Properties of colloidal solutions & Coagulation

(I)                Colligative Properties: – 

The values of colligative properties (osmotic pressure, lowering in vapor pressure, depression in freezing point, elevation in boiling point) are of small order as compared to values shown by true solution at same concentration.

(II)             Tyndall effect: – 

It is may be defending as the scattering of the light by the colloidal particles present in the colloidal solution.

(III)          Colour: – 

The colour of the colloidal solution depends on the wavelength of the light scatter by the dispersed particles. The wavelength of light further depends on the size and nature of the particles.

(IV)          Brownian Movement: –

It may be defined as continuous zigzag movement of the colloidal particles in a colloidal solution is known as Brownian movement.

(V)             Charge as colloidal particles:-

Charge as colloidal particles always carry is electric charge. The nature of this charge is the same on all the particles in a given colloidal solution and may be either +ve  or –ve.

Example:-

(a)     +ve charged solution:-

1. Haemoglobin
2. Hydrated Metallic oxides

E.g. AS₂S₃.xH₂O

(b)    –ve charged solution:-

1. Metals

E.g. Cu, Ag, Au

2. Metallic sulphides

E.g. AS₂S_3, Sb₂S₃, CdS solutions.

(VI)          Electrophoresis: –  

The movement of colloidal particles under the influence of an electric field is called electrophoresis. –ve charged particles move towards the cathode and +ve charged particles moves towards anode.

Coagulation: – 

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

The coagulation of the lyophobic sols can be carried out in the following ways:-

1. By electrophoresis: – The colloidal particles move towards oppositely changed electrodes get discharged and precipitate.
2. By mixing two oppositely charged sols: – Oppositely charged sols when mixed almost equal proportional neutralise then changed and get partially or completely precipitated.
3. By Boiling: – When a sol is boiled the adsorbed layer is disturbed due to increased collisions with the molecules of the dispersion medium. This reduces the charge on the particles and ultimately led to setting down in the form of a precipitate.
4. By Persistent dialysis: – On prolonged dialysis, traces of the electrolyte present in the sol are removed almost completely collides become unstable and ultimately coagulate.
5. By addition of electrolyte: –  When excess of electrolyte is added the colloidal particles precipitated, the reason is that colloids interact with ions carrying change opposite to that present on themselves, this causes neutralisation leading to their coagulation.

Coagulation of lyophilic sols: – 

There are two factors which are responsible for stability or the lyophilic sols. These factors are change and salvation of the colloidal particles. When these two factors are removed, a lyophilic sol can be coagulated. This is done,

(i)     By addition of an electrolyte

(ii)   By adding a suitable solvent

Protection of colloids:-

Lyophilic sols are more stable than lyophobic sols. Lyophilic colloids have a unique property of protecting lyophobic colloids. When a lyophilic sol is added to lyophobic sol, the lyophilic particles (colloids) covering up the particles of lyophobic sol.

Emulsions:-

An emulsion is a colloidal dispersion in which both the dispersion medium and dispersed phase are liquids generally; one of the two liquids is water. There are two types of emulsions.

• Oil dispersed in water (o/w type) and
• Water dispersed in oil (w/o type)

1. O/W type– water act as a dispersion medium.

Example: – Milk and vanishing cream.

2. W/O type– oil act as dispersion medium.

Example: – Butter and cream.

Colloids around us: – 

Most of the substances all we come across in our daily life are colloids, for example meals, clothes, wooden, furniture, houses, newspaper are largely composed of colloids.

Application of colloids: – 

Colloids are widely used in the industrial sector.

Example:-

• Electro precipitation of smoke: – 

The smoke, before is comes out from the chimney, is led through a chamber containing plates having a charged opposite to that carried by smoke particles. The particles on coming in contact with these plates lose their charge get precipitated, the particles thus settle down on the floor of the chamber. The precipitator is called Cottrell precipitator.

Purification drinking water: – 

Alum is added to water (that contain impurities) to coagulate the suspended impurities make water fit for colloidal in nature.

Example: – Argyrols is silver sol is used as an eye lotion.

• Medicines :-

Most of the medicines are colloidal in nature Ex:- Argyrols is a silver sol used as an eye lotion.

• Tanning:- 

when a hide an Animal skin  which has +ve  charged particles is soaked in tanning (or chromium salt) which contains – ve  charged particles , mutually coagulation take place. Result in the hardening of leather. This type of  process is termed as tanning.

• Photographic plates and films: – 

Photographic plates and films are prepared by coating an elision of the light sensitive silver bromide in gelatin over glass plates or celluloid films.

• Rubber industry: – 

Latex is a colloidal solution of rubber particles which are negative charged Rubber is obtained by coagulation of latex.

• Industrial products: – 

Paints inks, synthetic plastics, rubber, cement, graphite lubricants, etc are all colloids solution.