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Friday, 11 December 2020

Colloid - definition, properties and examples

By Andrew Joseph     December 11, 2020     Chemistry     No comments   

  A colloid is a type of a mixture containing two phases, in which one phase of the mixture is dispersed throughout  the other in form of a very small microscopic droplets or particles. In simpler terms, a colloid is a system in which one substance is dispersed as a very fine particle in another. A colloid appears to posses homogeneous features when viewed from the normal human eye, but when viewed using a microscope, it clearly composed of tiny particles of macromolecular structures, scattered throughout another medium which could be a liquid or a gas.

A typical example of a colloid is a fog. Here we can consider two phases of a mixture where liquid crystals are scattered around a gas, which in this case is air. 


Dispersed phase and dispersion medium

In a colloid, the phase which consist of substances which are scattered or present in form of colloidal particles is called the dispersed phase while the medium in which the colloidal particles are dispersed on is called the dispersion medium. 


The dispersed phase of a colloid is composed of very small minute particles, and it dispersed or suspended throughout another substance. These particles sometimes cannot be distinguished from the mixture with the naked eye, but only with the use of a strong microscope. They can range from 1 to 1000 nanometer in diameter and are larger than molecules, but tiny enough to remain evenly distributed throughout the mixture. Unlike a suspension, colloidal particles do not settle to the bottom of the container when left undisturbed overtime. 

Properties of a colloid

1. A colloid is a mixture composed of very tiny particles dispersed in a medium. 

2.  A colloid is made up of two phases namely; The dispersed phase and the dispersion phase (or medium).

3. BROWNIAN MOTION: The particles presents in a colloid neither settle, rest nor affected by gravity. They are constantly at a random zigzag type of movements. This type of uniformless or directionless movements is known as brownian motion. This therefore means that, unlike a regular suspension, no matter how long a colloid is left undisturbed, it dispersed particles do not separate out or settle at the bottom due to gravity.

4. TYNDALL EFFECTS: Colloids do exhibit the tyndall effect due to reasons that it particles although very tiny are large enough to scatter a beam of light shinning through it, making the path of the light appear visible.

5. Colloids are unable to pass through a porous membrane without filtering at least some of it particles, especialy if the pores have a size smaller than their dimension.

6. ELECTROSTATIC INTERACTION: particles in a colloid often posses electric charges, making them to attract or repel each other.

Types of colloids

Colloids are classified based on the physical states of it dispersion medium and it dispersed phase. Below are the types of colloids :

  • SOLID SOLS
  • SOLS
  • SOLID AEROSOLS
  • LIQUID AEROSOLS
  • GELS
  • FOAMS
  • EMULSIONS

To understand it a little more simpler, let arrange them into three distinct groups, namely:

1. Solid Dispersion Medium : When the dispersion medium is solid, the dispersed phase can also be solid, liquid or gas. Based on the state of dispersed phase, colloids are either called Solid Sols or Gels.

  • Solid Sols - solids dispersed on a solid. 
                                ► Gemstones, Pearls, some colored Glasses

  • Gels - liquids dispersed on a solid.
                                 ►Butter, cheese, Jellies, Jam, Shoe polish
  • Solid Sols - Gases dispersed on a solid.
                                 ►Pumice Stone, Foam Rubber


2. Liquid Dispersion Medium :  When dispersion medium is liquid, the dispersed phase can be solid, liquid or gas. Based on the state of dispersed phase, colloids are either called Sols,Emulsions or Foam.

  • Sols - solids dipersed on a liquid.
                                  ► Paints, Gum, Muddy water 

  • Emulsion - liquids dispersed on a liquid.
                                  ► Milk, Hair cream, Cod-liver oi

  • Foam - gases dispersed on a liquid.
                                   ► Froth, soap lather, Whipped cream


3. Gas Dispersion Medium : When dispersion medium is gas, the dispersed phase can be solid or liquid but not the gas**. Colloids of gas with dispersed phase as solids or liquids are called Aerosols.

  • Solid Aerosols
                                   ► Smoke, Dust, Clouds (Ice particle clouds)

  • Liquid Aerosols
                                    ► Insecticide spray, Fog, clouds (water                                                                    particles/droplet clouds)  

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