Preparation and properties of colloids – Tyndall effect.

Introduction

Colloids can be formed by  various techniques involving physical, chemical as well as by using some dispersion methods. These techniques rely on different principles. Let us explore more about these techniques for Preparation of Colloids.

Ways of preparation of colloids 

Preparation of Lyophilic Sols: For preparation of the lyophilic sol, the dispersed phase is directly put into the dispersion medium in cold or in warm situations.

Colloidal solutions of starch, gum, gelatin, etc. with water can be formed by this method. Solutions of colloidal electrolytes such as soaps and dyestuffs can also be generated by this method.

Preparation of Lyophobic Sols:

For preparation of the lyophobic sol, the substance in bulk is broken down into smaller particles of colloidal dimensions (also known as Dispersion) or by collecting smaller particles into particles of colloidal dimensions (also known as condensation). To increase the stability of sol, certain substances are put into the sol, the substances added are termed as stabilisers.

Dispersion methods:

In this method particles of larger size are broken down to smaller I.e. the colloidal size in dispersion medium. Starting with the material in greater amounts a colloidal solution is prepared by using suitable devices to make it into particles of colloidal size. Normally this is done by physical methods.

Electrical Dispersion or Bredig’s Arc Method

This procedure is used to make metal sols like platinum, silver, gold, copper in water. A very high voltage is introduced and then an electrical arc is passed between the tips of electrodes. This generates large heat  which results in the metal rods melting, evaporating and suddenly then it is cooled due to freezing point, the mixture gives rise to the colloidal solution of the metal.

Peptization or Chemical Dispersion:

Repeated dispersion of freshly prepared precipitate into the sol by putting an electrolyte containing common ion is called peptization. An electrolyte used for peptization is called the peptizing agent. Peptization is a reverse process of coagulation. The peptization action is due to the selected adsorption of one of the many ions of the electrolyte on the precipitate.

Double Decomposition Technique: 

When hydrogen sulphide is driven through a solution of arsenious oxide within distilled water, what we get is a colloidal solution of arsenious chloride.

As2O3 + 3H2S → As2S3 +  3H2O

Oxidation Technique

A solution of colloids of Sulphur is made by passing it through an aqueous solution of sulphur dioxide. It can even be prepared by passing the gas through a solution of an oxidation agent for example bromine water also with nitric acid.

SO2 + 2H2S →  2H2O + 3S

Reduction Technique:

Another method of preparation of colloidal solutions of metals such as silver, gold along with platinum involves the use of a reducing agent for reducing the salt solutions of the metals. 

Hydrolysis Technique:

 It includes the use of hot boiling water to obtain a reduced solution of ferric chloride.

Physical Methods of Preparation

Variety of physical methods can also be used to obtain colloidal dispersions. Some of them are listed below:

Exchange of Solvent

It involves the preparation of a colloidal solution of a metal by addition of its alcoholic solution to a large  amount of water. This colloidal preparation can take place only when the element is more dissolvable in alcohol than compared to water. Example: When an alcoholic solution of Sulphur is driven to pass through a large amount of water it forms a colloidal solution of Sulphur. This is due to the solubility of Sulphur being greater in water as opposed to alcohol.

The Technique Involving Excessive Cooling

This technique involves freezing down a solution of water in organic solvents such as chloroform, ether, etc to form solutions of colloids of ice. It is impossible for the molecules of water to exist separately and combine to form colloidal molecules.

Properties of colloids- Tyndall effect

When light is passed from a true solution, the dissolved particles(dispersed phase) are too small to deflect the light.  However, the dissolved particles of a colloid, being larger than the true solution, do deflect light.  The Tyndall effect is the scattering of light in the visible spectrum by colloid particles.  You must have definitely “seen” a light beam passing through a small hole in a room or, from fog, smoke, or a scattering of dust particles suspended in air.  All three are examples of the Tyndall effect of colloids.  Suspensions might scatter light, but if the  suspended particles are unnecessarily  large, the suspension might be not visible as the light scattering will not occur at all.

Tyndall Effect Examples

Shining a flash of light beam into a glass of milk is an outstanding demonstration of the Tyndall effect. You may use skim milk or dilute the milk with a little bit of water so that you can see the effect of the colloid particles on falling of the light. beam.

Examples of how the Tyndall effect scatters red light may be seen in the red colour of smoke from motorcycles or two-stroke engines.

The visible beam of headlights of the vehicles in fog is caused by the Tyndall effect. The water droplets might scatter the light, making the headlight beams visible and blur.

Conclusion

Colloids are very different solutions because their dispersed particles are much larger than those of a true solution.  The dispersed particles of a colloid cannot be separated by the method of filtration, but they can scatter light, a phenomenon known as the Tyndall effect.