A group of students took an old shoe box and covered it with a black paper from all sides. They fixed a source of light (a torch) at one end of the box by making a hole in it and made another hole on the other side to view the light. They placed a milk sample contained in a beaker/tumbler in the box as shown in the below Fig. They were amazed to see that milk taken in the tumbler was illuminated. They tried the same activity by taking a salt solution but found that light simply passed through it.

(a) Explain why the milk sample was illuminated. Name the phenomenon involved.
(b) Same results were not observed with a salt solution. Explain.
(c) Can you suggest two more solutions which would show the same effect as shown by the milk solution?"

AcademicChemistryNCERTClass 9

(a) Milk is a colloidal solution. The particulate matter present in the milk makes the light scatter which results in the Tyndall effect. Hence, milk gets illuminated.

(b) Salt is a homogenous solution. Small particles present in salt solution do not scatter light rays but simply pass through them.

(c) Detergent mixture and sulphur solution exhibit the Tyndall effect.

[Extra information: Scattering of light through heterogeneous mixture is called the Tyndall effect. It is shown by mixture, where the mixture has a tendency to scatter the light of a shorter wavelength and transmit the light of a larger wavelength.] 
Updated on 10-Oct-2022 13:29:39