Colloids and Their Properties

 

A colloid is a mixture in which one substance of microscopically dispersed insoluble particles is suspended throughout another substance. Owing to this peculiar structure of colloid, it has varied physical and chemical properties.

 

Colloidal Properties can be divided into following categories

Physical Properties

Colligative Properties

Optical Properties-Tyndall effect

Mechanical Properties - Brownian Movement

Electrical Properties

 

Physical Properties of Colloidal Solutions

·         Stability: Colloids are relatively stable in nature. The particles of the dispersed phase are in a state of continuous motion and remain suspended in the solution.

·         Filterability:  Silver colloidal products require specialized filters known as ultrafilters for filtration. They readily pass through ordinary filter papers without yielding any residue.

·         Heterogenous nature: Since colloids consist of two phases, the dispersed phase as well as the dispersion medium, they are known as heterogeneous in nature.

·         Homogenous appearance: Even though colloids have suspended particles and are heterogeneous in nature, they appear as if a homogenous solution. This is because the suspended particles are so tiny that they are not conceivable by the naked eye.

 

Colligative Properties

The particles of the dispersed phase come together to form associate molecules. The formation of these associate molecules renders the solution certain special properties such as

·         a decrease in vapour pressure

·         elevation in boiling point

·         depression in freezing point

·         a decrease in osmotic pressure

 

Optical Properties of Colloidal Solutions: Tyndall Effect

Colloids exhibit a phenomenon known as the Tyndall effect observed by Tyndall in 1869. When we pass an intense converging beam of light through a colloidal solution kept in dark, the path of the beam gets illuminated with a bluish light. This phenomenon of scattering of light by colloidal particles is called Tyndall effect and the illuminated path is known as Tyndall cone. The dispersed colloidal particles scatter the light falling on them resulting in emissions that are comparable to ultraviolet and visible radiations. These scattered radiations get illuminated.

The zone of scattered light is observed to be much larger than the particle itself. This makes the colloidal particles to appear as tiny bright spots when viewed under a microscope. This has to be done at right angles to the beam of light.

True solutions do not exhibit Tyndall effect. This is because the size of particles (ions or molecules) present in a true solution are too small to scatter light. Thus, Tyndall effect can be used to distinguish a colloidal solution from a true solution.