Colloids: Properties of Colloids, Coagulation and Precipitation (For CBSE, ICSE, IAS, NET, NRA 2022)

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Properties of Colloids

Heterogeneous Character

  • A colloidal system is a heterogeneous mixture of two phases.
  • Colloidal particles remain within their own boundary surfaces which separates them from the dispersion medium.

Brownian Movement

Brownian Motion or Movement is the zig-zag movement of colloidal particles in continuous and random manner (Fig) .

Brownian Movement

Brownian movement arises because of the impact of the molecules of the dispersion medium on the particles of dispersed phase. The forces are unequal in different directions and causes the particles to move in a zig-zag way.

Tyndall Effect

  • If a strong beam of light is through a colloidal solution, then the path of light is illuminated. This phenomenon is called Tyndall Effect.
  • This phenomenon is due to scattering of light by colloidal particles.
  • This is same effect which occur when beam of light enters a dark room through a slit and becomes visible.

Electrical Properties

  • The colloidal particles are electrically charged and carry the same type of charge, either negative or positive.
  • The dispersion medium has an equal and opposite charge.
  • The colloidal particles therefore repel each other and do not cluster together to settle down.
  • For example, arsenious Sulphide sol, gold sol, silver sol, etc. contain negatively charged colloidal particles whereas ferric hydroxide aluminum hydroxide etc. contain positively charged colloidal particles.

Origin of charge on colloidal particles is due to:

  • Preferential adsorption of cations or anions by colloidal particles.
  • Miscellas carry a charge on them due to dissociation.

Charge on a colloidal particle is verified by a process called electrophoresis.

Electrophoresis involves the movement of colloidal particles either towards cathode or anode under the influence of Electric Field. This is shown in fig.

Electrical Properties

Coagulation and Precipitation

The process of settling of colloidal particles is called coagulation or precipitation of the sol.

Method of Coagulation of Lyophobic Sols

By Electrophoresis

The colloidal particles move towards oppositely charged electrodes, get discharged and precipitated.

By Mixing Two Oppositely Charged Sols

  • Opposite charge sol when mixed in almost equal proportions, neutralize their charges and get partially or completely precipitated . This type of coagulation is called mutual coagulation.
  • For example, Mixing of hydrated ferric oxide and arsenious sulphide bring them in the precipitated forms.

By Boiling

  • When a sol is boiled, the adsorbed layer is disturbed due to increased collisions with the molecules of dispersion medium.
  • This reduces the charge on the particles and ultimately lead to settling down in the form of precipitates.

By Addition of Electrolyte

  • When excess of an electrolyte is added, the colloidal particles are precipitated.
  • The reason is that colloids interact with ions carrying charge opposite to that present on themselves. This causes neutralization leading to their coagulation.

Hardy-Schulze Rule

According to this rule, the greater the valence of the flocculating ion added, the greater is its power to cause precipitation.

In the coagulation of a negative sol, the flocculating power is in the order:

In the coagulation of a positive sol, the flocculating power is in the order:

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