(i) No particle is visible to the naked eye if its diameter is less than half the wavelength of light used. The shortest wavelength of light is about 4000Å or 400 mμ. Hence no particle of diameter less than 200 mμ can be seen. The size of the colloidal particles is less than 200 mμ.
(ii) The size of the colloidal particles are smaller than the pores in the filter paper and hence, they pass through the filter paper.
(iii) This is due to the Tyndall effect. The colloidal particles absorb light energy and then scatter in all directions.
(iv) The colour of the colloidal solution depends as the wavelength of scattered radiation by dispersed particles. The wavelength of light further depends on the size and nature of particles. The finest gold sol is red in colour. As the size of particles increase, it becomes purple, then blue and finally golden yellow.
