Why is smoke passed through charged plates before allowing it to come out of chimneys in factories?

Answer:

Smoke coming out of the chimneys contain as lot of unburnt carbon particles. Smoke is the colloidal solution of the solid particles such as carbon arsenic dust particles etc.These particles are injurious to health and have to be precipitated.

Smoke contains electrically charged particles of carbon along with other fuel gas. When it is passed through electrically charged plates, the charged carbon particles are attracted to the charged plate, where they lose their charge, coagulate and drop down as dust. The smoke coming out of chimneys now is free from carbon particles and hence do not cause pollution. The apparatus used for the purpose is known as Cotrell precipitator.