit volume in a colloidal solution is small.


Because colloidal particles are typically associated, the number of particles in the solution decreases as a result of association, as does the colligative property. As a result, given an equal concentration of the true solution and colloidal solution, the latter's colligative values will be less than the former's.

Kinetic Properties of Colloids

  • Brownian movement

Colloidal particles appear to move in a continuous zig-zag pattern when viewed through an ultramicroscope. The Brownian movement was named after Robert Brown, who first observed this type of motion. It is unaffected by the nature of the colliding particles but is affected by their size and the viscosity of the solution. The smaller the particle size, the lower the viscosity and the faster the particle movement. At higher temperatures, the motion becomes more intense.


Cause: 

The unbalanced bombardment of particles by the molecules of the dispersion medium causes the Brownian movement. The probability of uneven bombardment decreases as particle size increases, and Brownian movement slows. The Brownian movement has a stirring effect that keeps particles from settling and is thus responsible for sol stability.

Importance:

1. It provides a direct demonstration of the kinetic theory's postulated ceaseless motion of molecules.

2. It counteracts the gravitational force acting on colloidal particles and thus aids in the stability of colloidal sols by preventing them from settling.


  • Diffusion

Colloidal particles diffuse from a higher concentration region to a lower concentration region until the system's concentration becomes uniform. Colloidal particles, on the other hand, diffuse at a slower rate due to their large size and high molecular mass.

  • Sedimentation

Under the influence of gravity, colloidal particles settle down very slowly. The ultracentrifuge can increase the rate of sedimentation or settling down.

Optical Properties of Colloids

The colour of the colloidal solution depends on the wavelength of light scattered by the dispersed particles which in turn depends on the size and nature of the particles. 

Example: Gold sol. The finest gold sol is of red colour. As the particle size of the sol increases, the colour of the sol also changes. 


Also, the colour of the colloidal solution depends on how the observer receives the light. 

Example: A mixture of milk and water when viewed by reflected light appears blue and when viewed by transmitted light it appears red.


Tyndall Effect

The dispersion of light as a light beam travels through a colloidal solution is known as the Tyndall effect. The Tyndall effect occurs when light beams are focused on particles in a colloid. This effect can be observed in all colloidal solutions, including very tiny suspensions. The light is dispersed when it collides with the colloidal particles. It deviates from its natural path, which is a straight line.

Conditions for Tyndall Effect

1. The dispersed particle's diameter should not be much smaller than the wavelength of light. Particles that generate the Tyndall effect can have sizes ranging from 40 - 900 nm.

2. The magnitude of the refractive indices of the dispersed phase and dispersion medium should differ greatly.

Application of Tyndall Effect

It can be used to distinguish between a colloid and a true solution. Tyndall effect is not seen in true solutions.

Examples of Tyndall Effect

1. The path of light becomes visible when sunlight seeps in through the canopy of the forest. The scattering of light is observed in this situation is caused by water droplets present in the air.


2. Opaque glass has a bluish tint like appearance when viewed from the side. However, when light is shone through the glass, orange light is produced.

3. The light from the projector seems bright in a movie theatre because dust and smoke particles scatter light.

Scattering of light in a theatre by dust and smoke particles

4. Tyndall effect can be seen above sea due to the scattering of sunlight through the particles in the atmosphere. 


What causes blue eye colour?

Brown, blue, and black irises are distinguished by the amount of melanin present in one of the iris layers. The layer of a blue iris contains less melanin than the layer of a black iris, making it transparent. The light that strikes this translucent layer is dispersed due to the Tyndall effect.

Blue light has a shorter wavelength than red light, so it is more dispersed. Iris absorbs unscattered light from a deeper layer. Since majority of the light dispersed is blue, the colour of iris' is blue.

The scattering of light is involved in several processes. Mie scattering and Rayleigh scattering are two examples. 

Electric Properties of Colloids

Colloidal particles are always electrically charged. This charge can be either posive or negative, but it is the same on all particles in a given colloidal solution. The presence of charge is responsible for these solutions' stability. It should be noted that only the sol particles have some charge, whereas the dispersion medium has none. Some common sols and their charges are given in the table below.

 

Positively charged sols

Negatively charged sols

1.

Hydrated metallic oxides

(E.g. Al2O3.xH2O, Fe2O3.xH2O, etc.)

Metals

(E.g. Cu, Ag, Au sols)

2.

Basic dyes 

(E.g. Methylene blue sol)

Acid dyes 

(E.g. Eosin, congo red sols)

3.

Haemoglobin (blood)

Metallic sulphides

(E.g. As2S3, Sb2S3, CdS etc.)

4.

Oxides

(E.g. TiO2 sol )

Sols of starch, gum, gelatin, clay, charcoal etc.

The presence of colloidal particles with equal and similar charges causes repulsion between charged particles, preventing aggregation. As a result, the presence of like charges stabilises the sol.

Possible reasons for the charge on colloidal particles

A. Due to frictional electrification: The mutual rubbing of the dispersed phase particles with that of the dispersion medium results in some charge on the colloidal particles.

B. Due to dissociation of surfactants: The dissociation of surfactants like soap results in the formation of ions. 

C15H35COONa ⇌ C15H35COO- + Na+

The cations pass into the solution while the anion (C15H35COO-) aggregates due to weak attractive forces present in the hydrocarbon chains.

C.  Due to electro dispersion of metals in which the electrons are captured by the sol particles.

D.  Due to preferential adsorption of ions from solution: Preferential adsorption of ions is the most accepted reason. The sol particles acquire positive or negative charges by the preferential adsorption of positive or negative ions. When two or more ions are present in a dispersion medium, the preferential adsorption of the common ion to the colloidal particle usually takes place. 

Example - 1: When silver nitrate solution is added to potassium iodide solution, the precipitated silver iodide adsorbs iodide ions from the dispersion medium and results in a negatively charged colloidal solution. 

image


The precipitated silver iodide(AgI) adsorbs I- ions from the dispersion medium, to form AgI/I-.


However, when KI solution is added to silver nitrate solution, positively charged sol results due to the adsorption of Ag+ ions from the dispersion medium.

image

The precipitated silver iodide adsorbs Ag+ ions from the dispersion medium, to form AgI/Ag+.

Example - 2: If FeCl3 is added to an excess of hot water, a positively charged sol of hydrated ferric oxide is formed due to the adsorption of Fe3+ ions. However, when FeCl3 is added to NaOH solution, a negatively charged sol is obtained with adsorption of OH─ ions.

E. Due to the formation of an electrical double layer: There is an interaction between the charged colloidal particles and the counter (oppositely charged) ions. There is selective adsorption of common ions on the surface, such a colloidal particle acquires a positive or negative charge. This charged layer attracts counter ions from the medium to form a second layer. 

When a highly diluted AgNO3 solution is added to a highly diluted KI solution, the precipitated silver iodide(AgI) adsorbs I- ions from the dispersion medium, to form AgI/I-. K+ get adsorbed on the negatively charged layer.

The combination of the two layers of opposite charges around the colloidal particle is known as Helmholtz electrical double layer. 

The First (inner) layer of ions is firmly held and is fixed but the second (outer) layer of ions is mobile and is easily diffusible. The opposite charges on fixed and diffused layers result in a potential difference between the layers, known as electrokinetic potential or zeta potential. If two particles of an insoluble material (precipitate) do not have double layers they can come close enough and attractive van der Waals forces pull them together. When particles possess a double layer, the overall effect is that particles repel each other at large distances of separation. This repulsion prevents their close approach. They remain dispersed and the colloid is stabilised.


  • Electrophoresis

Electrophoresis experiment confirms the existence of charge on colloidal particles. Before an electric potential is applied, the particle concentration is higher in the middle of the tube. While negligible colloidal particles are present at the electrodes.


The colloidal particles move towaats the respective electrodes when an electric potential is applied across the platinum electrodes. The movement of colloidal particles under an applied electric potential is called electrophoresis.


If colloidal sol is positively charged colloidal particles move towards the cathode. If colloidal sol is negatively charged colloidal particles move towards the anode. 

  • Electro-osmosis

When electrophoresis, i.e., the movement of particles is prevented by some suitable means (semipermeable membrane), it is observed that the dispersion medium begins to move in an electric field. This phenomenon is termed electro-osmosis.


Practice Problems

Q1. A colloidal solution’s colour depends on the wavelength of 

A. Dispersed particles
B. Incident light
C. Light scattered by dispersed particles
D. None of the above

Solution: The colour of the colloidal solution depends on the wavelength of light scattered by dispersed particles which in turn depends on the size and nature of the particles. 

So, option C) is the correct answer.

Q2. Which of the following phenomena are observed as a result of the Tyndall effect?

A. Red sunsets
B. Blue colour of sky 
C. Scattering of light from a projector in a cinema hall.
D. All of the above

Solution: Dust in the atmosphere is colloidal. When the sun is low on the horizon, the light it emits must travel through a thick layer of dust before reaching your eyes. The light that is blue is scattered away from your sight. You can still see the red part of the spectrum. On a large scale, red sunsets represent the Tyndall effect. 

As light is scattered by the ir particles, the clear sky appears blue.

The light from the projector seems bright in a movie theatre because dust and smoke particles scatter light.

So, option D) is the correct answer.

Q3. Which of the following statements is/are correct?

A. Value of colligative properties is greater for a true solution than a colloidal solution
B. Brownian movement increases when the particle size decreases.
C. Presence of like charges provides stability to the sol.
D. All of the above

Solution: Colloidal particles are usually associated, hence the number of particles in the solution decreases due to association, and so is the colligative property. Therefore, given an equal concentration of a true solution and a colloidal solution, the value of the colligative properties of the latter will be less than the former. 

Brownian movement depends on the size of the colloid particles. The velocity of the particles increases when the particle size decreases. Thus, the velocity of colloidal particles is inversely proportional to the size of the particles.

The presence of equal and similar charges on colloidal particles leads to repulsion between charged particles, by preventing aggregation. Thus, the presence of like charges provides stability to the sol. 

So, option D) is the correct answer.

Q4. Which of the following is/are negative sol?

A. As2S3
B. Haemoglobin 
C. Cu, Ag, Au sols
D. Both A and C

Solution:

 

Positively charged sols

Negatively charged sols

1.

Hydrated metallic oxides

(E.g. Al2O3.xH2O, Fe2O3.xH2O, etc.)

Metals

(E.g. Cu, Ag, Au sols)

2.

Basic dyes 

(E.g. Methylene blue sol)

Acid dyes 

(E.g. Eosin, congo red sols)

3.

Haemoglobin (blood)

Metallic sulphides

(E.g. As2S3, Sb2S3, CdS etc.)

4.

Oxides

(E.g. TiO2 sol )

Sols of starch, gum, gelatin, clay, charcoal etc.

So, option D) is the correct answer.

Frequently Asked Questions - FAQ

Question 1. What are colloids?
Answer: A colloid is a heterogeneous system composed of one substance dispersed as very fine particles in another substance known as a dispersion medium. In a colloid, the dispersed phase may consist of particles of a single macromolecule or an aggregate of many atoms, molecules or ions. 

Question 2. Why does motorbike exhaust appear blue at times?
Answer: The intensity of dispersed light is dependent on the density and frequency of the suspended particles. The Tyndall effect, in a similar manner as Rayleigh scattering, scatters blue light more than red light. This is why motorbike exhaust might seem blue at times.

Question 3. What is considered a colloid?
Answer:
In chemistry, a colloid is a phase-separated combination in which one material is suspended within another with insoluble or soluble particles that are microscopically distributed. The dispersed-phase particles have a diameter of between 1 and 1000 nanometres.

Related Topics

Enzyme Catalysis

Adsorption

Colloidal Solutions

Coagulation of Colloids

Emulsion

Purification of Colloids

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