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Classification of Colloids

Chemical Bonding Assignment / Homework Help
Colloids can be classified as two types they are the lyophilic or reversible and lyophobic or irreversible. If water is the dispersion medium, the terms used are hydrophilic and hydrophobic.

Lyophilic colloids: A colloidal system that can be made readily by simple warming or shaking the substance in a suitable solvent is known as lyophilic colloid. Substances that form lyophilic colloids include gelatin, starch, proteins, gum and rubber. Lyophilic colloids are also termed as reversible colloids as on evaporating the dispersion medium such as water, the substance or the residue can be again reconverted into colloidal state simply by addition of the liquid i.e. water. These solutions are quite stable and cannot be easily precipitated.

Lyophobic colloids: colloids formed with difficulty are termed as lyophobic colloids. These colloids are formed by substances such as As2S3 , Fe(OH)3, gold and other metals which are sparingly soluble and thus their molecules do not pass readily into colloidal state. Lyophobic colloids are known as irreversible colloids since the residue cannot readily be reconverted into sol by ordinary means. These solutions are readily precipitated and hence are not stable.


Differences between lyophilic and lyophobic solutions

S.No Property Lyophilic solutions Lyophobic solutions
1 Reversibility These are reversible These are irreversible
2 Visibility The particles cannot be readily detected even under ultra-microscope The particles can be readily detected under ultra microscope
3 Surface tension Surface tension is usually lower than that of the dispersing medium. Surface tension is of the same order as that of the dispersion medium
4 Viscosity Viscosity is much higher than that of the dispersing medium Viscosity range is about the same as that of the dispersion medium.
5 Action of electrolytes Much large quantities of electrolytes are required to cause precipitation. Addition of small quantities of electrolytes causes precipitation
6 Influence of electric field The particles may or may not migrate under the influence of an electric field. The particles migrate either to cathode or to anode under the influence of an electric field.
7 Hydration Due to presence of a number of polar groups in the molecules the particles are well hydrated The particles are not hydrated to an appreciable extent.

Solved problems
  • What are the two major types of colloidal systems?
    • Lyophilic and reversible
    • Lyophobic and reversible
    • Lyophilic and Lyophobic
    • None of the above.
    Ans: c
  • Colloidal solutions that can be formed very easily by simple warming and shaking are termed as
    • Lyophilic colloids
    • Lyophobic colloids
    • Irreversible Colloids
    • Colloids cannot be formed by these methods.
    Answer: a
  • Addition of small amount of electrolytes results in precipitation of which type of colloidal?
    • Lyophilic colloids
    • Lyophobic colloids
    • Reversible colloids.
    • Addition of electrolytes does not cause precipitation.
    Answer: b
  • In which type of colloids the solute molecules move either towards the cathode or anode?
    • Lyophilic colloids
    • Reversible colloids
    • Irreversible colloids.
    • Solute molecules remain stable and do not move at all.
    Answer: c

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