Chemistry > Surface Chemistry > 7.0 Colloidal System
Surface Chemistry
1.0 Introduction
2.0 Adsorption
3.0 Factors affecting adsorption of gases by solids
4.0 Adsorption Isotherms
5.0 Applications of Adsorption
6.0 Types of Solutions
7.0 Colloidal System
7.1 Different Colloidal Systems
7.2 Classification of Colloidal System
7.3 Preparation of Colloidal System
7.4 Purification of Colloidal System
7.5 Properties of colloidal system
8.0 Coagulation of colloidal solutions
9.0 Emulsions
10.0 Catalysis
11.0 Zeolites as shape-selective catalysts
12.0 Enzyme as catalysts
12.1 Characteristics of Enzymes
12.2 Mechanism of enzyme catalysis
12.3 Autocatalysis
12.4 Induced catalysis
7.5 Properties of colloidal system
7.2 Classification of Colloidal System
7.3 Preparation of Colloidal System
7.4 Purification of Colloidal System
7.5 Properties of colloidal system
12.2 Mechanism of enzyme catalysis
12.3 Autocatalysis
12.4 Induced catalysis
1. Heterogeneous character
They consists of two phases, viz. the dispersed phase and the dispersion phase.
2. Non-setting
Colloidal solutions are stable systems.
3. Filterability
Colloidal particles readily pass through ordinary filter papers. However, they can be retained by special filters known as ultrafilters.
4. Visibility
Owing to very small size the colloidal particles are not visible even under a most powerful microscope.
5. Colour
Colour of a colloidal solution depends upon the size and shape of the particles. It also depends on the way an observer recieves the light.
6. Colligative properties
Colloidal particles are not simple molecules but are physical aggregations of molecules. Thus for a given mass of sol, the number of particles will be very small as compared to the number of particles in true solution. Hence, unlike true solutions, the colloidal system gives very low osmotic pressure and show very small freezing point depression or boiling point elevation.
7. Tyndall effect
Tyndall, in $1869$ observed that when a beam of light is passed through a true solution, it cannot be seen unless the eye is placed directly in its path. On the other hand, when the same beam is passed through a colloidal suspension it becomes visible as a bright streak. This phenomenon is called Tyndall effect and the illuminated path is known as Tyndall cone. The Tyndall effect is owing to the scattering of light from the surface of colloidal particles.
Tyndall effect is observed only when the following two conditions are satisfied:
- The diameter of the dispersed particles is not much smaller than the wavelength of the light used.
- The refractive indices of the dispersed phase and the dispersion medium differ greatly in magnitude.
8. Brownian movement
Robert Brown, an English botanist, observed that pollen grains in aqueous solutions were in constant motion. When colloidal solutions were examined under the ultramiscroscope, it was found that the particles in these solutions were also in constant, random motion, transversing no definite set paths, but travelling in zig-zag directions all over the field of view. This movement of colloidal particles is called Brownian movement.
The Brownian movement is due to the bombardment of colloidal particles by the molecules of the dispersion medium which are in constant motion like molecules in a gas.
9. Electrical property
When an electric current is passed through a colloidal solution the solid particles and the liquid medium being oppositely charged move in opposite direction. In case the experiment is so arranged that only the particles can move, the phenomenon is known as cataphoresis or electrophoresis. On the other hand, when only the medium is allowed to migrate and not the particles, the phenomenon is known as electrosmosis.
Distance travelled by colloidal particles in one second under a potential gradient of one volt per cm is called electrophoretic mobility of colloidal particles.
