Question

Question 37
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 Figure. 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?

Answer 1

(a) The milk sample was illuminated because milk is a colloidal solution and its particles are big enough to scatter the light, hence; they scatter the light passing through it. The phenomenon observed is called "Tyndall effect'.

(b) As the salt solution is a true solution i.e., the solute particle size is too small to scatter the light; hence, it does not show "Tyndall effect".

(c) Examples of colloid are gold sol, arsenic sulphide $\left(A{s}_2{S}_3\right)$ sol., Blood etc.