What are colloids?
COLLOIDS
Colloids, which are also known as colloidal solutions or colloidal systems are mixtures in which micro-insoluble particles of one substance are suspended in another substance. Suspended particles in a colloid can range from 1 to 1000 nanometres that is 10–9 metre in size. Colloids usually contain substances that are evenly scattered in another. In such a mixture, the material that is dispersed is called dispersed state whereas the material through which it is dispersed is called continuous phase.
Classification of colloids
Colloids are classified into several types.
Classification based on physical condition
(A) Solid Solution- Solids and dispersion medium in this scattered phase. Such as: Gemstone.
(B) Aerosol- These colloids contain air as the dispersion medium.
Example 1: Cloud- It consists of air as the dispersion medium and water droplets as the dispersion phase.
Example 2: Dust-. It consists of air dispersion medium and dent particles as dispersion phases.
Example 3: Smoke-It contains carbon particles in the air.
(C) Gels- In gels, the dispersion medium is solid, and the dispersed phase is liquid. E.g., Butter, cheese
(D) Emulsions- In emulsions, both dispersed phase and medium are liquids. They are further of 2 types: oil in water type, e.g., Milk and water in oil type, e.g., vanishing cream.
On the basis of forces of attraction between the dispersion medium and dispersed phase, the colloids can be classified as lyophobic and lyophilic colloids. The characteristics of these colloids are summarised in the table below:
Property | Lyophilic | Lyophobic |
|---|---|---|
Definition | They are solvent loving colloids | They are solvent hating colloids |
Stability | More stable due to strong forces of attraction between dispersed phase and dispersion medium | Less stable due to less forces of attraction between the dispersed phase and dispersion medium |
Preparation | Readily formed by warming and mixing the dispersed phase with the dispersion medium | Preparation is difficult and involves long procedures such as oxidation, reduction, hydrolysis, double decomposition, exchange of solvent, bredie’s arc method, peptisation,excessive cooling, etc. |
Coagulation | For coagulation of lyophilic colloids, addition of both solvent as well as electrolyte is required, since it is very stable. | Addition of a small amount of electrolyte leads to coagulation of the lyophobic colloidal solution. |
Reversibility | These colloids are reversible in nature | These colloids are irreversible in nature |
Examples | Metallic sols such as Ag, gold etc. | Starch, gum, gelatin,proteins,rubber etc |
Classification of colloids based on size of molecules
Based on the size of constituent molecules, the colloids can be classified as multimolecular, macromolecular and associated colloids.
Preparation of colloid solution
Stable colloids are also known as lyophilic sols, strong forces of interaction exist between the dispersion phases. These are prepared by the following appropriate methods.
Compaction method
In this method, small solute particles are condensed as dispersed phase particles.
- Chemical Methods:
A) By Oxidation: Colloidal sulfur can be obtained by passing oxygen gas from the hydrogen sulphide solution. In this method any oxidizing agent such as HNO3, H3Br2 can be used.
2H2S + O2 → 2H2O + 2S (Sulfur Sol)
B) By Double Decomposition: In this method a solution of arsenic sulphide is obtained. In this process hydrogen sulphide is passed through an Arsenic oxide cold solution in water.
AS2O3 + 3H2S → AS2S3 + 3H2O Arsenic Sulphide (Sol)
C) By Cutting: Aqueous solutions of these salts are obtained by reacting with suitable reducing agents such as formaldehyde, phenylhydrazone, hydrogen peroxide, stainless chloride, and many metals such as gold, silver, and platinum.
Sncl2+AuCl3 -> SnCl4+Au (Gold sol)
AuCl3 + HCHO + H2O → Au + HCOOH + HCl
Gold prepared in the reduction of gold chloride solution is purple in sol and is called violet of Cassius.
D) Extremely Cold: In this process, the colloidal sol of ice is obtained. In an organic solvent like chloroform ether, we take ice. The ice sol is obtained by freezing the water solution in a solvent. Water molecules are no longer in separate solutions to separate colloidal-shaped particles.
E) By Exchange of Solvent: In this process, the colloidal sols of some substances, such as sulphur, phosphorus, which are soluble in alcohol, but insoluble in water, can be prepared by mixing their alcohol solution with water. When added to water, a milky colloidal solution of sulphur is given for a sufficient alcoholic solution of sulphur.
(i) By Change of Physical State: The vapor of a substance such as mercury and sulphur are prepared by passing through a stable stabilizer containing cold water containing salt of ammonia.
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