Theory
When water soluble substances like sodium chloride, copper sulphate, sugar etc., are put into water they dissolve and a homogeneous solution is obtained. The particles of the solute are not visible and their size is molecular size. Such mixtures are called molecular solutions or true solutions.
Now suppose we take muddy river water or an insoluble substance like lead sulphate or calcium sulphate is put into water. The particles of the solutes are visible even with the naked eye because their size is large. On keeping for some time, particles settle down. These mixtures are called suspension.
In between these two extremes, there are particles which are bigger than molecules but are too small to be seen even by a microscope. The colloidal state can thus be regarded as the intermediate state between molecules and particles of a coarse suspension.
A substance is said to be in the colloidal state, when it is dispersed in another medium in the form of very small particles having diameter between 10-4 to 10-7 cm.
COLLOIDAL SOLUTIONS :
They considered as a heterogeneous system consisting of the following three essential components:
- A dispersed phase: It is also known as discontinuous or inner phase. It consists of discrete particles significantly larger than ordinary molecules and in this small particles of solute is diffused in solvent.
- A dispersion medium or continuous phase or the outer phase : It is the medium in which dispersed phase is present. This consists of continuously interlinked molecules.
- A stabilising agent: This is a substance which tends to keep the colloidal particles apart. Some colloids are self stabilizers.
Dispersed phase + Dispersion medium = Dispersion system (Colloidal solution)
Types of colloids:
PROPERTIES OF COLLOIDS:
- Heterogeneous nature : Colloidal solutions are heterogeneous in nature consisting of two distinct phases viz. the dispersed phase and the dispersion medium. Experiments like dialysis, ultrafiltration and ultracentrifuging, clearly indicate the heterogeneous character of colloidal system.
- Non- settling nature : Colloidal solutions are quite stable. The suspended colloidal particles remain suspended in the dispersion medium indefinitely.
- Filtrability : Colloidal particles readily pass through ordinary filter papers. It is because the size of the pores of the filter paper is larger than that of the colloidal particles.
- Diffusibility : Colloidal suspensions unlike true solutions do not readily diffuse through fine membranes and have a little power of diffusion. This is due to the large size of the colloidal particles as compared to ordinary solute particles.
- Colour : The colour of the sol is not always the same as the colour of the substance in the bulk. The colour of the colloidal solution changes as the size and shape of the particles change. It may exhibit different colours when seen by reflected and transmitted light.
- Shape of the colloidal particles : Different sol particles have different shapes, for example, red gold sol, silver sol, platinum sol.
- Visibility : Most of the sols appear to be true solutions with a naked eye, but the colloidal particle can be seen through an ultramicroscope.
- Colligative properties : Sol particles because of their free suspensions amongst the molecules of the medium, share kinetic energy with them in the same manner as molecules of regular solutes do. This gives rise to colligative properties like osmotic pressure lowering, depression of freezing point and elevation of boiling point. As the magnitude of colligative properties depends upon the number of solute particles present in the solvent, their values are smaller.
- Optical properties : Tyndall effect -If a homogeneous solution is observed in the direction of light, it appears clear and when it is observed from a direction at right angles to the direction of the light beam, it appears perfectly dark. But when a beam of light passes through colloidal solutions, it is scattered, the maximum scattered intensity being in the plane at right angles to the path of light. The path beam becomes visible. The effect was first observed by Faraday but was studied in detail by Tyndall and the effect is not commonly known as Tyndall effect.
