2. COLLOIDS
Colloid, suspension of tiny particles of one substance, called the
dispersed phase, in another phase, called the dispersion medium. The
particles are so small that they remain in suspension indefinitely, unaffected
by gravity.
Both the suspended, or dispersed, phase and the dispersion medium may be
solid, liquid, or gaseous, although the dispersal of one gas in another is not
known as colloidal dispersion. An aerosol is a colloidal dispersion of either a
solid colloid (such as cigarette smoke) or a liquid (such as insecticide spray)
in a gas, the air. An emulsion is a colloidal dispersion of liquid particles in
another liquid; mayonnaise, for example, is a suspension of tiny globules of
oil in water. A sol is a colloidal suspension of solid particles in a liquid;
paints, for example, are a suspension of minute solid pigment particles in an
oily vehicle. A gel is a sol in which the suspended particles are organized in a
loose, but definite three-dimensional arrangement, giving some rigidity and
elasticity to the mixture, as in jellies.
3. COLLOIDS
Colloids are mixtures with properties between solutions and
suspensions. To understand what colloids are, it is necessary to know what
are solutions and suspensions. Colloids consists of a dispersed phase and
dispersion medium.
The dispersed phase consists of the colloidal particles,
comparable to the solute in a solution.
The dispersion medium is the substance in which the colloidal
particles are distributed, comparable to the solvent in a solution.
In colloids, the dispersed particles do not settle out on standing.
4. TYPES OF COLLOIDS
Colloids may be grouped according to the phase (solid, liquid and
gas) of the dispersed substance and the dispersion medium. You can
prepare eight types of colloids.
e.g., A gas may be dispersed in a liquid to form foam (shaving
lather) or in a solid to form solid foam (Styrofoam or marshmallow).
A liquid may be dispersed in a gas to form an aerosol (fog or
aerosol spray), in another liquid to form a gel (jellies or cheese).
A solid may be dispersed in a gas to form a solid aerosol (dust or
smoke in air), in a liquid to form a sol (ink or muddy water), or in a solid to
form a solid sol(certain alloys).
A further classification is as lyophilic (solvent attracting), lyophobic (solvent
repelling) or association colloids (a mixture of the two).
If water is the dispersing medium, it is often known as a hydrosol.
5. TYPES OF COLLOIDS
Dispersed Dispersion
NAME Examples
Phase Medium
Sol solid liquid Paints, fruit jellies, gels, dyes
Mayonnaise, milk, salad,
Emulsion liquid liquid cream
Beer froth, soap suds,
Foam gas liquid whipped cream
Alloys, gemstones, ruby,
Solid Sol solid solid glass
Solid Emulsion liquid solid Butter, cheese
Pumice, marshmallow,
Solid Foam gas solid meringue
Smoke, dust in air, bacteria in
Solid Aerosol solid gas air
Liquid Aerosol liquid gas Fog, mist, cloud, sprays
6. PREPARING COLLOIDS
(1) Preparation of Lyophilic colloids
(i) The lyophilic colloids have strong affinity between particles of dispersed
phase and dispersion medium.
(ii) Simply mixing the dispersed phase and dispersion medium under ordinary
conditions readily forms these colloidal solutions.
(iii) For example, the substance like gelatin, gum, starch, egg, albumin etc.
pass readily into water to give colloidal solution.
(iv) They are reversible in nature become these can be precipitated and
directly converted into colloidal state.
(2) Preparation of Lyophobic colloids : Lyophobic colloids can be prepared
by mainly two types of methods.
(i) Condensation method : In these method, smaller particles of dispersed
phase are condensed suitably to be of colloidal size. This is done by the
following methods.
7. PREPARING COLLOIDS
(a) By oxidation : A colloidal solution of sulphur can be obtained by bubbling
oxygen (or any other oxidising agent like HNO3, Br2 etc.) through a solution of
hydrogen sulphide in water.
2H2S + O2 (or any other agent) → 2H2O + 2S
(b) By reduction : A number of metals such as silver, gold and
platinum, have been obtained in colloidal state by treating the aqueous
solution of their salts, with a suitable reducing agent such as
formaldehyde, phenyl hydrazine, hydrogen peroxide, stannous chloride etc
2AuCl3 + 3SnCl2 → 3SnCl + 2Au
Gold sol
2AuCl3 + 3HCHO + 3H2O → 2Au + 3HCOOH + 6HCl
Gold sol
The gold sol, thus prepared, has a purple colour and is called purple of
cassius.
8. PREPARING COLLOIDS
(ii) Dispersion methods : In these methods, larger particles of a substance
(suspensions) are broken into smaller particles. The following methods are
employed.
(a) Mechanical dispersion
• In this method, the substance is first ground to coarse particles.
• It is then mixed with the dispersion medium to get a suspension.
• The suspension is then grinded in colloidal mill.
• It consists of two metallic discs nearly touching each other and rotating in
opposite directions at a very high speed about 7000 revolution per minute.
• The space between the discs of the mill is so adjusted that coarse
suspension is subjected to great shearing force giving rise to particles of
colloidal size.
• Colloidal solutions of black ink, paints, varnishes, dyes etc. are obtained by
this method.
