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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Adsorption and Colloids Know More

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The word colloid originates from the Greek word ("kolla" meaning glue-like). This leads to a new branch of science called "colloid science".
A colloidal system is a two phase system, consisting of a continuous phase called the dispersion medium in which extremely minute particles, are present. The second phase is the dispersed phase or discontinuous phase.
Colloids are classified as lyophilic colloids and lyophobic colloids. The main difference between them in terms of various properties are summed up in the following table:

Sr.
No.

Property

Lyophobic Sols

Lyophilic Sols

1.

Surface tension

It is the same as that of the medium.

Generally lower than the medium.

2.

Viscosity

Same as the medium.

Much higher than the medium.

3.

Visibility

Particles can be detected by an ultra-microscope.

Particles cannot be detected by an ultra-microscope.

4.

Migration in an electric field

Particles migrate either towards anode or cathode in an electric field.

Particles migrate in either direction and not at all in an electric field.

5.

Action of electrolytes

Addition of small quantities of electrolyte causes precipitation.

Addition of electrolyte has little effect.


Characteristics of True Solutions, Colloidal Solutions and Suspensions

Sr.
No.

True Solutions
(Molecular Solutions)

Colloidal Solutions
(Colloidal Dispersions)

Suspensions
(Coarse Dispersions)

1.

Particle size between 1mm or 10A

Particles lie in the range of 1mm - 200 mm.

Particles have a size greater than 200 mm or 2000 A.

2.

Particles are invisible under all circumstances.

Particles are invisible under most powerful microscope.

Particles are visible with naked eyes or with a microscope.

3.

Diffuse readily through parchment membrane.

Diffuse slowly through parchment paper.

Generally do not diffuse.

4.

Pass readily through filter paper and parchment membrane.

Pass readily through filter paper but slowly through parchment paper.

Do not pass through filter paper or parchment paper.

5.

Do not scatter light.

Particles scatter light - the phenomenon being Tyndall effect.

Do not show Tyndall effect.

 

Surfactants
One of the most important examples of colloids
- used in day-to-day use is the surfactants. Surfactants are substances which get preferentially 'adsorbed' at the air-water, oil-water, solid-water interfaces. Surfactant molecules have two ends:

         i.            Hydrophilic - in contact with water

       ii.            Hydrophobic - away from water

This dual nature of surfactant molecules makes them very interesting.

This property makes them useful in cleaning, washing action of clothes, dirty objects, etc.
The surfactant is a good emulsifier. When two immiscible liquids, for example, hydrocarbon oil and water are shaken well, it results in a milky solution. However, the droplets remain suspended in water for a short time. On standing, the two liquids separate forming two phases. So in order to get a stable emulsion, an emulsifier is added. They, i.e., surfactants, get adsorbed between the two layers, i.e., dispersed phase (tiny droplets) and dispersion medium (water) and lower the interfacial tension between oil and water and facilitate mixing of two layers.


Role Of Surfactant In Stabilising
An Oil-Water Emulsion

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