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Question

Which of the following are true with respect to the colour of a colloid?
  1. The colour depends on the wavelength of the light
  2. The colour depends on both the size and nature of the dispersed particles
  3. In a light source containing many different wavelengths, a colloid scatters lower wavelengths to a much greater extent than the higher wavelengths
  4. The colour of a colloid depends on the manner in which the observer receives light. When viewed under transmitted light, it could appear one way and when viewed under reflected light, the colloid could appear in a different colour   


Solution

The correct options are
A The colour depends on the wavelength of the light
B The colour depends on both the size and nature of the dispersed particles
D The colour of a colloid depends on the manner in which the observer receives light. When viewed under transmitted light, it could appear one way and when viewed under reflected light, the colloid could appear in a different colour   
How will you differentiate a Colloidal dispersion from a true solution by using a light source? When we allow a beam of light to pass through a true solution, we can see that the beam tends to retain its shape. Also, we can’t see the path of the light beam as it passes through the bulk of the true solution. On the other hand, the particles in a colloid tend to scatter all wavelengths equally and this phenomenon is called Tyndall Scattering. This is the reason why we can clearly see the path of a beam passing through a colloidal dispersion. The nature of this scattering largely depends on the ratio of the size of the dispersed phase to the wavelength of the light.
In fact, the white colour of fog and milk are chiefly attributed to Tyndall scattering. The butterfat droplets in milk are actually transparent, but the scattering disperses the light in all directions so extensively that we are not that seeing through milk.
Further, the colour of colloidal solution also depends on the manner in which the observer receives the light. For example, when milk is viewed by reflected light it appears bluish. On the other hand, it appears reddish when viewed by transmitted light. 
Hence, options A, B and D are correct.
 

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