THE USEFUL COLLOIDSPrepared by: Group 1 Campers
COLLOIDSColloid, suspension of tiny particles of one substance, called thedispersed phase, in another phase, called the dispersion medium. Theparticles are so small that they remain in suspension indefinitely, unaffectedby gravity.Both the suspended, or dispersed, phase and the dispersion medium may besolid, liquid, or gaseous, although the dispersal of one gas in another is notknown as colloidal dispersion. An aerosol is a colloidal dispersion of either asolid 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 inanother liquid; mayonnaise, for example, is a suspension of tiny globules ofoil 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 anoily vehicle. A gel is a sol in which the suspended particles are organized in aloose, but definite three-dimensional arrangement, giving some rigidity andelasticity to the mixture, as in jellies.
COLLOIDS Colloids are mixtures with properties between solutions andsuspensions. To understand what colloids are, it is necessary to know whatare solutions and suspensions. Colloids consists of a dispersed phase anddispersion 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 colloidalparticles are distributed, comparable to the solvent in a solution. In colloids, the dispersed particles do not settle out on standing.
TYPES OF COLLOIDS Colloids may be grouped according to the phase (solid, liquid andgas) of the dispersed substance and the dispersion medium. You canprepare eight types of colloids. e.g., A gas may be dispersed in a liquid to form foam (shavinglather) or in a solid to form solid foam (Styrofoam or marshmallow). A liquid may be dispersed in a gas to form an aerosol (fog oraerosol 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 orsmoke in air), in a liquid to form a sol (ink or muddy water), or in a solid toform a solid sol(certain alloys).A further classification is as lyophilic (solvent attracting), lyophobic (solventrepelling) or association colloids (a mixture of the two).If water is the dispersing medium, it is often known as a hydrosol.
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 glassSolid Emulsion liquid solid Butter, cheese Pumice, marshmallow, Solid Foam gas solid meringue Smoke, dust in air, bacteria inSolid Aerosol solid gas airLiquid Aerosol liquid gas Fog, mist, cloud, sprays
PREPARING COLLOIDS(1) Preparation of Lyophilic colloids(i) The lyophilic colloids have strong affinity between particles of dispersedphase and dispersion medium.(ii) Simply mixing the dispersed phase and dispersion medium under ordinaryconditions 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 anddirectly converted into colloidal state.(2) Preparation of Lyophobic colloids : Lyophobic colloids can be preparedby mainly two types of methods.(i) Condensation method : In these method, smaller particles of dispersedphase are condensed suitably to be of colloidal size. This is done by thefollowing methods.
PREPARING COLLOIDS(a) By oxidation : A colloidal solution of sulphur can be obtained by bubblingoxygen (or any other oxidising agent like HNO3, Br2 etc.) through a solution ofhydrogen sulphide in water.2H2S + O2 (or any other agent) → 2H2O + 2S(b) By reduction : A number of metals such as silver, gold andplatinum, have been obtained in colloidal state by treating the aqueoussolution of their salts, with a suitable reducing agent such asformaldehyde, phenyl hydrazine, hydrogen peroxide, stannous chloride etc2AuCl3 + 3SnCl2 → 3SnCl + 2Au Gold sol2AuCl3 + 3HCHO + 3H2O → 2Au + 3HCOOH + 6HCl Gold solThe gold sol, thus prepared, has a purple colour and is called purple ofcassius.
PREPARING COLLOIDS(ii) Dispersion methods : In these methods, larger particles of a substance(suspensions) are broken into smaller particles. The following methods areemployed.(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 inopposite directions at a very high speed about 7000 revolution per minute.• The space between the discs of the mill is so adjusted that coarsesuspension is subjected to great shearing force giving rise to particles ofcolloidal size.• Colloidal solutions of black ink, paints, varnishes, dyes etc. are obtained bythis method.