Give information about colloids. Explain the classification of colloids.

(N/A) We know that solutions are homogeneous systems. We also know that sand in water when stirred gives a suspension,which slowly settles down with time. Between the two extremes of suspensions and solutions,we come across a large group of systems called colloidal dispersions or simply colloids.
$A$ colloid is a heterogeneous system in which one substance is dispersed as very fine particles in another substance called the dispersion medium.
The essential difference between a solution and a colloid is that of particle size. In a solution,the constituent particles are ions or small molecules. In a colloid,the dispersed phase may consist of particles of a single molecule (such as protein or synthetic polymer) or an aggregate of many atoms,ions,or molecules.
Colloidal particles are larger than simple molecules but small enough to remain suspended. Their range of diameters is between $1$ to $1000$ $nm$ ($10^{-9}$ to $10^{-6}$ $m$).
Colloidal particles have an enormous surface area per unit mass as a result of their small size. Consider a cube with $1$ $cm$ side. It has a total surface area of $6$ $cm^{2}$. If it were divided equally into $10^{12}$ cubes,the cubes would be the size of large colloidal particles and have a total surface area of $60,000$ $cm^{2}$ or $6$ $m^{2}$.
This enormous surface area leads to some special properties of colloids.
Colloids are classified on the basis of the following criteria: $(i)$ Physical state of dispersed phase and dispersion medium. $(ii)$ Nature of interaction between dispersed phase and dispersion medium. $(iii)$ Type of particles of the dispersed phase.
Classification based on physical state of dispersed phase and dispersion medium: Depending upon whether the dispersed phase and the dispersion medium are solids,liquids,or gases,eight types of colloidal systems are possible. $A$ gas mixed with another gas forms a homogeneous mixture and hence is not a colloidal system.

$No$. \text{Type of colloid}	\text{Dispersed phase} / \text{Dispersion medium}	\text{Examples}
$(1)$ \text{Solid sol}	\text{Solid} / \text{Solid}	\text{Coloured glasses,gem stones}
$(2)$ \text{Sol}	\text{Solid} / \text{Liquid}	\text{Paints,cell fluids}
$(3)$ \text{Aerosol}	\text{Solid} / \text{Gas}	\text{Smoke,dust}
$(4)$ \text{Gel}	\text{Liquid} / \text{Solid}	\text{Cheese,jellies}
$(5)$ \text{Emulsion}	\text{Liquid} / \text{Liquid}	\text{Milk,hair cream,butter}
$(6)$ \text{Aerosol}	\text{Liquid} / \text{Gas}	\text{Fog,mist,cloud,insecticide sprays}
$(7)$ \text{Solid sol}	\text{Gas} / \text{Solid}	\text{Pumice stone,foam rubber}
$(8)$ \text{Foam}	\text{Gas} / \text{Liquid}	\text{Froth,whipped cream,soap lather}