What is meant by a pure substance?

Fractional distillation is suitable for the separation of miscible liquids with a boiling point difference of about $25\, K$ or less. What part of the fractional distillation apparatus makes it efficient and provides an advantage over a simple distillation process? Explain using a diagram.

Which of the following are homogeneous in nature?

$A$ child wanted to separate the mixture of dyes constituting a sample of ink. He marked a line by the ink on the filter paper and placed the filter paper in a glass containing water as shown in the figure. The filter paper was removed when the water moved near the top of the filter paper.
$(i)$ What would you expect to see,if the ink contains three different coloured components?
$(ii)$ Name the technique used by the child.
$(iii)$ Suggest one more application of this technique.

Why is the path of light not visible in a solution when a beam of light is passed through it?

Give some examples of the Tyndall effect observed in your surroundings.