Colloids Chemistry Notes
→ Thomas Graham (1861) found that gum, gelatin etc., are not diffused from animal membrane while aqueous solution of sugar, salt etc., are diffused easily from animal membrane. On this basis, he classified the substances into two classes.
→ Crystalloids : The substances whose solution readily diffuse through parchment paper or animal or plant membranes are known as crystalloids. Examples: Sugar-urea, glucose, sodium chloride, fructose etc.
→ Colloids : The substances whose solution either do not diffuse or diffuse very slowly through parchment membrane, are called colloids. Examples: Glue, gelatin, albumin, starch, gum, proteins etc. The colloids are different from the crystalloids in their particle size. This classification of substance is not rigid because a substance can behave like a crystalloid or a colloid under different conditions and circumstances.
→ For example: sodium chloride behaves as crystalloid in water but as a colloid in benzene. On the other hand, sulphur behaves as crystalloid in alcoholic solution while as a colloid in water. According to modern concept, colloid is a state of a system which depends upon the particle size of matter. Thus on the basis of the size of particles, substances are classified into three classes :
- True solution
- Colloidal solution
→ True Solution : True solution is a homogeneous mixture. The shape of particles of solute and solvent is same in this solution. The diameter of particles is less than 10 cm. Their particles cannot be seen by ultra microscope. Their particles are present in the form of ion or molecules.
→ Suspension : It is a heterogeneous mixture which contains large insoluble particles. The diameter of particles is more than 10 cm. These particles are accumulated at the surface of apparatus due to gravitational force. These particles can be seen by eyes.
→ Colloidal Solution : It is an intermediate state of above both conditions. The size of their particles is less than 10- cm and more than 10 cm. These particles can not be seen by eyes but can be seen under the microscope. These particles are not diffused rapidly due to their larger size as compared to particles of true solution and are also not accumulated in the bottom by gravitational force due to smaller size as compared to suspension. Such type of heterogeneous solutions are called colloidal solutions.
Differences between True Solution, Colloidal Solution and Suspension
Phases of Colloid :
It is a biphase or heterogeneous system. It has two phases which are given below:
Dispersed phase :
In colloidal system, the particles of soluble substances are known as dispersed phase. This phase is discontinuous. So it is also known as inner phase or discontinuous phase. The size of particles of dispersed phase should be in between 1 nm-1000 nm. The particles of this phase are known as colloidal particles.
Dispersion medium :
This is the medium in which the particles of dispersed phase are distributed in colloidal form. This phase is continuous hence it is known as continuous phase or dispersion medium. This phase is of solvent phase hence it is known as outer phase or external phase. For example : In a gold solution, the colloidal particles of gold constitute the dispersed phase while water constitute the dispersion medium, On the basis of dispersion medium, following specific names are given to colloidal solutions.
- Aerosol : In this type of colloidal system, the dispersion medium is air or a gas.
- Hydrosol : In this type of colloidal system, the dispersion medium is water. It is also known as aqua sol.
- Alcosol : In the type of colloidal system, the dispersion medium is alcohol.
- Benzosol : In this type of colloidal system, the dispersion medium is benzene.
- Organosol : In this type of colloidal system, the dispersion medium is organic liquid.
Methods of Preparation of Colloidal Solution :
A few important methods for the preparation of colloids are as follows:
→ Methods of preparation of Lyophilic sols : In lyophilic colloid the dispersed, phase has great affinity with dispersion medium hence such colloids can be easily prepared by just shaking or warming dispersed phase in contact with dispersion medium. Examples : Colloidal solution of gum, gelatin, agar-agar, proteins etc. can be obtained just shaking them with water.
→ Methods of preparation of Lyophobic sols Condensation Methods
- By exchange of solvent
- Super cooling method
- Vapour condensation method
- Double decomposition method
- Mechanical dispersion
- Electrical dispersion
- Ultrasonic dispersion
Condensation methods for the preparation of Lyophobic sols :
→ In condensation method, the particles of size smaller than colloidal particles (<1 of size in true solutions) are condensed into bigger particles of colloidal dimensions. It may be carried out by the following methods :
Physical Methods: It is of following types:
→ By exchange of solvents : When the solution of any substance is mixed with such solvent in which the solubility of solute is lesser than colloidal solution then thissolute is obtained in that solvent. For example: If a solution of sulphur or phosphorus prepared in alcohol is mixed into water then a colloidal solution of sulphur or phosphorus is obtained due to low solubility in water.
→ By supercooling : If the mixture of water and chloroform is highly cooled then the minute particles of ice form a colloidal solution with chloroform where ice particles behave as dispersed phase and chloroform behave as dipersion medium.
→ Vapour Condensation Method : Colloidal solution of substance is obtained by passing vapour of boiled substance in liquid. This method can be used to form aqueous colloids of sulphur and mercury.