Colloids are found mainly in nature and can also be made by humans. Since colloids have such a wide range of industrial applications, it’s critical to have a comprehensive categorisation system in place. A dispersed phase and a dispersion medium constitute colloids. Colloids are also called colloid dispersion because in a colloidal solution the particles do not settle down in the bottom of the container and remain dispersed. The phase in which substances are being dispersed is called dispersed phase, and the substance in which it is dispersed is called the continuous phase. Classification of colloids is based on the dispersed phase and the medium features. 

What is a Colloid?

A colloid is a heterogeneous system in which one component is scattered as extremely small particles (dispersed phase) in another material called the dispersion medium. The primary distinction between a solution and a colloid is particle size. Colloidal particles are bigger than simple molecules yet still tiny enough to float. Their sizes range from 1 to 1000 nm (10-9 to10-6 m).

Classification of Colloids

The following criteria are used to categorise colloids:

• The physical condition of the dispersed phase and dispersion medium- Eight colloidal systems depend on whether the dispersed phase and dispersion medium are liquids, solid or gases. A colloidal system is one in which a gas is combined with another gas to generate a homogenous combination. Colloids are found in a variety of commercial and natural goods. 

• An Emulsion is defined as a light dispersion of minute droplets of one liquid in another immiscible liquid. The droplets’ diameters range from 10-4 to 10-6 cm. Examples – Milk and butter
• Solid Aerosols – A solution in which extremely minute small particles of solid particles are spread into a gas is a solid aerosol. Examples – Smoke and dust
• Solid Sols – The particles of one solid material are distributed among the particles of another solid substance in a colloidal system. Examples – Colored gas and rock salt 
• Gels – Gels are colloidal solutions in which minute droplets of liquid are distributed in a solid dispersion medium. Cheese and curd are good examples.
• Solid Foam – A colloidal solution is a solid dispersion medium in which minute gas particles are scattered. Rubber and Cake are its examples.

• The nature of the dispersed phase’s interaction with the dispersion medium – Colloidal sols are classified as lyophilic (solvent attracting) or lyophobic (solvent repelling) depending on the nature of the dispersed phases’s interaction with the dispersion medium. The words hydrophilic and hydrophobic are used when the dispersion medium is water.

• Liquid-loving colloids are referred to as lyophilic colloids. Lyophilic sols are colloidal sols made by combining gum, gelatine, starch, rubber, and others with a suitable liquid (the dispersion medium). If the dispersion medium is detached from the dispersed phase (for example, by evaporation), the sol may be rebuilt by simply mixing it with the dispersion medium; this is a key feature of these sols. These sols are referred to as reversible sols because of their nature. 
• Colloids that move away from the water are lyophobic colloids or liquid-hating colloids. Metals, sulphides, and other substances do not form colloidal sol when combined with the dispersion medium. Only particular processes are used to make these colloidal sols. These sols rapidly precipitate (or solidify) when small quantities of electrolytes are added or heated, or shaken and are not stable. Furthermore, they do not simply add the dispersion medium to restore the colloidal sol once precipitated. These sols are referred to as irreversible sols because of their nature. For the preservation of lyophobic sols, stabilising agents are required.

• Type of particles in the dispersed phase – Colloids are characterised as multi-molecular, macromolecular, or related colloids depending on the type of particle. 

• In multi-molecular colloids, large numbers of atoms or small molecules of a substance combine to group together during dissolution to produce colloidal species (1–1000 nm in size). Example – A silver colloid may include a huge number of atoms as well as a wide range of particle sizes.
• Macromolecules in appropriate solvents generate macromolecular colloids, which are solutions in which the macromolecules’ sizes are in the colloidal range. These colloids are extremely stable and in many ways, resemble real solutions. Glucose and lipids are examples of macromolecules found in nature. Polythene, nylon, polystyrene, and synthetic rubber are examples of artificial macromolecules.
• Associated colloids (Micelles) – Certain chemicals behave as strong electrolytes at low concentrations but display colloidal behaviour at greater concentrations due to aggregates development. Micelles are the aggregated particles produced by this process. 

Applications of Colloids 

Colloids have a wide range of applications in the industry. Here are a few instances:

1. Smoke precipitation is caused by electricity
2. Drinking water purification
3. Medicines
4. Tanning
5. Soaps and detergents have a cleansing function
6. Plates and films for photography
7. Rubber is a large industry
8. Products for the industry

Conclusion

In chemistry, a colloid is a material composed of large molecules combined with another substance. Colloids cover a large variety of products, many of which you may already have in your home, which is why we need to divide them down into categories. The difference between a real solution and a colloidal solution is that a true solution doesn’t scatter a beam of light travelling through it. Still a colloidal solution scatters a beam of light travelling through it and makes its path apparent.