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 Fig.2.4. 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?

Question

Philoid · Accepted Answer

(a) The milk sample was illuminated because milk is a colloidal solution and its particles are big and hence, scatter the light passing through it. The phenomenon observed is called the “Tyndall effect” or “Tyndall scattering”.

(b) As the salt solution is a true solution i.e., the solute particle size is too small and hence they cannot scatter the light. That is why the salt solution does not show “Tyndall effect”.

(c) Examples of colloid are gold sol (colloidal gold), arsenic sulphide (AS₂S₃) sol, blood, detergent solution and sulphur solution. Sols have good stability and show the Tyndall effect.