Question

Write notes on : 

a. Tyndall effect 

b. Brownian motion 

c. Types of emulsion 

d. Hardy-Schulze rule

Answer 1

a. Tyndall effect :

i. Tyndall observed that when light passes through true solution, the path of light through it cannot be detected.

ii. However, if the light passes through a colloidal dispersion, the particles scatter some light in all directions and the path of the light through colloidal dispersion becomes visible to observer standing at right angles to its path.

iii. The phenomenon of scattering of light by colloidal particles and making path of light visible through the dispersion is referred as Tyndall effect and the bright cone of the light is called Tyndall cone.

iv. Tyndall effect is observed only when the following conditions are satisfied.

• The diameter of the dispersed particles is not much smaller than the wavelength of light used.
• The refractive indices of dispersed phase and dispersion medium differ largely.

 v. Significance of Tyndall effect :

• It is useful in determining number of particles in colloidal system and their particle size.
•  It is used to distinguish between colloidal dispersion and true solution.

 

b. Brownian motion : 

i. The colloidal or microscopic particles undergo ceaseless random zig-zag motion in all directions in a fluid. This motion of dispersed phase particles is called Brownian motion.

ii. Cause of Brownian motion :

• Particles of the dispersed phase constantly collide with the fast-moving molecules of dispersion medium (fluid).
• Due to this, the dispersed phase particles acquire kinetic energy from the molecules of the dispersion medium.
•  This kinetic energy brings about Brownian motion.

 c. Types of emulsion :

There are two types of emulsions :

a. Emulsion of oil in water (o/w type) : 

An emulsion in which dispersed phase is oil and dispersion medium is water is called emulsion of oil in water.

e.g. 1. Milk consists of particles of fat dispersed in water.

2. Other examples include vanishing cream,paint, etc.

b. Emulsion of water in oil (w/o type) : 

An emulsion in which dispersed phase is water and dispersion medium is oil is called emulsion of water in oil. 

e.g. 1. Cod liver oil consists of particles of water dispersed in oil. 

2. Some other examples of this type include butter, cream, etc.

d. Hardy-Schulze rule : 

i. Generally, greater the valency of the flocculating ion added, greater is its power to cause precipitation. This is known as Hardy-Schulze rule. 

ii. In the coagulation of negative sol, the flocculating power follows the following order :

Al³⁺ > Ba²⁺ > Na⁺

iii. Similarly, in the coagulation of positive sol, the flocculating power is in the following order :

[Fe (CN)₆]^4- > PO₄^3- > SO₄^2- > Cl^-