How is a pure substance different from a mixture?

Three students $A, B$ and $C$ prepared mixtures using chalk powder,common salt and milk respectively in water. Whose mixture
$(i)$ would not leave residue on filter paper after filtration?
$(ii)$ would show Tyndall effect?
$(iii)$ would give transparent/clear solution?
$(iv)$ would settle down at the bottom when left undisturbed?
$(v)$ could be filtered by filter paper?

Chromatography is the technique which is used for separation of those solutes that

What is meant by a pure substance?

$A$ group of students took an old shoe box and covered it with 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 the 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?

Tincture of iodine has antiseptic properties. This solution is made by dissolving