Colloidal State Topics  

   
________________________________________________________________________________

Introduction to Colloidal State


THE COLLOIDAL STATE: 

While studying the diffusion of solutions through an animal membrane, ThomasGraham (1861) observed that certain substances such as sugar, urea, sodium chloride etc. in the dissolved state passed through the membrane, while the solutions of substances such as glue, gelatin, gum Arabic etc. did not. This observation led him to classify the soluble substances into two categories: 

                (i) Crystalloids                            (ii) Colloids

According to Graham, crystalloids were those substances which could be obtained in crystalline form and whose solutions were able to pass through an animal membrane. On the other hand, colloids were those substances which were amorphous in nature and whose solutions were unable to pass through the membrane. However, it was soon realized that the classification of dissolved substances made by Graham was not tenable because certain substances could act both as crystalloids and colloids. 

Later on it was found that the diffusibility of crystalloids and non-diffusibility of colloids through an animal membrane was due to the difference in the size of their particles. Crystalloids formed smaller particles in solutions and therefore passed through the membrane. On the other hand, colloids formed larger particles (larger than the dimensions of the pores of the membrane) in solutions and were unable to pass through the membrane. 

On the basis of the size of particles, the systems containing dispersed particles can be divided into following three categories. 

1. True solutions: True solutions are homogeneous system and have the size of dispersed particles less than 1 nm, i.e. 10-9. The particles of solute present in a true solution are either single molecules or irons and are homogeneously distributed throughout the solutions. These particles are invisible and cannot be seen even with a microscope. Due to very small size of dispersed particles, true solutions pass through ordinary filter paper as well as through animal membranes. Sodium chloride, sugar, urea etc. form true solutions in water. 


2. Colloidal solutions: Colloidal solutions are heterogeneous systems and have the size of dispersed particles lying between 1 nm-1000 nm (i.e. 10-9-10-6m). The particles in a colloidal solution are thus larger particles and are referred to as colloidal particles. Although colloidal particles are larger in size, yet they are not large enough to be seen with naked eye. However, they can be seen with the help of an ultra microscope. Colloidal solutions can pass through ordinary filter paper but not through an animal membrane, Gum Arabic, gelatin, glue etc. form colloidal solutions when dispersed in water. 

3. Suspensions: Suspensions are also heterogeneous system and have still larger particles. The size of particles present in a suspension is more than 1000 nm (i.e., >10-6m). These particles are either visible to naked eye or can be seen under a microscope. The suspensions neither pass through an animal membrane nor through an ordinary filter paper. Stirred muddy water is an example of suspensions.    

               types-of-dispersed-systems 
                       
                      The three types of systems containing 
                               dispersed particle of different size. 


The three types of dispersed systems are diagrammatically shown in figure. 

The important characteristics of true solutions, colloidal solutions and suspensions are summarized in the following table. 

From the above discussion it is clear that colloidal solutions are intermediate of true solutions and suspensions. Colloidal solutions are not only formed by certain specific substances as mentioned above but they can be obtained from any substance by subdividing or aggregating its particles in the size range 1 nm-1000 nm. Practically, all substances can be made to exist in colloidal form. Therefore instead of talking of colloidal solution, it would be more appropriate to talk of the colloidal state of matter. The colloidal state of matter may be defined as follows:


The colloidal state matter is the state in which the size of the particles lies in between 1nm (10-9m or 104) and 1000nm (10-6) and the systems consisting of dispersed particles in this range are called colloidal systems. 

Table: Important Characteristics of True solutions,
Colloidal solutions and Suspensions

S.No.

Property

True solution

Colloidal solution

Suspension

1.

Particle size

Less than 1 nm (i.e. 10-9m)

Between 1 nm-1000nm (i.e. 10-9m-10-6m)

Greater than 100nm (i.e. 10-6m)

2.

Nature

Homogeneous

Heterogeneous

Heterogeneous

3.

Visibility of particles

Invisible

Visible under ultra-microscope

Visible to naked eye or under microscope

4.

Appearance

Transparent

Generally transparent but may show translucence

Opaque

5.

Filterability

Passes easily through ordinary filter paper as well as animal membranes

Passes easily through ordinary filter paper but not through animal membranes

Does not pass either through ordinary filter paper or through animal membranes.

6.

Setting of particles under gravity

Particles do not settle

Colloidal particles do not settle under gravity. However, they can be made to settle under high speed centrifugation

Particles settle on standing

7.

Diffusion of particles

Diffuses rapidly

Diffuses slowly

Does not diffuse

8.

Scattering of light by particles (Tyndall effect)

Does not scatter light

Scatters light and exhibits Tyndall effect

Tyndall effect may be exhibited


 

Related Resources
Applications of Colloids

Applications of Colloids Colloids play a very...

Colligative Properties of Colloidal Solutions

Colloidal State Topics Introduction to Colloidal...

Preparation of Colloidal Solutions

Colloidal State Topics Introduction to Colloidal...

Electrical Properties of Colloidal Solutions

Electrical Properties of Colloidal Solutions Some...

Summary of Surface Chemistry

Colloidal State Topics Introduction to Colloidal...

General Physical Properties of Colloidal Solutions

General Physical Properties of Colloidal Solutions...

Mechanical Properties of Colloidal Solutions

Colloidal State Topics Introduction to Colloidal...

Classification of Colloids

Colloidal State Topics Introduction to Colloidal...

Classification of Colloids based on Nature of Interaction

Classification of Colloids based on Nature of...

Keywords of Surface Chemistry

Keywords of Surface Chemistry 1. Adsorption 2....

Protective Colloids and Gold Number

Protective Colloids and Gold Number Lyophobic sols...

Emulsions

Colloidal State Topics Introduction to Colloidal...

Optical Properties of Colloidal Solutions

Optical Properties of Colloidal Solutions Optical...

Purification of Colloidal Solutions-Part1

Purification of Colloidal Solutions (Sols)...