Write four uses of colloids?

Hint:Colloids particles get coagulated by the addition of a suitable coagulating agent.
Colloidal particles possess charge in them.

Complete step-by-step answer:In the question we are asked to explain four applications of colloidal system.
From the lower classes we are studying different types of solutions. Now let’s brush up our memory about colloidal solutions and briefly describe the applications of colloids.
Colloids are an example for heterogeneous mixture. They are those solutions in which minute particles are dispersed along a uniform medium and the minute particles which are dispersed in the medium are called a dispersed phase and the medium is called a dispersion medium.
Colloids are classified into many depending on the nature of the dispersed phase and dispersion medium, based on the physical state, on the interactive force present etc.
Now let’s discuss the application of colloids.
Colloids play a very important role in our day to day life, we know that in the colloidal solution, the particles are dispersed in the medium and they don’t settle down without an application of an external aid.
Applications:
-Sewage disposal- we know that the sewage discharge is the colloidal solution of dirt, mud, sand etc.
And we know that the colloidal solution possesses a charge and when the sewage discharge is treated by passing them through plates which are kept at high potential, the particles get coagulated by the process of electrophoresis and the particles aggregated can be easily removed.
-Filtration of water-We know that the water may contain many impurities and these impurities will be in a colloidal state with the water and the impurities possess charge and these impurities can be removed by adding a coagulating agent, by the colloidal particles aggregates and settles down due to gravity and the impurities can be removed by simple decantation process.
For example in the water negatively charged impurity species were present, they can be coagulated by adding alum. The $A{{l}^{3+}}$ions present in alum neutralizes the negatively charged colloids and the colloids particles aggregates by the attraction force and settles down and is decanted off.
-Cleansing action of soap solution-The soap solution is a charged colloidal solution, which cleanses the dirt present by the phenomenon of adsorption or emulsification which is a property of the colloidal solution.
-Applications in medicines-Many medicines we take are present in the colloidal state. The medicines which are prepared in colloidal form are easily absorbed by our body and hence show effective results with short span of time.

Note:The dispersed phase cannot be removed or separated from the dispersion medium by simple filtration and these limits its applications in many areas.
The can only be removed by the techniques like coagulation, electrophoresis, centrifugation etc.