Classification of Colloids: Properties & Phases

In 1861, Thomas Graham divided various substances into two categories namely, crystalloids and colloids on the basis of his experiment on diffusion of dissolved substances through parchment paper or an animal membrane. Substances like sugar, urea, and common salt which are crystalline and in dissolved state which passes through the animal membrane or the parchment paper were called crystalloids. Substances like starch, glue, gelatin which are non-crystalline and in the dissolved state that do not pass through the membrane were named as the colloids.

Key Terms: Colloids, Crystalloids, Colloidal dimensions, Colloidal sols, Salt, Sugar, Urea, Surface Chemistry

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Colloids

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Colloids and Classification of Colloids are an important section from the Unit of Surface Chemistry. Students must know that from Unit 1 to Unit 5, the total weightage is of 23 marks as seen in the sample papers. 

Classification of Colloids

This division of substances into crystalloids and colloids was soon proved to be wrong because a crystalloid could behave as a colloid under variable conditions and vice versa. For example, common salt, a typical crystalloid, in an aqueous solution, behaves as a colloid in the benzene medium while soap, a typical colloid in water behaves as a crystalloid in alcohol. Thus colloids do not represent a separate class of substances.

The most important difference between the crystalloid and colloidal state is that of the particle size. A substance behaves as a crystalloid when, if in the dissolved state it exhibits the particle size of molecular dimensions, i.e. less than 1nm.

In this state the homogenous mixture is called as true solution. But, when the particle size in the dissolved state lies between 1nm to 1000nm, the substance is said to form a colloidal system which is heterogeneous in nature. If the particle size is more than 1000nm in the dissolved state, it is termed as suspension. The colloidal state is the intermediate state between the true solution and suspension.

A distinction between true solutions, colloidal solutions and suspensions.

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Phases of Colloidal Solution 

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A colloidal solution is heterogeneous in nature and it consists of two necessary phases that are the dispersed phase and the dispersion medium.

(i) The dispersed phase: it is the component present in small proportions and consists of particles of colloidal dimensions (1-1000nm)

(ii) The dispersion medium: The medium in which colloidal particles are dispersed is called dispersion medium.

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Classification of Colloids

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Colloids may be classified into different types on the basis of the following criteria:

1. Classification on the basis of physical state of dispersed phase and dispersion medium.

The two phases that are dispersed phase and the dispersion medium can be solid, liquid or a gas. Hence, different types of colloidal solutions are possible depending upon the physical state of the two different phases. The colloidal solutions having fluid- like appearance is called sols in which the dispersion medium is generally liquid. Some common examples of colloids depending upon their dispersion medium are shown in the table.

2. Classification based on the nature of interactions between dispersed phase and dispersion medium.

On such basis, the colloidal sols are classified into two type namely lyophilic sols and lyophobic sols.

The word lyophilic implies liquid- loving. In this type of colloidal sols, the dispersed phase has great affinity for the dispersion medium. In such colloids, the dispersed phase does not get easily precipitated and the sols are quiet stable.

The word lyophobic means liquid-hating. In this type of sols, the dispersed phase has very little affinity for the dispersion medium. The sols are relatively less stable than the lyophilic sols.

3. Classification on the basis of the type of the particles of the dispersed phase.

Depending upon the nature of particles, the colloidal solutions are classified as:

• MULTIMOLECULAR COLLOIDS: the colloidal particles are aggregates of atoms or small molecules with molecular size less than 1nm. The aggregated particles are held together by Van der Waal’s forces.
• MICROMOLECULAR COLLOIDS: the colloidal particles are themselves large molecules having colloidal dimensions. They have very high molecular masses varying from thousands to millions. These colloidal solutions are quite stable and resemble true solutions in some aspects.
• ASSOCIATED COLLOIDS: There are certain substances which behave as normal, strong electrolytes at low concentration but at higher concentration they exhibit colloidal behavior due to the formation of aggregated particles. Soaps and detergents are examples of such colloids. The cluster or aggregate formed in the solutions are called micelles.

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Properties of Colloidal Solutions 

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The colloidal solutions carry the properties on the basis of their heterogeneous nature, colligative properties, filterability, optical properties and mechanical properties.

1. HETEROGENEOUS NATURE: the colloidal system is of two phases (a) Dispersed phase and (b) Dispersion phase. In a colloidal solution, each particle is contained within its own boundary surfaces which separate it from the dispersion medium.
2. COLLIGATIVE PROPERTIES: Due to high average molecular masses of colloidal particles, they give very low mole fractions in Colloidal Solutions. Hence, they possess very low values of colligative properties such as relative lowering of vapor pressures, elevation in boiling point, depression in freezing point and osmotic pressure.
3. FILTERABILITY: The size of the colloidal particles is less than the pores of the filter paper, and therefore, they easily pass through a filter paper. Colloidal particles, however, cannot pass through the parchment paper or an animal membrane or ultrafilter.
4. OPTICAL PROPERTIES:

     (a) TYNDALL EFFECT: Tyndall in 1869 observed that if a strong beam of light is passed through a colloidal solution placed in dark room, the path of the beam gets illuminated by a bluish light. This effect was first observed by Faraday and later studied in details by Tyndall and is called Tyndall effect. The illuminated path of beam is called Tyndall cone. The phenomenon is due to the scattering of light by the colloidal particles in all the directions.

     (b) COLOUR: Color of colloidal sol depends upon the wavelength of the light scattered by the dispersed particles. The wavelength further depends on the size and nature of colloidal particles. The color of colloidal sol also depends upon the manner by which the observer receives the scattered light. For example, a mixture of milk and water appears blue when viewed by the reflected light and red when viewed by transmitted light.

5. MECHANICAL PROPERTIES:

     (a) Brownian movement: Robert Brown, a botanist discovered in 1827 that pollen grains placed in water do not remain at rest but move about continuously and randomly. Later on, this phenomenon was observed in case of colloidal particles when they were seen under the ultra-microscope. The particles were seen to be in constant motion in zig-zag path in all possible direction. This zig-zag motion of colloidal particles is called Brownian movement.

The presence of equal and similar charges on colloidal particles are also largely responsible in providing stability to the colloidal solution, because the repulsive forces between similarly charged particles prevent them from aggregating when they come closer to one another.

Brownian Movement

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Things To Remember

• The CBSE Class 12 Chemistry paper has a duration of 3 hours. 
• Students must manage their time according to the sections to finish their papers on time. 
• The Chemistry paper is divided into the practical and theory parts. 
• The theory section of the chemistry paper is of 70 marks. 
• 30 marks has been allotted to the practical part. 

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